DE102005041584B4 - Differential measurement method for determining concentration differences for supersaturation determination - Google Patents

Abstract

Method for determining the degree of saturation of a solution (12) to be investigated of a substance dissolved in a solvent at a specific temperature (T), which comprises the following steps:
- Forming a first interface (28) between a first optical element (26) and the solution to be examined (12);
- irradiating at least a first incident light beam (44) of a specific first Einstrahlpolarisation on the first interface (28) such that the first incident light beam (44) substantially by total reflection at the first interface (28) into a first reflected light beam (48). is reflected with a first expiring polarization, wherein the first Einstrahlpolarisation at least temporarily both a component in parallel and a component perpendicular to the plane of incidence of the first incident light beam (44);
- forming a second interface (32) between a second optical element (30) and a saturated solution (16) of the solute in the solvent at the determined temperature (T);
- irradiation of at least one second incidental ...

Description

The The present invention relates to a method and an apparatus for determining the degree of supersaturation or undersaturation a solution or suspension. The present invention is concerned with the control crystallization over the degree of supersaturation.

crystallization out of solution finds manifold application in the nutrition, chemical, pharmaceutical and basic industries. In the crystallization is made of a two- or multi-component solution (watery or organically) crystallized out a component which then, for example by filtration with as possible high yield and purity can be obtained.

The Kristallisat is usually intended to meet certain quality requirements Crystal size distribution, Purity and crystal form meet. For example, in the food industry Sugar industry of fulfillment these quality requirements highest Meaning attributed. Here these requirements are elaborately developed Procedures for process control (seeding, cooling / evaporation programs, intermittent, semi-continuous and continuous driving) Fulfills.

Out a solution crystals are formed by nucleation and subsequent growth. For both operations must be a supersaturation be present as a driving thermodynamic potential. A solution is then oversaturated, though a larger share a component in the solvent solved is, as it corresponds to the thermodynamic equilibrium. A supersaturation can for example by lowering the temperature or by withdrawal of solvents be achieved by evaporation. Because nucleation and growth rate by the amount of supersaturation determined is the mean supersaturation and its local distribution an important process parameter to be controlled.

So far Crystallization processes are not based on supersaturation guided, because no suitable in-line sensors are available.

As in 3 represented, the supersaturation or sub-saturation of crystallizing solutions can in principle be determined by determining the current concentration X of the crystallizing component in the solution using a suitable measurement method and their calibration. By simultaneously measuring the temperature of the crystallizing solution T _K , the supersaturation ΔX of the crystallizing component can be calculated with knowledge of the saturation curve of the substance system.

In the literature, for example, the measurement of the supersaturation or subsaturation, for example, the density measurement by means of "bending oscillator", refractometry, the measurement of the ultrasonic velocity and the ATR-FTIR spectroscopy are given as suitable measurement methods. For all these methods the exact knowledge of the saturation curve in the form X * = f (T _K ) is necessary. These measurement methods are also not applicable for poorly defined solutions, ie multicomponent or contaminated solutions. The methods are only suitable for two-component systems, ie pure solutions. In addition, the measurements are disturbed by crystals present, that is, before taking a measurement, it is necessary to separate the crystals. The accuracy of these methods for measuring the concentration difference .DELTA.X is also questionable if the solution to be examined is contaminated. In this case, a new calibration would have to be performed in which a new saturation curve X * = f (T _K ) is created for the contaminated solution.

Another method of supersaturation measurement is in DE 43 40 383 C2 and DE 43 45 163 C2 shown. In this method, the determination of solubility is not required. The method is in principle applicable to multi-component, contaminated solutions. In the supersaturated solution a temperature-sensitive sensor surface is immersed. The supercooling of this surface, in which a shortening sets in, is considered a measure of the prevailing supersaturation. The onset of the shortening is either registered by the associated heat of reaction or determined by the fact that the surface is designed as a vibrating piezoelectric crystal whose natural frequency changes due to adhering particles. Both versions of this method are in principle only applicable to material systems whose solubility depends significantly on the temperature. For this reason, such a sensor is for example not suitable for aqueous NaCl solutions, since in this case the solubility hardly depends on the temperature.

US Pat. No. 6,717,663 B2 describes an optical configuration for use in measuring the refractive index difference between a test sample and a reference sample. In this case, a divergent light beam is at least partially reflected on a surface on which a sample to be tested and a reference sample are arranged side by side. This results in two reflected partial beams, one of which was reflected at the interface to the test sample and the other at the interface to the reference sample. Depending on the respective refractive index transition at the surface, a part of the divergent light beam is totally reflected at the surface. A different refractive index of the two samples leads to a different proportion of To Talreflexion at the two reflected partial beams. By determining the different proportions of total reflection, a refractive index difference can be determined.

DE 693 17 8874 T2 describes a method and apparatus for measuring the difference in refractive index between two media each traversed by a beam of light. For this purpose, a polarized light beam is split into two sub-beams, one of which traverses a container with a composition to be measured and the other a container with a reference composition. After passing through the respective container, the two partial beams are brought back into interference. The resulting interference pattern depends on the refractive index difference between the composition to be measured and the reference composition, ie the difference in transit time or the difference in the optical path length of the two partial beams. Thus, the refractive index of the composition to be measured can be determined from the interference pattern.

Shock RAW describes in "Development of a supersaturation monitor" (Int Cryst., Prog. Oc. Symp., 8 ^th (1982), meeting date 1981, 363-4) a method for measuring a supersaturation in a crystallization based on the Measurement of refractive index differences. A refractive index difference between a supersaturated liquid sample from a crystallizer and a saturated sample at equilibrium at the local temperature is determined.

US 5,742,382 B2 describes a device for measuring a refractive index by means of the determination of the critical angle of total reflection.

It There is therefore an increasing need for industrial suspension crystallizers better able to control and regulate than before. By an improved Control and regulation can be more demanding product quality with higher Reproducibility be realized. The necessary in-line sensor technology is not yet available. It is therefore the task of the present invention, a method and an apparatus for the Improvement of the measurement of a degree of saturation a solution or to provide suspension.

These The object is solved by the features of claims 1 and 8, respectively.

