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WO2000026665A1 - Procede et dispositif pour mesurer par voie optique la concentration et la temperature de liquides - Google Patents

Procede et dispositif pour mesurer par voie optique la concentration et la temperature de liquides Download PDF

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Publication number
WO2000026665A1
WO2000026665A1 PCT/US1998/023179 US9823179W WO0026665A1 WO 2000026665 A1 WO2000026665 A1 WO 2000026665A1 US 9823179 W US9823179 W US 9823179W WO 0026665 A1 WO0026665 A1 WO 0026665A1
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WO
WIPO (PCT)
Prior art keywords
sample
refractive index
liquid mixture
component
exit side
Prior art date
Application number
PCT/US1998/023179
Other languages
English (en)
Inventor
Jon P. Longtin
Original Assignee
The Research Foundation Of State University Of New York
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The Research Foundation Of State University Of New York filed Critical The Research Foundation Of State University Of New York
Priority to US09/830,735 priority Critical patent/US6549276B1/en
Priority to PCT/US1998/023179 priority patent/WO2000026665A1/fr
Priority to CA002348541A priority patent/CA2348541A1/fr
Publication of WO2000026665A1 publication Critical patent/WO2000026665A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/43—Refractivity; Phase-affecting properties, e.g. optical path length by measuring critical angle

Definitions

  • the present invention relates to measurement of optical properties of materials, and, more particularly, to precision optical measurement of refractive index, concentration and temperature of materials.
  • Double-Diffusive Thermohaline Systems 56 Rev. Sci. Instrum. 291-96 (1985) and T. L. Bergman, D. R. Munoz, F. P. Incropera and R. Viskanta, "Measurement of Salinity Distributions in Salt- Stratified, Double-Diffusive Systems by Optical Deflectometry," 57 Rev. Sci. Instrum. 2538-41 (1986).
  • Other techniques include a planar laser- induced fluorescence technique, as described in A Lozano, S. H. Smith, M. G. Mungal, and R. K.
  • Refractometers are routinely used to evaluate the refractive index to determine the concentration of a liquid mixture, as described in J. E. Geake, "Linear Refractometers For Liquid Concentration Measurement," Chemical Engineer 305-08 (1975). Still other techniques reported in the literature for accurately measuring the refractive index of solids and gases include the minimum deviation method as set forth in I. H. Malitson, "Refractive Properties of Barium Fluoride,” Journal of the Optical Society of America 628-32 (1964) and B. C. Platt, H. W. Icenogle, J. E.
  • the present invention which substantially overcomes the shortcomings of the currently known techniques, provides a method for determining the concentration and temperature of a transparent liquid.
  • the method includes the steps of causing the liquid to be contained in a vessel having a transparent entrance side and a transparent exit side; causing a beam of light to impinge on the entrance side; and then calculating the refractive index of the liquid using Snell's law.
  • the vessel can have an entrance side and an exit side with a known angular relationship therebetween, and can be immersed in known surroundings.
  • the impingement can be at an angle ⁇ , with the normal to an outer surface of the entrance side.
  • the beam can then pass through the material, and then through the exit side, from which it exits at an angle ⁇ e with respect to the normal to an outer surface of the exit side impinged by the light beam.
  • the calculation of the refractive index of the material using Snell's law can be based on the angles ⁇ j and ⁇ e , and can be done by applying Snell's law at the interface between the surroundings and the entrance side, the interface between the entrance side and the material, the interface between the material and the exit side, and the interface between the exit side and the surroundings.
  • the present invention also provides a method for determining the concentration of a given component (e.g., a solute) in a sample of a multi-component liquid mixture (e.g., a solution).
  • the method includes the step of determining the refractive index of the sample of the multi-component liquid mixture as described above and then comparing the determined refractive index of the sample of the multi- component liquid mixture to predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture. In this manner, the concentration of the given component of the sample of the multi-component liquid mixture can be determined from the refractive index of the sample of the multi- component liquid mixture determined as described above.
  • the present invention yet further provides a method for determining a change in concentration of a given component of a sample of a multi-component liquid mixture from an initial concentration of the given component which corresponds to an initial refractive index of the sample of the multi-component liquid mixture, at an initial sample temperature.
  • the method includes determining an initial and a subsequent refractive index of the sample of the multi-component liquid mixture in the manner described above, and then determining a change in the refractive index by subtracting the initial refractive index from the subsequent refractive index.