• – Ausbilden einer ersten, vorzugsweise im wesentlichen planaren, Grenzfläche zwischen einem ersten, vorzugsweise zumindest teilweise im wesentlichen optisch transparenten, optischen Element, welches vorzugsweise eine Linse und/oder ein Prisma umfasst, und der zu untersuchenden Lösung bzw. Suspension;
• – Einstrahlen zumindest eines ersten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren ersten Einstrahlpolarisation auf die erste Grenzfläche derart, dass der erste einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der ersten Grenzfläche in einen ersten reflektierten Lichtstrahl mit einer ersten auslaufenden Polarisation reflektiert wird, wobei die erste Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des ersten einfallenden Lichtstrahls aufweist;
• – Ausbilden einer zweiten, vorzugsweise im wesentlichen planaren Grenzfläche zwischen einem zweiten, vorzugsweise zumindest teilweise im wesentlichen optisch transparenten, optischen Element, welches vorzugsweise eine Linse und/oder ein Prisma umfasst, und einer gesättigten Lösung des in dem Lösungsmittel gelösten Stoffes, also einer gesättigten Lösung der zu untersuchenden Lösung, bei der bestimmten bzw. bestimmbaren Temperatur;
• – Einstrahlen zumindest eines zweiten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren zweiten Einstrahlpolarisation auf die zweite Grenzfläche derart, dass der zweite einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der zweiten Grenzfläche in einen zweiten reflektierten Lichtstrahl mit einer zweiten auslaufenden Polarisation reflektiert wird, wobei die zweite Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des zweiten einfallenden Lichtstrahls aufweist;
• – Ermitteln eines Unterschieds zwischen einer ersten Polarisationsänderung von der ersten Einstrahlpolarisation zur ersten auslaufenden Polarisation und einer zweiten Polarisationsänderung von der zweiten Einstrahlpolarisation zur zweiten auslaufenden Polarisation;
• – Bestimmen des Brechungsindexunterschieds zwischen der zu untersuchenden Lösung bzw. Suspension und der gesättigten Lösung bzw. Suspension der zu untersuchenden Lösung aus dem ermittelten Unterschied zwischen den Polarisationsänderungen; und
• – Bestimmen des Sättigungsgrads, insbesondere der Übersättigung und/oder Untersättigung der zu untersuchenden Lösung bzw. Suspension aus dem ermittelten Brechungsindexunterschied.

In particular, according to the present invention, a method for determining a degree of saturation, in particular a supersaturation and / or a supersaturation, of a solution or suspension of a substance dissolved in a solvent at a specific or determinable temperature, preferably the operating temperature of a crystallization process, is provided which comprises the following steps:

• - Forming a first, preferably substantially planar, interface between a first, preferably at least partially substantially optically transparent, optical element, which preferably comprises a lens and / or a prism, and the solution or suspension to be examined;
• - At least a first incident light beam of a certain or determinable first Einstrahlpolarisation on the first interface such that the first incident light beam is reflected substantially by total reflection at the first interface in a first reflected light beam having a first outgoing polarization, wherein the first Einstrahlpolarisation at least temporarily has both a component in parallel and a component perpendicular to the plane of incidence of the first incident light beam;
• - Forming a second, preferably substantially planar interface between a second, preferably at least partially substantially optically transparent, optical element, which preferably comprises a lens and / or a prism, and a saturated solution of the substance dissolved in the solvent, ie a saturated Solution of the solution to be tested, at the determined or determinable temperature;
• - Injecting at least a second incident light beam of a certain or determinable second Einstrahlpolarisation on the second interface such that the second incident light beam is reflected substantially by total reflection at the second interface in a second reflected light beam having a second outgoing polarization, wherein the second Einstrahlpolarisation at least temporarily has both a component in parallel and a component perpendicular to the plane of incidence of the second incident light beam;
• Determining a difference between a first polarization change from the first single beam polarization to the first outgoing polarization and a second polarization change from the second single beam polarization to the second outgoing polarization;
• Determining the refractive index difference between the solution or suspension to be investigated and the saturated solution or suspension of the solution to be investigated from the difference determined between the polarization changes; and
• Determining the degree of saturation, in particular the supersaturation and / or supersaturation of the solution or suspension to be investigated the determined refractive index difference.

The saturated solution thus comprises the same solvent and the same solved Substance like the solution to be investigated, with the difference that the saturated solution is essentially in one Equilibrium state, so essentially neither a supersaturation still a sub saturation having. The saturated solution serves according to the present Invention as a reference solution.

When Plane of incidence of an incident on an interface or surface Light beam is called the plane passing through the light beam and the surface normals the interface or surface is determined at the point of impact of the light beam. Also the reflected Beam of light lies in this plane. Thus, the first and the second Einstrahlpolarisation at least temporarily each a component parallel and a component perpendicular to that through the first or second incident light beam and the corresponding first or second outgoing light beam fixed plane. As parallel polarized a light beam or one of its components is called, if the oscillation plane of the electric field, ie the electric Field vector of the electromagnetic radiation parallel to and in particular lies in the plane of incidence. For a vertically polarized component however, see the electric field vector, so the vibration level of the electric field perpendicular to the plane of incidence.

The Irradiation of the first and second incident light beam with a particular or determinable polarization does not mean that the polarization is fixed as constant, ie unchanged in time is. In this context, the irradiation of a "definite" or "determinable" polarization means rather that the polarization and in particular the change in the polarization of incident light rays is known or at least so far evaluated can be that they are so measured with the polarization of the reflected light rays can be compared, that is a determination a change the polarization is made possible by the total reflection. It's even special advantageous to the polarization of the incident light beams time-dependent change and in particular to periodically modulate. It's just that required that the incident light rays at least temporarily have parallel and vertical polarization components.

If a ray of light on the interface between two media at an angle greater than the critical angle the light is totally reflected, with a phase shift occurs at least individual components of the light waves, which is polarization-dependent. In particular, the phase shift of the vertical differ polarized components and phase shift of the parallel polarized Components of each other. The difference in these phase shifts depends on from the relationship the refractive indices of the materials forming the interface. The relative phase shift of parallel and vertical component provides a measure of the refractive index ratio of these two media. If one refractive index is known, the other one be calculated.

The simultaneous measurement of the solution or suspension to be investigated and one saturated solution or suspension is unnecessary the exact knowledge of the equilibrium curve or saturation curve and allowed as well In-line measurements. For the differential refractive index determination will turn the light on two interfaces totally reflected: an interface exists between the first optical element and the one to be examined Solution, and the other between the second optical element and the saturated one solution or reference solution. The here resulting difference in the rotation of the vibration levels is a measure of the differential Refractive index.

According to the present Invention will now transfer the principle of a differential refractometer, by preferably the total reflection of a polarized laser beam on the surface in particular supersaturated solution is measured against a reference cell, in which then the original solution of Saturated reactor is present. Crystals or other particles in the solution disturb the Measurement not. The solution in the reference cell is preferably exchanged regularly against the reactor solution. To to set the saturation equilibrium passes vorzugswei se less than a minute. From the different change the degree of polarization of the reflected laser beam on the one hand at the measuring cell and on the other hand, at the reference cell, the supersaturation level of the solution can be directly in the reactor with a time delay usually measured under one minute. This allows a very precise control of crystallization.