  • the method further includes measuring the initial and subsequent temperatures of the sample of the multi-component liquid mixture and then determining the change in the concentration of the given component in the sample of the multi-component liquid mixture from the initial concentration according to the following approximate formula: ⁇ C a ( ⁇ n - ( ⁇ n/ ⁇ T) ⁇ T)(an/ ⁇ C)- 1 , (1) where:
  • ⁇ n is the change in the refractive index of the sample
  • ⁇ C is the change in the concentration of the given component in the sample of the multi-component liquid mixture from the initial concentration
  • ⁇ WdT is partial derivative of index of refraction
  • n of the multi-component liquid mixture, with respect to temperature, determined from known data (can be evaluated at sample temperature and is approximately constant for a ⁇ T up to 10- 15°C, and can be treated as a function of temperature for larger temperature changes)
  • ⁇ T is the difference between the subsequently measured temperature of the sample and the initial temperature
  • ⁇ n/3C is partial derivative of index of refraction, n, of the multi-component liquid mixture, with respect to concentration of the given component, evaluated from known data in a region near the initial concentration and near the sample temperature. Also provided is a method for determining the temperature of a given sample of a liquid, which can be either a pure liquid or a multi-component liquid having a substantially constant concentration of the components. The method includes the step of determining the refractive index of the sample of the liquid as initially described above, and then comparing the refractive index of the sample of the liquid to predetermined data relating different temperatures of the liquid to corresponding values of the refractive index of the liquid.
  • the present invention yet further provides an apparatus for determining the concentration of a solute and the temperature of a light transmitting liquid.
  • the apparatus includes a vessel which contains the liquid, a light source, and a light beam position sensor.
  • the vessel has a planar entrance side and a planar exit side with a known angular relationship therebetween, and is immersed in known surroundings.
  • the light source is located so as to cause a beam of light to impinge on the entrance side at an angle ⁇ j with respect to the normal to the outer surface of the entrance side where the beam impinges, to pass through the material, and to then pass through the exit side, from which it exits at an angle ⁇ e with respect to the normal to the outer surface of the exit side where the beam exits.
  • the sensor determines the exit angle ⁇ e , thus permitting the refractive index of the material to be determined, based on the angles ⁇ j and ⁇ e , by applying Snell's law.
  • the apparatus includes a computer which is suitably programmed to calculate the refractive index of the liquid, and to determine the unknown concentration or temperature of the liquid sample based on comparison with predetermined data.
  • the present invention thus provides methods and an apparatus capable of carrying out precision refractive index, concentration and temperature measurement of liquids.
  • the apparatus and methods overcome the disadvantages of prior systems and methods.
  • the present method and apparatus permit real time, non- invasive, remote measurements with high resolution and can be carried out using simple and inexpensive components.
  • the apparatus is compact and easy to operate.
  • measurements can be based on readily quantifiable and measurable parameters, rather than visual, subjective parameters.
  • Real-time remote monitoring of liquid component concentration such as real-time mixture monitoring, precision solute and contaminant analysis, and measurement of diffusion coefficients of components of a multi-component liquid mixture, for example, are all possible with the present invention.
  • Figure 1 A is a schematic diagram of an exemplary embodiment of an apparatus according to the present invention
  • Figure IB is a detailed view of a vessel of the apparatus of Figure 1A, showing various parameters of interest
  • Figure 2 is a plot of refractive index of an ethanol- water mixture versus ethanol concentration, determined using the present invention and compared to known values;
  • Figure 3 is a figure similar to Figure 2 for a methanol-water mixture
  • Figure 4 is a graph of measured concentration vs. true concentration for a NaCl- water solution
  • Figure 5 is a figure similar to Figure 4 for a MgCl 2 -water solution; and Figure 6 is a plot similar to that of Figure 5, but for very small concentration changes.
  • FIG. 10 An exemplary apparatus for determining the refractive index of a material, according to the present invention, is designated generally by reference numeral 10.
  • the apparatus includes a vessel 12 which contains a multi-component liquid 14 (e.g., a solution).
  • a multi-component liquid 14 e.g., a solution
  • the vessel 12 which can be, for example, a precision cuvette, has a planar entrance side 16 and a planar exit side 18.
  • the vessel 12 can be immersed in known surroundings 20, such as ordinary atmospheric air.
  • the apparatus 10 also includes a laser light source 22 which is located so as to cause a collimated beam of coherent light 24 to impinge on the entrance side 16 at an angle ⁇ , with respect to a normal 26 to the outer surface of the entrance side 16 where the light beam impinges.
  • Light source 22 is located with respect to vessel 12 such that beam 24 then passes through the liquid 14 and then through the exit side 18, from which it exits at an angle ⁇ e with respect to a normal 28 to the outer surface of the exit side 18 where the beam exits.
  • Apparatus 10 further includes a beam position sensor, designated generally as 30, which determines the exit angle ⁇ e , thus permitting the refractive index of the material 14 to be determined, based on the angles ⁇ j and ⁇ e , by using Snell's law.
  • Snell's law is applied at an interface 32 between the surroundings 20 and the outer surface of the entrance side 16, an interface 34 between the inner surface of the entrance side 16 and the liquid 14, an interface 36 between the liquid 14 and the inner surface of the exit side 18, and an interface 38 between the outer surface of the exit side 18 and the surroundings 20.