In the steps of forming the first and / or second boundary surface, the first and / or second optical element preferably has a refractive index at least in the region of the first or second boundary surface which is greater than at least one wavelength of the first or second incident light beam Refractive index of the solution to be examined or the saturated solution. More preferably, the first and / or second incident light beam is irradiated on the first and second interface, respectively, so as to be incident on them from the side of the first and second optical elements meets.

at this preferred embodiment a ray of light not through the fluids to be examined sent, but penetrates during the Process of total reflection preferably only a few 100 nm in the solution one. This must be the solution to be investigated and the saturated one Solution not necessarily transparent and particle-free. Thus, the Measurement of the degree of saturation no separation of crystals from the solutions or suspensions needed.

• – Bereitstellen zumindest einer Messzelle, die vorzugsweise als Kristallisationszelle oder Kristallisator dient, zur Aufnahme der zu untersuchenden Lösung;
• – Bereitstellen zumindest einer Sättigungszelle bzw. Referenzzelle zur Aufnahme der gesättigten Lösung bzw. Referenzlösung, wobei die gesättigte Lösung in der Referenzzelle bzw. Sättigungszelle mit der zu untersuchenden Lösung in der Messzelle bzw. im Kristallisator über eine filtrierende Membran derart in Verbindung gebracht wird, dass die filtrierende Membran für die flüssige Phase der Lösung bzw. Suspension im wesentlichen durchlässig und für die kristalline Phase im wesentlichen undurchlässig ist.

In a particularly preferred embodiment, the method according to the invention further comprises the steps:

• - Providing at least one measuring cell, which preferably serves as a crystallization cell or crystallizer, for receiving the solution to be examined;
• Providing at least one saturation cell or reference cell for receiving the saturated solution or reference solution, wherein the saturated solution in the reference cell or saturation cell is brought into contact with the solution to be investigated in the measuring cell or in the crystallizer via a filtering membrane such that the filtering membrane is substantially permeable to the liquid phase of the solution or suspension and substantially impermeable to the crystalline phase.

In a preferred embodiment the measuring cell is independent or formed separately from a crystallizer. In another preferred embodiment the measuring cell is arranged on or in a crystallizer and in particular connected to the crystallizer. In the most preferred embodiment the measuring cell is formed by the crystallizer itself. In this most preferred embodiment Thus, the entire crystallizer serves as a measuring cell. This can be the procedure is especially good for In-line measurements during the Use crystallization process.

• – Erzeugen von linear polarisiertem Licht, vorzugsweise mittels eines Lasers;
• – periodisches Modulieren der Polarisation des linear polarisierten Lichts, vorzugsweise mittels eines elektrooptischen Modulators (EOM);
• – Aufspalten des in seiner Polarisation periodisch modulierten Lichts in den ersten einfallenden Lichtstrahl und den zweiten einfallenden Lichtstrahl, vorzugsweise mittels eines Strahlteilers.

Preferably, the method further comprises the steps:

• - generating linearly polarized light, preferably by means of a laser;
• Periodically modulating the polarization of the linearly polarized light, preferably by means of an electro-optic modulator (EOM);
• - Splitting the polarized in its periodically modulated light in the first incident light beam and the second incident light beam, preferably by means of a beam splitter.

Preferably is determined in the step of determining the difference in the polarization changes of first and / or second reflected light beam after the passage by a first and second analyzer of a first or detected second photodetector.

Especially preferred is the saturated solution additionally with a crystallizate of the solute wherein the crystals preferably with a crystals Grain size of preferably substantially less than 0.1 mm, more preferably less than 10 μm, most preferably substantially at about 1 micron. This crystals is preferably in the saturation cell or reference cell introduced.

Preferably is at least temporarily by means of an ultrasonic transmitter, the particular is preferably arranged in the region of the reference cell, ultrasound in the saturated solution coupled. This achieves a good mixing of the solution with the crystals and preferably a substantially uniform distribution of the crystals in the saturated Solution. this leads to in a change the composition or the temperature of the solution to achieve rapid a new balance.

• – zumindest ein vorzugsweise zumindest teilweise im wesentlichen optisch transparentes, erstes optisches Element, welches vorzugsweise eine Linse und/oder ein Prisma umfasst, mit zumindest einer vorzugsweise im wesentlichen planaren Oberfläche, die zumindest teilweise mit der zu untersuchenden Lösung bzw. Suspension unter Bildung einer vorzugsweise im wesentlichen planaren ersten Grenzfläche in Kontakt steht bzw. gebracht werden kann;
• – zumindest ein vorzugsweise zumindest teilweise im wesentlichen optisch transparentes; zweites optisches Element mit zumindest einer vorzugsweise im wesentlichen planaren Oberfläche, die zumindest teilweise mit einer gesättigten Lösung bzw. Suspension des in dem Lösungsmittel gelösten Stoffes unter Bildung einer vorzugsweise im wesentlichen planaren zweiten Grenzfläche in Kontakt steht bzw. gebracht werden kann;
• – zumindest ein erstes Lichteinstrahlmittel zum Einstrahlen zumindest eines ersten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren ersten Einstrahlpolarisation derart, dass der erste einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der ersten Grenzfläche in einen ersten reflektierten Lichtstrahl mit einer ersten auslaufenden Polarisation reflektiert wird, wobei die erste Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des ersten einfallenden Lichtstrahls aufweist;
• – zumindest ein zweites Lichteinstrahlmittel zum Einstrahlen zumindest eines zweiten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren zweiten Einstrahlpolarisation derart, dass der zweite einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der zweiten Grenzfläche in einen zweiten reflektierten Lichtstrahl mit einer zweiten auslaufenden Polarisation reflektiert wird, wobei die zweite Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des zweiten einfallenden Lichtstrahls aufweist;
• – zumindest eine Polarisationserfassungseinrichtung zum Ermitteln eines Unterschieds zwischen einer ersten Polarisationsänderung von der ersten Einstrahlpolarisation zur ersten auslaufenden Polarisation und einer zweiten Polarisationsänderung von der zweiten Einstrahlpolarisation zur zweiten auslaufenden Polarisation; und
• – zumindest eine Auswerteeinrichtung zum Bestimmen eines Brechungsindexunterschieds zwischen der zu untersuchenden Lösung bzw. Suspension und der gesättigten Lösung bzw. Suspension aus dem ermittelten Unterschied zwischen den Polarisationsänderungen und zum Bestimmen des Sättigungsgrads, insbesondere der Übersättigung und/oder Untersättigung, der zu untersuchenden Lösung bzw. Suspension aus dem ermittelten Brechungsindexunterschied.