  • the vessel 12 need not necessarily be completely filled with the liquid 14, it is only necessary that liquid 14 be present in sufficient quantity to contain the path of the beam of light 24.
  • the light source can be a laser, so as to provide a collimated beam of coherent light 24, and that the laser may be a helium-neon laser having a wavelength of 632.8 nm.
  • the laser may be a helium-neon laser having a wavelength of 632.8 nm.
  • other types of light sources emitting collimated light beams at other wavelengths can be employed.
  • Sensor 30 preferably includes a light beam position sensor 40 which senses a position of the beam of light 24 after the beam exits the exit side 18 of the vessel 12.
  • the beam is designated as 42 in this region.
  • Position sensor 40 preferably has a known geometrical positional relationship to the vessel 12 so that the sensed position of the beam 42 can be related to the exit angle ⁇ e .
  • the beam position sensor 40 can simply be a suitable flat surface wherein the position of illumination by the beam 42 can be detected and related to an initial position.
  • a semiconductor beam position sensor to be discussed below, is employed. It is envisioned that the present invention will have particular utility for measurements of temperature and concentration to be carried out with materials 14 which are in the liquid phase. Measurement of index of refraction of gases may be accomplished by containing the gases in a sealed vessel at a desired pressure (in practice, the near-unity index of refraction of most gases may limit the utility of the method except, possibly, at high pressures).
  • the entrance and exit sides 16, 18 of the vessel 12 preferably have a substantially uniform thickness, t, as shown in Figure IB, and are preferably made from the same light transmitting material such that they have substantially the same refractive index.
  • t is shown on a side of the vessel other than the entrance and exit sides 16, 18 in Figure IB, it will appreciated that only these particular sides need have the uniform thickness and refractive index to obtain the desired benefits.
  • all sides are uniformly thick and have a substantially uniform refractive index, and it is such a cuvette which is envisioned as being one preferred form of vessel for use with the present invention.
  • W 0 is a reference value of distance between an inner corner 60 formed by the intersection of the planar entrance and exit sides 16, 18, and a point where the beam of light 42' exits the exit side 18 (see interface 38'), determined by measurement and calculation, as discussed below, ⁇ e 0 is a reference value of the angle at which the beam of light 42' exits the exit side 18, determined during calibration,
  • D is distance from the point where the beam of light exits the exit side to the position sensor 40, measured during calibration
  • L distance from the inner corner 60 to a point where the beam of light passes from the entrance side into the liquid (interface 34), determined by measurement and Snell's-law calculation
  • n, refractive index of liquid which is to be determined
  • t thickness of vessel walls
  • n g refractive index of vessel walls
  • ⁇ e (n,) is the value of ⁇ e , during measurement of a given liquid, defined according to the equation: ⁇ e (n,)
  • the position sensor 40 is preferably a semiconductor light beam position sensor, such as a photodiode beam position sensor to be further described below, coupled to a position-to-voltage converter 46 which produces an output voltage which is substantially linearly proportional to the displacement of the beam 42 from its reference position 42' determined during calibration.
  • computer 44 is preferably a general purpose digital computer, although those of skill in the art will appreciate that an application specific integrated circuit (ASIC) can also be employed.
  • Apparatus 10 can further include a first digital voltmeter 48 which receives the output voltage from the position-to-voltage converter 46 and which supplies a corresponding digital signal to the computer 44 as the aforementioned representation of the position of the beam of light 42.
  • Apparatus 10 can further include a temperature sensor 50 which is positioned to sense the temperature of the liquid 14 and which is coupled to the computer 44 so as to supply the computer 44 with a digital signal representative of the temperature of the liquid 14.
  • the coupling between the temperature sensor 50 and the computer 44 can be, for example, a second digital voltmeter 52.
  • Sensor 50 is preferably a thermistor, but could also be a thermocouple, or any other type of suitable temperature sensor known to those of skill in the art.
  • Apparatus 10 can also include a rotation stage 54 on which the vessel 12 is mounted.
  • Rotation stage 54 permits the angle of incidence of the beam of light 42 onto the vessel 12 to be adjusted during calibration of the apparatus 10, such that the reference position of the beam from the exit side 18 of the vessel 12 during calibration is substantially perpendicular to the plane of the beam position sensor 40 and impinges the sensor 40 at its midpoint.
  • the beam 42' shown in Figures 1 A and IB represents a reference beam during a calibration process, which has been adjusted to be substantially perpendicular to the beam position sensor 40, and will be described more fully below.
  • apparatus 10 can further include a scale 58 interposed between the rotation stage 54 and a mounting surface 56.