Another object of the present invention relates to an apparatus for determining a degree of saturation, in particular a supersaturation and / or a supersaturation, of a solution or suspension of a substance dissolved in a solvent, which comprises:

• At least one preferably at least partially substantially optically transparent, first optical element, which preferably comprises a lens and / or a prism, with at least one preferably substantially planar surface which is at least partially in contact with the solution or suspension to be examined, preferably substantially planar first interface is in contact or can be brought;
• At least one preferably at least partially substantially optically transparent; second optical element having at least one preferably substantially planar surface which is at least partially in contact with a saturated solution of the solute dissolved in the solvent to form a preferably substantially planar second interface;
• At least a first Lichteinstrahlmittel for irradiating at least a first incident light beam of a certain or determinable first Einstrahlpolarisation such that the first incident light beam is reflected substantially by total reflection at the first interface in a first reflected light beam having a first outgoing polarization, wherein the first Einstrahlpolarisation at least temporarily se has both a component in parallel and a component perpendicular to the plane of incidence of the first incident light beam;
• At least one second light irradiation means for irradiating at least one second incident light beam of a definable second Einstrahlpolarisation such that the second incident light beam is reflected substantially by total reflection at the second interface in a second reflected light beam having a second outgoing polarization, wherein the second Einstrahlpolarisation at least temporarily has both a component in parallel and a component perpendicular to the plane of incidence of the second incident light beam;
• At least one polarization detection device for determining a difference between a first polarization change from the first single-beam polarization to the first outgoing polarization and a second polarization change from the second single-beam polarization to the second outgoing polarization; and
• At least one evaluation device for determining a refractive index difference between the solution or suspension to be examined and the saturated solution or suspension from the determined difference between the polarization changes and for determining the degree of saturation, in particular the supersaturation and / or supersaturation, of the solution to be investigated or Suspension from the determined refractive index difference.

Preferably has the first and / or second optical element at least in the area the first or second interface a refractive index which is at least one wavelength of the first and second incident light ray greater than the refractive index the solution to be examined or the saturated solution is. Furthermore is the first and / or second light irradiation means preferably designed the first and / or second incident light beam on the first or second interface to irradiate that he is on that interface from the side of the first or second optical element.

• – eine erste bzw. zweite Einstrahlfläche, durch die der erste bzw. zweite einfallende Lichtstrahl vor der Totalreflexion im wesentlichen senkrecht zu dieser Einstrahlfläche in das entsprechende optische Element eingestrahlt wird; und/oder
• – eine erste bzw. zweite Auskoppelfläche, durch die der erste bzw. zweite reflektierte Lichtstrahl nach der Totalreflexion im wesentlichen senkrecht zu dieser Auskoppelfläche aus dem entsprechenden optischen Element austritt.

Particularly preferably, the first and / or second optical element comprises

• A first or second incident surface through which the first or second incident light beam is radiated before the total reflection substantially perpendicular to this incident surface in the corresponding optical element; and or
• A first or second outcoupling surface, through which the first or second reflected light beam emerges from the corresponding optical element after the total reflection substantially perpendicular to this outcoupling surface.

Preferably the irradiation takes place in the respective optical element via a Inlet outside the to be examined or the saturated solution.

• – zumindest eine Messzelle, die vorzugsweise als Kristallisationszelle oder als Kristallisator ausgestaltet ist bzw. dient, zur Aufnahme der zu untersuchenden Lösung bzw. Suspension;
• – zumindest eine Referenzzelle bzw. Sättigungszelle zur Aufnahme der gesättigten Lösung bzw. Suspension, wobei die gesättigte Lösung in der Sättigungszelle mit der zu untersuchenden Lösung in der Messzelle bzw. im Kristallisatior über eine filtrierende Membran derart in Verbindung steht, dass die filtrierende Membran für die flüssige Phase der Lösung bzw. Suspension im wesentlichen durchlässig und für die kristalline Phase im wesentlichen undurchlässig ist.

In a preferred embodiment, a device further comprises:

• - At least one measuring cell, which is preferably configured or serves as a crystallization cell or as a crystallizer, for receiving the solution or suspension to be examined;
• - At least one reference cell or saturation cell for receiving the saturated solution or suspension, wherein the saturated solution in the saturation cell with the solution to be examined in the measuring cell or in the crystallizer via a filtering membrane in conjunction so that the filtering membrane for the liquid phase of the solution or suspension is substantially permeable and is substantially impermeable to the crystalline phase.

• – zumindest einen Laser zur Erzeugung linear polarisierten Lichtes,
• – zumindest einen elektrooptischen Modulator zur periodischen Modulation der Polarisation des linear polarisierten Lichts und
• – einen Strahlteiler zum Aufspalten des in seiner Polarisation periodisch modulierten Lichts in den ersten einfallenden Lichtstrahl und den zweiten einfallenden Lichtstrahl.

Particularly preferably, the light irradiation means

• At least one laser for producing linearly polarized light,
• - At least one electro-optical modulator for periodically modulating the polarization of the linearly polarized light and
• - A beam splitter for splitting the polarized in its periodically modulated light in the first incident light beam and the second incident light beam.

Preferably the first and / or second polarization detector comprises a first or second analyzer and a first and second photodetector.

Especially an ultrasonic transmitter is preferred in the region of the reference cell for coupling ultrasound into the saturated solution.

• – zumindest ein vorzugsweise zumindest teilweise im wesentlichen optisch transparentes, erstes optisches Element, das vorzugsweise im wesentlichen als Linsen- und/oder Prismenelement ausgebildet ist, mit zumindest einer vorzugsweise im wesentlichen planaren Oberfläche, die zumindest teilweise mit der Meßprobe unter Bildung einer vorzugsweise im wesentlichen planaren ersten Grenzfläche in Kontakt steht bzw. gebracht werden kann;
• – zumindest ein vorzugsweise zumindest teilweise im wesentlichen optisch transparentes, zweites optisches Element mit zumindest einer vorzugsweise im wesentlichen planaren Oberfläche, die zumindest teilweise mit einer gesättigten Lösung bzw. Suspension des in dem Lösungsmittel gelösten Stoffes unter Bildung einer vorzugsweise im wesentlichen planaren, zweiten Grenzfläche in Kontakt steht bzw. gebracht werden kann;
• – zumindest ein erstes Lichteinstrahlmittel zum Einstrahlen zumindest eines ersten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren ersten Einstrahlpolarisation derart, dass der erste einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der ersten Grenzfläche in einen ersten reflektierten Lichtstrahl mit einer ersten auslaufenden Polarisation reflektiert wird, wobei die erste Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des ersten einfallenden Lichtstrahls aufweist;
• – zumindest ein zweites Lichteinstrahlmittel zum Einstrahlen zumindest eines zweiten einfallenden Lichtstrahls einer bestimmten bzw. bestimmbaren zweiten Einstrahlpolarisation derart, dass der zweite einfallende Lichtstrahl im wesentlichen durch Totalreflexion an der zweiten Grenzfläche in einen zweiten reflektierten Lichtstrahl mit einer zweiten auslaufenden Polarisation reflektiert wird, wobei die zweite Einstrahlpolarisation zumindest zeitweise sowohl eine Komponente parallel als auch eine Komponente senkrecht zur Einfallsebene des zweiten einfallenden Lichtstrahls aufweist;
• – zumindest eine Polarisationserfassungseinrichtung zum Ermitteln eines Unterschieds zwischen einer ersten Polarisationsänderung von der ersten Einstrahlpolarisation zur ersten auslaufenden Polarisation und einer zweiten Polarisationsänderung von der zweiten Einstrahlpolarisation zur zweiten auslaufenden Polarisation; und
• – zumindest eine Auswerteeinrichtung zum Bestimmen des Unterschieds zwischen dem ersten Brechungsindex der Meßprobe und dem zweiten Brechungsindex der Referenzprobe aus dem ermittelten Unterschied zwischen den Polarisationsänderungen.

Thus, the present invention also relates generally to a device for determining a difference of a first refractive index of a test sample, in particular a measuring liquid, and a second refractive index of a reference sample, in particular a reference liquid, which comprises:

• At least one preferably at least partially substantially optically transparent, first optical element, which is preferably formed substantially as a lens and / or prism element, with at least one preferably substantially planar surface at least partially with the test sample to form a preferably substantially planar first interface is in contact or brought who that can;
• At least one preferably at least partially substantially optically transparent, second optical element having at least one preferably substantially planar surface at least partially filled with a saturated solution or suspension of the solute in the solvent to form a preferably substantially planar second interface Contact stands or can be brought;
• At least a first Lichteinstrahlmittel for irradiating at least a first incident light beam of a certain or determinable first Einstrahlpolarisation such that the first incident light beam is reflected substantially by total reflection at the first interface in a first reflected light beam having a first outgoing polarization, wherein the first Einstrahlpolarisation at least temporarily has both a component in parallel and a component perpendicular to the plane of incidence of the first incident light beam;
• At least one second light irradiation means for irradiating at least one second incident light beam of a definable second Einstrahlpolarisation such that the second incident light beam is reflected substantially by total reflection at the second interface in a second reflected light beam having a second outgoing polarization, wherein the second Einstrahlpolarisation at least temporarily has both a component in parallel and a component perpendicular to the plane of incidence of the second incident light beam;
• At least one polarization detection device for determining a difference between a first polarization change from the first single-beam polarization to the first outgoing polarization and a second polarization change from the second single-beam polarization to the second outgoing polarization; and
• - At least one evaluation device for determining the difference between the first refractive index of the sample and the second refractive index of the reference sample from the determined difference between the polarization changes.

The The invention will be described below with reference to the accompanying drawings preferred embodiments described by way of example. Showing:

1 a first preferred embodiment of a device according to the present invention;

2 a second preferred embodiment of a device according to the present invention;

3 a schematic representation of an equilibrium curve or saturation curve X * = f (T _K )

1 shows a first preferred embodiment of a device for determining a refractive index difference and in particular a device for determining a degree of saturation of a solution to be examined or suspension. In a crystallizer 10 becomes a solution or suspension to be examined 12 provided. At least one particular substance is at least partially dissolved in a particular solvent. In the solution to be examined 12 should be generated by regulation or control of the process conditions, a crystallizate of the solute. For example, the solute may be NaCl (common salt) or sucrose (sugar) and the solvent may be water. By changing the temperature of the solution and / or by changing the concentration of solute in the solution, the formation of crystals should be influenced.

In the preferred embodiment shown, the device comprises a saturation cell 14 in which is a saturated solution or suspension 16 of the substance dissolved in the solution to be examined is in the same solvent. Preferably, the saturated suspension sets 16 from a liquid phase and a crystallizate of the solute together. In this case, the crystals are preferably in the form of very small crystals having a particle size of preferably substantially not more than 0.1 mm in the saturated solution 16 or in the saturation cell 14 in front. Particularly preferably, the grain size is substantially not greater than 10 microns. More preferably, the grain size of the crystals in the saturation cell is substantially about 1 micron. These small crystals act as additional solubilizing crystals or crystallization nuclei and, due to their large surface area, enable the equilibrium state of the saturated solution to be reached quickly 16 in the saturation cell 14 ie an occurring supersaturation or supersaturation of the solution in the saturation cell 14 is mined relatively quickly, so the solution 16 is essentially always saturated. Thus, a supersaturation or supersaturation in the saturation cell is preferably degraded faster than in the crystallizer, so the solution to be examined.

Preferably, the saturation cell is 14 with the crystallizer 10 or the saturated solution 16 with the solution to be examined 12 so in thermal contact that the temperature of the saturated solution 16 essentially the temperature of the solution to be examined 12 equivalent. Preferably, the saturation cell 14 at least one opening 18 on, passing through a filtering membrane 20 is closed. The filtering membrane 20 is preferably by means of a screw cap 22 at the saturation cell 14 attached. The filtering membrane 20 and especially their pore size is preferably so to the solution to be examined 12 or the saturated solution 16 adapted that the membrane 20 permeable to the liquid phase of the solutions, but impermeable to the crystalline phase of the solutions or suspensions. This ensures that changes in the substance system in the crystallizer 10 in the crystallization process (eg changes in composition in evaporation crystallization, concentration of impurity or temperature changes) also in the saturation cell 14 be transmitted. The into the saturation cell 14 introduced crystals ("body") are preferably at certain intervals by an ultrasonic transmitter 23 suspended and mixed.

In a housing 24 The preferred embodiment is a first optical element 26 preferably arranged as a hemispherical-shaped lens such that a preferably planar or planar surface of the first optical element 26 at least partially as the first interface 28 with the solution to be examined 12 in contact. At the first interface 28 Thus, preferably forms a direct transition from the material of the first optical element 26 to the solution to be investigated 12 out.

A second optical element 30 , which is preferably also designed as a hemispherical lens, is in the region of the saturation cell 14 preferably arranged such that it has a preferably planar or planar surface with the saturated solution 16 is in contact, which is analogous to the first interface 28 a second interface 32 between the lens body and the saturated solution 12 formed.