  • Scale 58 is preferably a digital balance. It can be provided in order to determine the mass of the liquid 14 being measured, and particularly changes in mass concentration of a liquid being measured, by weighing samples of the liquid 14 contained in the vessel 12 as additional mass of a given component is added to change the concentration. Again, the scale is not normally envisioned for use in a production environment, since the apparatus of the present invention would be employed to determine concentration.
  • concentration of a component in a multi- component liquid may be expressed in molar, volume percent or mass percent units in accordance with the present invention, however, mass percent is considered to be preferable.
  • the apparatus of the present invention can be employed to determine the unknown temperature of a sample of liquid 14 by measuring its index of refraction. The index of refraction of the material 14 can then be compared to predetermined data which relate different temperatures of the material to corresponding values of the refractive index of the material. Thus, the temperature, hitherto unknown, can be determined for a given sample of liquid by measuring the refractive index of the sample and converting the measured refractive index to a corresponding sample temperature.
  • Computer 44 can be programmed to carry out such calculations, although they can also be done manually.
  • Temperature determination is most practical for pure materials, or for multi-component materials having a substantially constant concentration of the components.
  • the refractive index of the unknown material is measured, and is then compared to known values of temperature vs. index of refraction, for example, using interpolation, a curve fit, or any other suitable means which will be apparent to those of skill in the art. Straight line, polynomial or spline curve fits can be employed.
  • the "Tablecurve 2D" program available from SPSS software has been found to be useful for such curve fitting.
  • the apparatus 10 according to the present invention can also be used to determine the unknown concentration of a given component of a multi-component liquid mixture.
  • the liquid 14 is a multi-component liquid mixture and the determination is carried out by comparing the measured refractive index of the multi- component liquid mixture to predetermined data which relate different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index. Again, this comparison can be carried out manually, or preferably by a suitably programmed computer 44.
  • an unknown value of the concentration of a given component of a sample of the multi-component liquid mixture can be determined by measuring the refractive index of the sample and converting the measured refractive index to a corresponding concentration of the given component in the sample of the multi-component liquid mixture.
  • a suitable interpolation between tabulated values, comparison with a curve fit, or any other desired method can be employed.
  • the refractive index of a multi - component liquid mixture is a function of the mixture temperature, mixture component concentration, and wavelength of the incident light. While the wavelength of the incident light is substantially constant when using a laser, the temperature and concentration can vary. When it is desired to determine the concentration of a given component in the mixture, in the presence of temperature changes, suitable correction must be carried out. For relatively large changes in concentration, values of refractive index as a function of concentration can be stored in computer 44 (or otherwise) for a number of different values of temperature. The measured refractive index of the specimen of unknown concentration can then be compared to predetermined data for at least two temperatures nearest the temperature of the specimen, which can be measured with the temperature sensor 50.
  • Interpolation can then be carried out between the values of the predetermined data at the at least two nearest temperatures so as to obtain the value of the concentration of the given component of the liquid 14 for the actual temperature of the liquid 14.
  • An ordinary bilinear interpolation familiar to thermodynamics students who have used the steam tables for the properties of water vapor, can be employed and can be programmed into computer 44, as will readily be appreciated by those of skill in the art.
  • Other suitable interpolation methods employing curve fitting and the like can also be used.
  • a given value of the refractive index of the mixture can correspond to two different values of concentration.
  • a unique value of concentration for a given a measured value of refractive index cannot be determined.
  • the correct value of the concentration can be selected.
  • computer 44 could be programmed to store values of the concentration of the given component of the liquid taken at previous proximate points in time and to employ the previous values to select the correct value of the concentration when the concentration is double valued with respect to the index of refraction.
  • apparatus 10 can be used for measurement of a multi- component liquid 14. Further, apparatus 10 can also include the aforementioned computer 44 and the aforementioned temperature sensor 50 which, as noted, can be positioned to sense temperature of the multi-component liquid 14 and which can be coupled to the computer 44 so as to supply the computer with a signal which is representative of the temperature of the multi-component liquid 14. When it is expected that relatively small concentration changes will be encountered, the apparatus 10 can be used to calculate changes in the concentration of given components of the multi-component liquid mixture from an initial value.
  • Computer 44 can be programmed to determine the refractive index of a sample of the multi- component liquid mixture based on the entrance and exit angles ⁇ j and ⁇ e , by applying Snell's law at the interfaces 32, 34, 36, 38 as discussed above. Computer 44 can further be programmed to determine a change in the refractive index of the multi- component liquid mixture by subtracting, from the refractive index determined by the computer, an initial refractive index of the sample of the multi-component liquid mixture corresponding to an initial concentration of a given component at a given temperature measured with the temperature sensor 50.
  • Computer 44 can be further programmed to receive the signal which represents the temperature of the given sample of the multi-component liquid mixture, and to determine the change, ⁇ C, in the concentration of the given component of the sample of the multi-component liquid mixture from the initial concentration according to Equation (1) above.
  • Equation (1) is advantageously used in cases where ⁇ C is relatively small.
  • Equation (1) is appropriately used for up to 5 or even 10% changes in C, although for substantially constant values of 3n/ ⁇ c (at a constant temperature), Equation (1) can be used for almost any value of ⁇ C.
  • the apparatus 10 can be employed to measure the index of refraction of a liquid, the temperature of a liquid, the concentration of a given component in a multi-component liquid, or a change in concentration of a given component in a multi-component liquid.
  • various calculations are referred to as being performed by computer 44, it will be appreciated that they could also be performed manually or by any other known means.
  • a method, according to the present invention, for determining the refractive index of a liquid 14, will now be discussed.
  • the method includes the steps of (a) causing the material to be contained in a vessel 12 having an entrance side 16 and an exit side 18 of known refractive index; (b) causing a beam of coherent light 24 to impinge on the entrance side 16 at an angle ⁇ j with respect to a normal 26 to the surface of the entrance side 16 where the beam impinges; and (c) calculating the refractive index of the liquid 14.
  • the vessel 12 can have entrance and exit sides 16, 18 with a known angular relationship therebetween and can be immersed in known surroundings 20.
  • the beam of light 24 can pass through the liquid 14, and then through the exit side 18 of the vessel 12, from which it exits at an angle ⁇ e with respect to a normal 28 to the surface of the exit side 18 where the beam exits.
  • the angle ⁇ e can be measured directly or indirectly within the scope of the present invention.
  • the refractive index of the material can then be calculated based on the angles ⁇ j and ⁇ e , by applying Snell's law at the interfaces 32, 34, 36, 38, as described discussed above.
  • a further step (d) can be performed, which comprises repeating steps (a) through (c) for samples of the multi-component liquid mixture having different concentrations of a given component. Accordingly, a curve of values of the refractive index of the multi- component liquid mixture corresponding to different concentrations of a given component can be obtained. Such curves are shown, for example, in Figures 2 and 3 and will be discussed further below. Throughout this application, "curve” should be understood broadly to include a physical, graphical plot, tabulated data values, parameters stored in a computer, and the like.
  • step (d) can be repeated under substantially isothermal conditions at a plurality of different temperatures, so as to obtain a family of curves of values of the refractive index of the multi-component liquid mixture corresponding to different concentrations of a given component for the plurality of different temperatures.
  • a family of curves similar to Figure 2 could be obtained at temperatures other than 20 °C.
  • the data obtained by this method could then be used, for example, by computer 44, to determine the unknown concentration of a given sample by measuring its index of refraction and, optionally, temperature.
  • the liquid 14 can be either a pure liquid or a multi-component liquid mixture having a substantially constant concentration of its components.
  • An additional step can be performed in this case, of repeating steps (a) through (c) for samples of the liquid at different temperatures, so as to obtain a curve of values of the refractive index of the liquid corresponding to different temperatures of the liquid. While such a curve is not shown in the drawing figures herein, it will represent a plot of refractive index versus temperature for either a pure liquid or a multi-component liquid mixture where the concentrations of all the components are constant. Further discussion on the refractive index as a function of temperature, concentration and wavelength will be presented below. However, as used herein, "substantially constant concentration" means that any changes in concentration of the components are small enough such that they may be neglected in determining a change in the refractive index.
  • the method can also include the step of providing an beam position sensor 40 as discussed above, which senses a position of the beam of light 42 following exit from the exit side 18. Just as for the vessel 12, the entrance and exit sides 16, 18 can be substantially perpendicular. Sensed position from the beam position sensor 40 can be used in calculating the refractive index.
  • Step (c) can include determining the refractive index of the liquid sample 14 according to equations (2), (3) and (4) set forth above.
  • the calibration can include the steps of causing a first liquid of known refractive index to be contained in the vessel 12.
  • a relative rotation between the vessel 12 and the position sensor 40 can then be carried out, for example, using the rotation stage 54, such that, for the first liquid, the beam of light 42 exits the exit side 18 substantially perpendicular to the beam position sensor 40 and impinges thereon at its midpoint. Accordingly, the aforementioned initial calibration value of d may be taken as 0.
  • the calibration method can further include recording W 0 and ⁇ e 0 , emptying the vessel 12 of the first liquid, and causing a second liquid of known refractive index to be contained in the vessel 12.
  • the refractive index of the second liquid is different than that of the first liquid.
  • Values of d, W and ⁇ e corresponding to the second liquid can then be determined, with the values of W and ⁇ e being respectively calculated from the equations (3) and (4) above.
  • the temperatures of the first and second liquids can be measured, during calibration, using temperature sensor 30 such that the refractive index of each liquid is accurately determined at well- defined temperatures. Calibration can be carried out under constant temperature conditions.