In the preferred embodiment shown, the device comprises a laser 34 which preferably produces linearly polarized light. The linearly polarized light is transmitted through a first polarization-maintaining optical fiber 36 to an electro-optical modulator 38 (EOM). Preferably, the electro-optical modulator 38 formed by a Pockels cell. It is a preferably a birefringent crystal of depending on an externally applied electric field causes a polarization-dependent phase shift of the incident light. In particular, the phase shift for polarization components perpendicular to an optical axis of the electro-optical modulator or of the Pockels cell or of the birefringent crystal preferably differs from the phase shift for polarization components parallel to this axis. Preferably, the difference of the phase shifts is particularly linearly dependent on the applied electric field. After passing a certain length in the Pockels cell, there is a relative phase shift, ie a difference in the phase shifts for parallel and perpendicular to the optical axis of the Pockels cell polarized light, which preferably changes in proportion to the applied electric field. Preferably, the externally applied electric field is periodically modulated so that the polarization and in particular the relative phase shift for the electro-optical modulator 38 leaving light is periodically modulated. Particularly preferred for generating the electric field to the Pockels cell a sawtooth voltage is applied, the one-time running leads to a relative phase shift by an integral multiple of 2π, so an integral multiple of the period of the light. This achieves a continuous and periodic change or modulation of the polarization for the light leaving the electrostatic modulator. For example, a frequency in the range between 300 Hz and 2 kHz is used as the modulation frequency of the polarization or as the frequency of the sawtooth voltage. Basically, lower or higher frequencies are possible. In the usual way, preferably a frequency is used which does not coincide with a multiple of a disturbing signal frequency, for example a radio frequency or the network frequency.

The modulated in its polarization laser light is then on a second polarization-maintaining fiber 40 in the case 24 passed the device. There it meets a beam splitter 42 and becomes a first incoming beam of light 44 and a second incident beam of light 46 split. Both incident light beams now have the same modulated polarization.

The first incident light beam 44 meets a preferably spherical surface of the first optical element 26 and penetrates into the first optical element 26 one. The entry surface is preferably designed such that the first incident light beam 44 perpendicular to this surface impinges and thus when entering the first optical elements 26 not broken. In the preferred embodiment shown, the first optical element 26 a refractive index n ₁ at the wavelength of the laser light, which is greater than the refractive index n _{m of} the solution to be examined 12 is. The first incident light beam 44 meets in the first optical element 26 at an angle Θ ₁ to the normal direction of the first interface 28 on this first interface 28 , wherein the angle of incidence Θ _{1 is} greater than the critical angle of total reflection. This becomes the first incident light beam 44 in a first outgoing light beam 48 totally reflected.

In the process of total reflection, the light undergoes a phase shift. The phase shift or the phase jump depends on the refractive index ratio between the first optical element 26 and the solution to be investigated 12 , the angle of incidence Θ ₁ and the polarization of the light. Due to the modulation of the polarization of the first incident light beam 44 this light beam has at least temporarily polarization components perpendicular to the plane of incidence and polarization components parallel to the plane of incidence. As the plane of incidence, the plane is denoted by the first incident light beam 44 and the first outgoing light beam 48 is fixed, ie the plane in which both the first incident light beam 44 as well as the first outgoing light beam 48 lie. Depending on the angle of incidence Θ ₁ and the refractive index difference or the refractive index ratio between the first optical element 26 and the solution to be investigated 12 The vertical polarization suffers a different phase shift or a different phase jump than the parallel polarization. The difference Δ _t in the phase shift or in the phase jump between vertical and parallel polarization fulfills the equation:

This differentiates the polarization of the first outgoing light beam 48 from that of the first incoming beam of light 44 , In particular, the relative phase position of the components of light polarized perpendicular and parallel to the plane of incidence changes. The relative shift of the phase position of the vertical and parallel components of the light depends on the refractive index ratio between the first optical element at a fixed angle of incidence Θ ₁ 26 and the solution to be investigated 12 from.

The first outgoing light beam strikes after leaving the first optical element 26 on a first analyzer 50 , Depending on the analyzer, only a part of the first outgoing light beam 48 , that is, a certain polarization component, the first analyzer 50 penetrate and then meets a first photodetector 52 , The other or the other polarization component (s) of the first outgoing light beam 48 are preferably by the first analyzer 50 filtered out. The first photodetector 52 detects the intensity of the incident there portion of the first outgoing light beam 48 and outputs a signal to an evaluation device 54 further.

The second incident light beam 46 hits from the beam splitter 42 in the embodiment shown on a mirror 56 and from there to the second optical element 30 , which is similar to the first optical element 26 preferably has a spherical surface through which the second incident light beam 46 in the second optical element 30 penetrates. Again, the second incident light beam is preferably perpendicular to the spherical surface to avoid refraction of the light at this point. The second optical element 30 has a refractive index at least at the wavelength of the laser light, which is greater than the refractive index of the saturated solution 16 is. The second incident light beam 46 meets the second interface at an angle of incidence Θ ₂ greater than the critical angle of total reflection 32 , As in the case of the first incident light beam 44 becomes also the second incident light beam 46 at the second interface 32 totally reflected, with a second outgoing light beam 58 arises after passing through a second analyzer 60 at least partially to a second photodetector 62 meets. Analogous to the first light beam, the relative phase angle of parallel and perpendicularly polarized components for the second incident light beam also depends on the refractive index ratio between the second optical element and the second incident angle Θ ₂ 30 and the saturated solution 16 from. The difference Δ _r in the phase shift between vertical and parallel polarization of the second light beam in total reflection fulfills the equation analogously to the first light beam:

Preferably, the second analyzer 60 and the second photodetector 62 analogous to the first analyzer 50 and the first photodetector 52 built up. Most preferred are the second optical element 30 , the second analyzer 60 and the second photodetector 62 to the first optical element 26 , the first analyzer 50 and the first photodetector 52 identical or identical.

Depending on the second outgoing light beam 58 on the second photodetector 62 incident light power sends the second photodetector 62 a signal to the evaluation device 54 , There, the incoming signals of the first and second photodetector are compared and the refractive index difference between the solution to be investigated and the saturated solution is calculated. The evaluation device determines from this calculated refractive index difference 54 the difference ΔX of the saturation of the two solutions. This difference thus corresponds to the over- or undersaturation of the solution to be examined.

In the device of 1 is preferred used a HeNe laser at a wavelength of 633 nm. Preferably, linearly polarized laser light having a plane of polarization enclosing with the plane of incidence an angle of 45 °, ie a polarization at an unchanged passage through the electro-optic modulator with a linear polarization of 45 ° to the plane of incidence on the first and second interface meets the electro-optical modulator whose optical axis encloses an angle other than 0 ° and 90 ° with the polarization of the laser light. Thus, the polarization of the light leaving the electro-optic modulator is modulated by the voltage applied to the electro-optic modulator. Particularly preferably, the optical axis of the electro-optical modulator makes an angle of 45 ° with the plane of polarization of the laser light, ie it is preferably perpendicular or parallel to the plane of incidence. This achieves a modulation of the light leaving the electrooptical modulator, which is of a linear polarization parallel to the polarization of the laser light via an elliptical, a first circular polarization, an elliptical, a linear polarization perpendicular to the polarization of the laser light, an elliptical, a second circular polarization opposite to the first circular polarization and an elliptical polarization leads back to the initial linear polarization.