  • the parameters L and W 0 can be determined as follows.
  • the value of L can be determined by measuring the position L' of the laser beam 24 on the outside of the vessel 12 and then using Snell's law in conjunction with the vessel wall thickness t to determine the value of L as shown in Figure IB. One subtracts from L' the downward displacement of the beam through the entrance side 16, determined from Snell's law. Once L is known, Snell's law can again be used to determine W 0 . Alternatively, the wall thickness t can be subtracted from W 0 ' to yield W 0 . Accordingly, the values of L and W 0 may be determined by measurements and calculations using Snell's law. The measurements of W 0 ' and L 1 can be made with a ruler. W 0 ' and L 1 can be measured from an actual corner of a "virtual" sharp corner where the planar sides would intersect when rounded corners are present.
  • the present invention also provides a method for determining the concentration of a given component in a sample of a multi-component liquid mixture.
  • the method includes the steps of determining the refractive index of the sample of the multi-component liquid mixture as described above, and then comparing the refractive index of the sample thus determined to predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component mixture, so as to convert the determined refractive index of the sample to a corresponding concentration of the given component in the sample of the multi-component liquid mixture.
  • This method can also include the additional step of recording the temperature of the sample of the multi-component liquid mixture.
  • the step of comparing the refractive index of the sample of the multi-component liquid mixture to the predetermined data can include comparing the refractive index of the sample previously determined to values of the predetermined data relating the different concentrations of the given component to the corresponding values of the refractive index for at least two temperatures near the recorded temperature of the sample, and then interpolating between those values.
  • the interpolation can be carried out at the at least two temperatures near the recorded temperature of the sample of the multi-component liquid mixture, so as to obtain the value of concentration of the given component in the sample of the multi-component liquid mixture which is determined for the recorded temperature.
  • the temperature can be determined with the temperature sensor 50.
  • the predetermined data relating different concentrations of a given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture may be double valued as described above, such that certain values of the refractive index correspond to two different values of concentration.
  • refractive indices from about 1.331 to about 1.342 each correspond to two different values of methanol concentration.
  • the method can further include storing previously measured values of concentration for the given component in the sample of the multi-component liquid mixture taken at proximate prior points in time, and then using such stored previously measured values of the concentration to indicate an expected range of a newly measured concentration value so that the correct newly measured concentration value can be selected.
  • the present invention further provides a method for determining a change in concentration of a given component in a sample of a multi-component liquid mixture from an initial concentration corresponding to an initial refractive index of the sample at an initial temperature. This method is advantageous for relatively small changes in concentration, as discussed above with respect to Equation (1).
  • This method includes the steps of (a) determining the refractive index of the sample of the multi-component liquid mixture as described above; (b) determining a change in the refractive index, ⁇ n, by subtracting the refractive index determined in the preceding step from an initial refractive index; (c) measuring the temperature of the sample of the multi-component liquid mixture, for example, with the temperature sensor 50; and (d) determining the change, ⁇ C, in the concentration of the given component in the sample of the multi-component liquid mixture, from the initial concentration, according to equation (1) given above.
  • the present invention yet further provides a method for determining the temperature of a sample of a liquid, wherein the liquid may be a pure liquid or a multi-component liquid having a substantially constant concentration of each component.
  • the method includes the steps of (a) determining the refractive index of the sample of the liquid as described above; and comparing the determined refractive index of the sample to predetermined data which relate the different temperatures of the liquid to corresponding values of the refractive index of the liquid, so as to convert the determined refractive index to a corresponding temperature of the sample of the liquid.
  • the above-described apparatus 10 can be employed in carrying out this method.
  • the above-described equations (2), (3) and (4) can be employed.
  • the foregoing description of calibration of the apparatus 10 is also applicable to this method.
  • the concentration of the mixture can be determined.
  • the refractive index of a liquid depends on its density, as described in J. D. Spear, R. E. Russo, and R. J. Silva, "Collinear Photothermal Deflection Spectroscopy with Light-Scattering Samples," 29 Applied Optics 4225-34 (1990), and the wavelength of the incident light, as set forth in G. M. Hale and M. R. Querry, "Optical Constants of Water in the 200-nm to 200- ⁇ m Wavelength Region," 12 Applied Optics 555-63 (1973). Fluctuations in both temperature and concentration will change the liquid density (with pressure effects neglected). Thus, for an isothermal, achromatic measurement, the change of the refractive index of a multi- component liquid is determined only by the concentration of the components.
  • C A , C B are concentration of components A and B, respectively
  • m A and m B are masses of components A and B, respectively.
  • Snell's law is employed to relate the incident and exit light beam angles, and the liquid refractive index as the beam passes through the vessel 12.
  • the laser beam 24 is sent into the vessel 12 at an incident angle ⁇ such that it passes through the wall 16 into the material 14 and then strikes and passes through the wall 18, which may be perpendicular to the entrance wall 16.
  • ⁇ e is more significant than W
  • a simplified, explicit formula for the unknown refractive index n can be used: n, - (sin 2 ⁇ j + sin 2 ⁇ e )' /j . (9)
  • lateral beam displacement on the vessel exit side 18 10 decreases with increasing distance D between the vessel 12 and beam position sensor 40.
  • D 2 cm
  • Eq. (1) temperature changes in the liquid will result in a 15 perceived concentration change. Rather than attempt to maintain an isothermal system, the liquid temperature can be monitored, and the concentration change can be compensated for temperature by using Eq. (1) and values of dn/dT such as those in Table 1 below. It will be seen that values of total derivative, dn/dT, are tabulated in Table 1 because the data therein is for pure liquids. Values for mixtures can be 20 estimated by linearly interpolating, using the values of dn/dT for each component.
  • dnld ⁇ may often be approximately constant with concentration, or can be treated as a function of concentration and a double interpolation carried out.
  • Table 1 Thermo-Optical Properties of Liquids n(589 nm, 20°C) n(632.8 nm, 20 °C) 10 4 x dn/dT (K- 1 ) water 1.3330 [17] 1.331 [21] -1.04 [18] ethanol 1.3614 [17] 1.358 [21] -3.9 [21] methanol 1.3290 [17] 1.325 [21] -3.9 [21]
  • the laser beam 24 was directed onto the cuvette wall 16 at a specified incident angle ranging from 70 to 80° (measure with respect to the normal to the wall) using a rotation stage 54.
  • the beam 42 exits the adjacent cuvette wall 18 at an angle ⁇ e that depends on the index of refraction, n,, of the liquid in the cuvette, and the exiting beam strikes a semiconductor beam position sensor 40.
  • a UDT LSC-5L linear beam position sensor with a 15 V bias voltage was used as sensor 40 to monitor this position change of the beam 42.
  • This highly linear, low noise position sensor was of the single photodiode type and was capable of detecting beam displacements as small as 2 ⁇ m. It was manufactured by United Detector Technologies, Inc. of San Jose, California.
  • the sensor output was sent to a position- to-voltage converter 46, which produced an output voltage linearly proportional to the beam position. This voltage output was in turn measured using a 6.5 digit Keithley Model 2000 digital voltmeter (DVM), shown in Figure 1 A as first voltmeter 48.
  • DVM digital voltmeter
  • the measured sensor sensitivity was 3.89 mV/ ⁇ m.
  • the DVM readings were sent via a Hewlett-Packard industry standard general purpose bus (GPIB) interface to computer 44, which was a personal computer running Visual Basic for data acquisition. Typically, 10 readings were taken over a 1 sec interval and averaged to obtain a final reading.
  • GPS general purpose bus
  • the UDT SL5-2 beam position sensor has three leads.
  • the center lead can be grounded while the other to leads A and B each provide a current I A and I B , respectively.
  • the currents I A and I B can be converted to respective voltages V A and V B , respectively, using a well known operational amplifier technique.
  • the differential voltage, V A - V B directly relates to the beam position. To avoid errors in measuring caused by fluctuations in light beam intensity, the voltage difference (V A - V B ) is normalized by the voltage sum (V A + V B ).
  • V A and V B can be added by adding V A and V B by using a well known operational amplifier technique and by using an AD734 4-quadrant multiplier-divider manufactured by Analog Devices, Inc. of Norwood, Mass. to provide a quantity (V A -V B )(V A +V B) , which is also directly proportional to beam position relative to the center of the LSC- 5L beam position sensor.
  • the concentration of the given component in the liquid was determined using an Ohaus laboratory digital balance, shown in Figure IA as scale 54, with a resolution of 0.01 g.
  • the glass cuvette 12 and rotation stage 54 were mounted on scale 58. Very small concentration changes that would otherwise not be resolvable with the scale 58 were made by first preparing a diluted mixture of known concentration, and then adding this diluted mixture to the cuvette, so that the total mass was large enough to be measured by the scale 58. To minimize vibration effects, the system was assembled on a vibration-isolated optical bench. Vibration isolation is believed to be desirable in all embodiments of the present invention.
  • a 2 mm diameter thermistor was used as temperature sensor 50 and was placed under the test liquid surface inside the cuvette to monitor the liquid temperature.
  • the liquid temperature was recorded simultaneously with the beam position using a second Keithley multimeter, shown in Figure 1 A as voltmeter 52.
  • a second Keithley multimeter shown in Figure 1 A as voltmeter 52.
  • the pure water was first placed in the vessel 12, and the beam position sensor 40 was oriented perpendicular to the exit laser beam 42, which impinged on the center of the sensor 40.
  • the beam position W 0 and exit angle ⁇ e 0 required in Eq. (2) were also recorded.