Preferably the analyzers are at an angle of 45 ° to the plane of incidence arranged. In this arrangement, a particularly high measurement signal and thus a particularly good measurement resolution achievable. The analyzers Thus, preferably only polarization components of the reflected Beams of light hit the photodetectors, making an angle from 45 ° to Incidence level. By modulating the polarization of the incident Receives light rays a modulation of the incident on the photodetectors light intensity. In the shown preferred embodiment is a maximum intensity reached especially when the caused by the total reflection relative Phase shift which caused relative to the electro-optical modulator Phase shift exactly canceled, so exactly opposite to this acts. As a result, linearly polarized light strikes the analyzer, where the polarization plane is equal to the polarization of the laser light and thus in the preferred embodiment shown, equal to the polarization direction of the analyzer. On the other hand you get a minimal signal, when the two relative phase shifts in the total reflection and in the electro-optical modulator to a total relative phase shift of π add. Thereby This results in a linear polarization perpendicular to the analyzer polarization of the reflected light beam and the measured intensity decreases to zero.

The force voltage conditions at the electro-optical modulator to reach a maximum thus depend from the phase shift in the total reflection and differentiate for the first incident light beam and the second incident light beam dependent on from the refractive index ratios at the first and second interfaces, respectively. The two detected intensity signals thus have a phase shift, which is a measure of the difference of the relative phase shifts at the first and second interfaces and preferably coincides with the difference in the relative phase shifts. It must only additional Phase shifts, caused by further reflections Mirrors and beam splitters, taken into account but by appropriate calibrations or calibrations considered can be.

Thus, the linearly polarized laser beam is periodically modulated by the electro-optical modulator (EOM) in its polarization and by means of a beam splitter 42 decomposed into a reference beam and a measuring beam. The intensity / _{r of} the reference beam becomes after the total reflection at the interface 32 between the second optical element 30 and the reference solution 16 and the passage through the analyzer 60 with the photodetector 62 measured. The intensity / _{m of} the measuring beam becomes after total reflection at the interface 28 between prism 26 and test solution 12 and the passage through the analyzer 50 with the photodetector 52 measured. The measured intensities / _r and / _t preferably have the following course: I r = 1 2 [1 - cos (Γ + Φ S + Φ M + Δ r )] I t = 1 2 a 2 [1 - cos (Γ + Δ t )], where Γ the phase of the electro-optical modulator and a ² the relative intensity of the two beams to each other, Φ _S the phase shift by the reflection at the beam splitter 42 and Φ _{M is} the phase shift due to reflection at the mirror 56 is. The measured intensities / _r and / _t show this a phase difference Δ = Δ _r - Δ _t, which represents a measure for the differential refractive index .DELTA.n of the test solution. From the known dependence of the refractive index on the degree of saturation of a solution, it is thus possible to determine the supersaturation or undersaturation of the solution.

2 shows a second preferred embodiment of a device for determining a refractive index difference and in particular for determining a degree of saturation of a solution to be examined according to the present invention. Instead of lenticular optical elements having substantially spherical entrance and exit surfaces, the second preferred Ausfüh form a first optical element 26 and a second optical element 30 on, which are designed as prisms. Both optical elements have substantially the same shape with substantially planar entrance and exit surfaces for the incident and outgoing light beams. In addition, they have additional, substantially planar surfaces, on which further total reflections of the incident or outgoing light rays take place. Notwithstanding the first preferred embodiment, in the second embodiment shown, the first 44 and the second incident beam of light 46 at the same angle to the first 28 or second interface 32 , ie Θ ₁ , = Θ ₂ . As a result, calibration of the system due to different polarization changes for the first and second light beams due to different angles of incidence, as required in the first embodiment, is no longer necessary in this embodiment. The polarization changes due to the additional total reflections in this embodiment also preferably balance out for the two light beams. Only changes in the polarization at additional mirrors 56 . 64 must be calibrated. This is preferably done by a measurement in which the same solution or liquid is present in both cells, ie in the crystallizer and in the saturation cell or reference cell. As such liquid, for example, water could be used, in which there is preferably substantially no solute.

In addition to the examples shown, a number of other embodiments and variations are possible. While in the embodiments shown both the beam splitter 42 as well as the optical elements 26 . 30 , Analyzers 50 . 60 and photodetectors 52 . 62 in the common housing 24 housed, alternatively, the beam guide of the first incident or outgoing light beam on the one hand and the second incident and outgoing light beam on the other hand be performed in separate housings or in glass fibers. In particular, the first can also 26 and second optical element 30 be arranged at the ends of a glass fiber. Such a device can be used very flexibly in various systems of crystallizers and reference cells.

deviant of the embodiments shown can also other angles for an electro-optical modulator an incident laser light and / or an analyzer can be used. It does not necessarily have to Analyzers for linear polarization can be used. Also, the measurement of circular Polarization would be possible and leads in analogous to the goal. Furthermore it is different from the preferred embodiments shown not necessary to modulate the polarization of the incident light. It is only necessary that the incident light be vertical and parallel polarization components. It is then however, an absolute measurement of intensities and a comparison of the absolute ones Intensities of Measuring and Reference signal instead of a phase shift of measured Intensitätsoszillationen required. Thus, the embodiments shown have a periodic one Modulation of polarization has the advantage that in the manner of a lock-in technique noise can be filtered out and the sensitivity increases significantly.

10

Crystallizer or measuring cell or crystallization cell

12

to investigative solution

14

saturation cell or reference cell

16

saturated solution

18

opening

20

filtering membrane

22

screw cap

23

ultrasonic transmitter

24

casing

26

first optical element

28

first interface

30

second optical element

32

second interface

34

laser

36

first glass fiber

38

electro-optical modulator

40

second glass fiber

42

beamsplitter

44

first incident light beam

46

second incident light beam

48

first reflected or outgoing light beam

50

first analyzer

52

first photodetector

54

evaluation

56

mirror

58

second reflected or outgoing light beam

60

second analyzer

62

second photodetector

64

mirror

Claims (14)