  • EXAMPLE 2 To measure concentration, aqueous salt solutions of NaCl and MgCl 2 were used. Such solutions have a substantially linear relationship between n and C, i.e., dn/dC is substantially constant, as described in D. R. Lide, Ed., CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton, 1998). The values of dn/dC for NaCl-H 2 O and MgCl 2 -H 2 O solutions at 20°C are 1.71 x 10 "3 and 2.60 x 10 "3 %, respectively. Measurements on aqueous salt mixtures permit very small concentration changes to be made easily, and without effects such as evaporation losses and cooling that the alcohol mixtures would incur.
  • the MgCl 2 -H 2 O results have an uncertainty of 0.09% in Figure 5, which is smaller than 0.14% for the NaCl-H 2 O solution in Figure 4.
  • the thermistor used as temperature sensor 50 was calibrated in a separate temperature-controlled water bath using a NIST-traceable platinum- resistance RTD standard thermometer with a reported accuracy of 0.01 °C, resulting in a thermistor uncertainty of less than 0.1 °C.
  • the power of the HeNe laser used as light source 22 was 1 mW and the absorption coefficient of the test liquids, , is on the order of 10 "3 , as described in D.

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Abstract

L'invention concerne un procédé et un dispositif pour mesurer avec précision par voie optique la concentration d'un soluté et la température d'un liquide transparent. Ce dispositif comprend une cuve (12) contenant un échantillon de liquide (4) dont l'indice de réfraction doit être déterminé, une source lumineuse (22) et un détecteur de position de rayonnement (30). La cuve (12) présente un côté entrée et un côté sortie, qui forment un angle l'un par rapport à l'autre, et est située dans un milieu connu. La source lumineuse (22) est placée de sorte qu'un rayon lumineux arrive sur le côté entrée en formant un angle par rapport à la normale à une surface extérieure du côté entrée, traverse le liquide (14) puis traverse le côté sortie, d'où il sort en formant un angle par rapport à la normale à une surface extérieure du côté sortie. Le détecteur de position de rayonnement (30) fournit des données servant à déterminer l'angle de sortie et permettant ainsi de déterminer l'indice de réfraction du liquide, d'après lesdits angles.
PCT/US1998/023179 1998-10-30 1998-10-30 Procede et dispositif pour mesurer par voie optique la concentration et la temperature de liquides WO2000026665A1 (fr)

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PCT/US1998/023179 WO2000026665A1 (fr) 1998-10-30 1998-10-30 Procede et dispositif pour mesurer par voie optique la concentration et la temperature de liquides
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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2003083457A1 (fr) * 2002-03-28 2003-10-09 Qinetiq Limited Detection de perturbation
US6731380B2 (en) * 2001-06-18 2004-05-04 Applied Optics Center Of Delaware, Inc. Method and apparatus for simultaneous measurement of the refractive index and thickness of thin films
CN102507506A (zh) * 2011-11-18 2012-06-20 江苏大学 激光混药浓度在线检测的方法及其装置
CN111337454A (zh) * 2020-04-17 2020-06-26 湖南文理学院 基于激光干涉技术的溶液浓度快速检测方法
CN112345493A (zh) * 2020-10-20 2021-02-09 广西壮族自治区计量检测研究院 液体折射率测量方法
CN115219562A (zh) * 2022-07-14 2022-10-21 湖北工程学院 溶液浓度检测方法、装置、设备及存储介质

Citations (1)

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Publication number Priority date Publication date Assignee Title
US5548393A (en) * 1993-07-05 1996-08-20 Nippondenso Co., Ltd. Oil deterioration detection apparatus and apparatus for detecting particles in liquid

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5548393A (en) * 1993-07-05 1996-08-20 Nippondenso Co., Ltd. Oil deterioration detection apparatus and apparatus for detecting particles in liquid

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6731380B2 (en) * 2001-06-18 2004-05-04 Applied Optics Center Of Delaware, Inc. Method and apparatus for simultaneous measurement of the refractive index and thickness of thin films
WO2003083457A1 (fr) * 2002-03-28 2003-10-09 Qinetiq Limited Detection de perturbation
US7333919B2 (en) 2002-03-28 2008-02-19 Qinetiq Limited Perturbation detection
CN102507506A (zh) * 2011-11-18 2012-06-20 江苏大学 激光混药浓度在线检测的方法及其装置
CN111337454A (zh) * 2020-04-17 2020-06-26 湖南文理学院 基于激光干涉技术的溶液浓度快速检测方法
CN112345493A (zh) * 2020-10-20 2021-02-09 广西壮族自治区计量检测研究院 液体折射率测量方法
CN115219562A (zh) * 2022-07-14 2022-10-21 湖北工程学院 溶液浓度检测方法、装置、设备及存储介质

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