Method for determining the degree of saturation of a solution to be investigated ( 12 ) of a substance dissolved in a solvent at a certain temperature (T), comprising the following steps: - forming a first interface ( 28 ) between a first optical element ( 26 ) and the solution to be investigated ( 12 ); - irradiation of at least one first incident light beam ( 44 ) of a certain first Einstrahlpolarisation on the first interface ( 28 ) like that, that the first incident light beam ( 44 ) essentially by total reflection at the first interface ( 28 ) in a first reflected light beam ( 48 ) is reflected with a first outgoing polarization, wherein the first Einstrahlpolarisation at least temporarily both a component in parallel and a component perpendicular to the plane of incidence of the first incident light beam ( 44 ) having; Forming a second interface ( 32 ) between a second optical element ( 30 ) and a saturated solution ( 16 ) of the substance dissolved in the solvent at the determined temperature (T); Irradiation of at least one second incident light beam ( 46 ) of a certain second Einstrahlpolarisation on the second interface ( 32 ) such that the second incident light beam ( 46 ) essentially by total reflection at the second interface ( 32 ) into a second reflected light beam ( 58 ) is reflected with a second outgoing polarization, the second Einstrahlpolarisation at least temporarily both a component in parallel and a component perpendicular to the plane of incidence of the second incident light beam ( 46 ) having; Determining a difference between a first polarization change from the first single beam polarization to the first outgoing polarization and a second polarization change from the second single beam polarization to the second outgoing polarization; Determining the refractive index difference between the solution to be investigated ( 12 ) and the saturated solution ( 16 ) from the determined difference between the polarization changes; and determining the degree of saturation of the solution to be investigated ( 12 ) from the determined refractive index difference. The method of claim 1, wherein in the steps of forming the first ( 28 ) and second interface ( 32 ) the first ( 26 ) and second optical element ( 30 ) at least in the area of the first ( 28 ) or second interface ( 32 ) has a refractive index which is at least one wavelength of the first ( 44 ) or second incident light beam ( 46 ) greater than the refractive index of the solution to be investigated ( 12 ) or the saturated solution ( 16 ) and wherein the first ( 44 ) and second incident light beam ( 46 ) to the first ( 28 ) or second interface ( 32 ) is irradiated on it from the side of the first ( 26 ) or second optical element ( 30 ) meets. The method of claim 1 or 2, further comprising the steps: - providing at least one measuring cell ( 10 ) for receiving the solution to be investigated ( 12 ); Providing at least one reference cell ( 14 ) for receiving the saturated solution ( 16 ); where the saturated solution ( 16 ) in the reference cell ( 14 ) with the solution to be investigated ( 12 ) in the crystallization cell ( 10 ) via a filtering membrane ( 20 ) in such a way that the filtering membrane ( 20 ) is substantially permeable to the liquid phase of the solution or suspension and substantially impermeable to the crystalline phase. Method according to one of claims 1 to 3, further comprising the steps of: - generating linearly polarized light; Periodically modulating the polarization of the linearly polarized light; and splitting the light which is periodically modulated in its polarization into the first incident light beam ( 44 ) and the second incident light beam ( 46 ). Method according to one of claims 1 to 4, wherein in the step of determining the difference in the polarization changes, the first ( 48 ) and / or second reflected light beam ( 58 ) after passing through a first ( 50 ) or second analyzer ( 60 ) from a first ( 52 ) or second photodetector ( 62 ) is detected. Method according to one of claims 1 to 5, wherein the saturated solution ( 16 ) is additionally mixed with a crystallizate of the solute. The method of claim 6, wherein at least temporarily Ultrasound in the saturated solution is coupled. Device for determining the degree of saturation of a solution to be investigated ( 12 ) of a substance dissolved in a solvent, comprising: - at least one first optical element ( 26 ) with at least one surface which at least partially coincides with the solution to be investigated ( 12 ) to form a first interface ( 28 ) is in contact or can be brought into contact; At least one second optical element ( 30 ) having at least one surface at least partially saturated with a solution ( 16 ) of the solvent dissolved in the solvent to form a second interface ( 32 ) is in contact or can be brought into contact; At least a first light irradiation means for irradiating at least a first incident light beam ( 44 ) of a certain first Einstrahlpolarisation such that the first incident light beam ( 44 ) essentially by total reflection at the first interface ( 28 ) in a first reflected light beam ( 48 ) is reflected with a first expiring polarization, wherein the first Einstrahlpolarisation at least temporarily both a component in parallel and a component perpendicular to the plane of incidence of he most incident light beam ( 44 ) having; At least one second light irradiation means for irradiating at least one second incident light beam (US Pat. 46 ) of a certain second Einstrahlpolarisation such that the second incident light beam ( 46 ) essentially by total reflection at the second interface ( 32 ) into a second reflected light beam ( 58 ) is reflected with a second outgoing polarization, the second Einstrahlpolarisation at least temporarily both a component in parallel and a component perpendicular to the plane of incidence of the second incident light beam ( 46 ) having; At least one polarization detection device for determining a difference between a first polarization change from the first single-beam polarization to the first outgoing polarization and a second polarization change from the second single-beam polarization to the second outgoing polarization; and - at least one evaluation device ( 54 ) for determining a refractive index difference between the solution to be investigated ( 12 ) and the saturated solution ( 16 ) from the determined difference between the polarization changes and for determining the degree of saturation of the solution to be investigated ( 12 ) from the determined refractive index difference. Apparatus according to claim 8, wherein the first ( 26 ) and second optical element ( 30 ) at least in the area of the first ( 28 ) or second interface ( 32 ) has a refractive index which is at least one wavelength of the first ( 44 ) or second incident light beam ( 46 ) greater than the refractive index of the solution to be investigated ( 12 ) or the saturated solution ( 16 ) and the first and second light irradiation means is designed, the first ( 44 ) and second incident light beam ( 46 ) to the first ( 28 ) or second interface ( 32 ) to point out that this is from the side of the first ( 26 ) or second optical element ( 30 ) meets. Apparatus according to claim 9, wherein the first ( 26 ) and second optical element ( 30 ) - a first or second Einstrahlfläche through which the first ( 44 ) or second incident light beam ( 46 ) is irradiated before the total reflection substantially perpendicular to this Einstrahlfläche in the corresponding optical element; and / or - a first or second outcoupling surface through which the first ( 48 ) or second reflected light beam ( 58 ) after the total reflection substantially perpendicular to this decoupling surface from the corresponding optical element ( 26 . 30 ) exit. Device according to one of claims 8 to 10, further comprising: - at least one measuring cell ( 10 ) for receiving the solution to be investigated ( 12 ); At least one reference cell ( 14 ) for receiving the saturated solution ( 16 ); where the saturated solution ( 16 ) in the reference cell ( 14 ) with the solution to be investigated ( 12 ) in the measuring cell ( 10 ) via a filtering membrane ( 20 ) in such a way that the filtering membrane ( 20 ) is substantially permeable to the liquid phase of the solution and substantially impermeable to the crystalline phase. Device according to one of claims 8 to 11, wherein the light irradiation means comprise: - at least one laser ( 34 ) for producing linearly polarized light, - at least one electro-optical modulator ( 38 ) for the periodic modulation of the polarization of the linearly polarized light and - at least one beam splitter ( 42 ) for splitting the light which is periodically modulated in its polarization into the first incident light beam ( 44 ) and the second incident light beam ( 46 ). Device according to one of claims 8 to 12, wherein the first and / or second polarization detector a first ( 50 ) or second analyzer ( 60 ) and a first ( 52 ) or second photodetector ( 62 ). Device according to one of claims 8 to 13, wherein in the region of the reference cell ( 14 ) an ultrasonic transmitter ( 23 ) for coupling ultrasound into the saturated solution ( 16 ) is provided.