EP1920237A1

EP1920237A1 - Differential measuring method for determining variations in concentration for determining oversaturation

Info

Publication number: EP1920237A1
Application number: EP06776888A
Authority: EP
Inventors: Matthias Kind; Miguel Postigo; Wolfgang Heering
Original assignee: Universitaet Karlsruhe
Current assignee: Karlsruher Institut fuer Technologie KIT
Priority date: 2005-09-01
Filing date: 2006-08-16
Publication date: 2008-05-14
Also published as: WO2007025648A1; DE102005041584A1; DE102005041584B4

Abstract

The invention relates to a specific method for determining a degree of saturation of a to be analyzed solution or suspension (12) of a substance dissolved in a solvent at a specific or specifiable temperature (T) and to a corresponding device therefor.

Description

Differential measurement method for determining concentration differences for supersaturation determination

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

Crystallization from solution is widely used in the food, chemical, pharmaceutical and basic industries. In the crystallization, a component is crystallized from a two- or multi-component solution (aqueous or organic), which can then be recovered, for example, by filtration with the highest possible yield and purity.

The crystals should generally meet certain quality requirements in terms of crystal size distribution, purity and crystal form. For example, in the sugar industry, which is part of the food industry, the fulfillment of these quality requirements is of the utmost importance. Here, these requirements are met by elaborately developed procedures for process control (seeding, AbkühMEindampfprogramme, intermittent, semi-continuous and continuous driving).

From a solution crystals are formed by nucleation and subsequent growth. For both processes, supersaturation must be present as a driving thermodynamic potential. A solution is supersaturated when a greater proportion of a component in the solvent is dissolved than corresponds to the thermodynamic equilibrium. Supersaturation can be achieved, for example, by lowering the temperature or by removing solvents by means of evaporation. Since nucleation and growth rate are determined by the level of supersaturation, the mean supersaturation and its local distribution is an important process parameter to be controlled. So far crystallization processes are not guided by the supersaturation, since no suitable in-line sensors are available.

As shown in FIG. 3, the supersaturation or sub-saturation of crystallizing solutions can in principle be determined by determining the actual concentration X of the crystallizing component in the solution by means of a suitable measuring method and its calibration. By simultaneously measuring the temperature of the crystallizing solution TK, it is possible, with knowledge of the saturation curve of the substance system, to calculate the supersaturation ΔX of the crystallizing component.

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 ATR-FTI R spectroscopy are given as suitable measurement methods. For all these methods the exact knowledge of the saturation curve in the form X ^* = f (T _κ ) is necessary. These measuring methods are also not applicable to poorly defined solutions, ie multi-component 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 (Tκ) is created for the contaminated solution.

Another method for supersaturation measurement is shown in DE 43 40 383 C2 and DE 43 45 163 C2. 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 hypothermia of this surface, which causes a shortening is used as 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.

There is therefore an increasing need to be able to control and regulate industrial suspension crystallizers better than before. Through improved control and regulation, more sophisticated product qualities can be realized with higher reproducibility. However, the necessary in-line sensor technology is currently not available. It is thus an object of the present invention to provide a method and a device for improving the measurement of a degree of saturation of a solution or suspension.

This object is achieved by the embodiments characterized in the claims.

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 , comprising 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 investigated;

Irradiating at least one first incident light beam of a specific or determinable first single-beam polarization onto the first boundary Such that the first incident light beam is reflected substantially by total reflection at the first interface into a first reflected light beam having 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 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 the solution to be tested, at the determined or determinable temperature;

Irradiating at least one second incident light beam of a specific 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 having 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 examined from the determined refractive index difference. The saturated solution thus comprises the same solvent and the same solute as the solution to be tested, with the difference that the saturated solution is substantially in an equilibrium state, ie essentially has neither a supersaturation nor a sub-saturation. The saturated solution serves as a reference solution according to the present invention.

The plane of incidence of a light beam incident on an interface or surface is defined as the plane defined by the light beam and the surface normal of the interface or surface at the point of impact of the light beam. The reflected light beam is also in this plane. Thus, the first and the second Einstrahlpolarisation at least temporarily each have a component in parallel and a component perpendicular to the plane defined by the first and second incident light beam and the corresponding first and second outgoing light beam plane. A light beam or one of its components is referred to as being parallel polarized if the plane of oscillation of the electric field, that is to say the electric field vector of the electromagnetic radiation, lies parallel to and in particular in the plane of incidence. In the case of a component polarized perpendicularly, on the other hand, the electric field vector, ie the plane of oscillation of the electric field, is perpendicular to the plane of incidence.

The irradiation of the first or second incident light beam with a certain or determinable polarization does not mean that the polarization is fixed as constant, ie unchanged in time. In this context, the irradiation of a "determined" or "determinable" polarization means that the polarization and in particular the change in the polarization of the incident light beams is known or at least can be evaluated to such an extent that they correspond to the measured polarization of the Reflected light rays can be compared, so that a determination of a change in the polarization is made possible by the total reflection. It is even particularly advantageous to change the polarization of the incident light beams in a time-dependent manner and in particular to periodically modulate them. It is only required that the incident light rays have at least temporarily parallel and vertical polarization components.

When a light beam strikes the interface between two media at an angle greater than the critical angle, the light is totally reflected, causing a phase shift of at least individual components of the lightwaves, which is polarization-dependent. In particular, the phase shift of the vertically polarized components and phase shift of the parallel polarized components differ from each other. The difference in these phase shifts is dependent on the ratio of refractive indices of the materials forming the interface. The relative phase shift of parallel and perpendicular components provides a measure of the refractive index ratio of these two media. If one refractive index is known, the other can be calculated.

The simultaneous measurement of the solution or suspension to be examined and of a saturated solution or suspension eliminates the need for exact knowledge of the equilibrium curve or saturation curve and also permits in-line measurements. For the differential refractive index determination, the light is totally reflected at two interfaces: one interface exists between the first optical element and the solution to be examined, and the other between the second optical element and the saturated solution or reference solution. The resulting difference in the rotation of the vibration planes is a measure of the differential refractive index.

According to the present invention, the principle of a differential refractometer is now transferred by preferably measuring the total reflection of a polarized laser beam on the surface of the particularly supersaturated solution against a reference cell in which the original solution of the reactor is then saturated. Crystals or other particles in the solution do not interfere with the measurement. The solution in the reference cell is preferably exchanged regularly with the reactor solution. Until the saturation equilibrium is set, preferably less than one minute passes. From the different changes tion of the degree of polarization of the reflected laser beam on the one hand to the measuring cell and on the other hand to the reference cell, the degree of supersaturation of the solution can be measured directly in the reactor with a time delay of usually less than a minute. This allows a very precise control of the 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 or second interface in such a way that it strikes the latter from the side of the first and second optical element.

In this preferred embodiment, a light beam is not sent through the liquids to be examined, but penetrates during the process of total reflection, preferably only a few 100 nm into the solution. Thus, the solution to be tested and the saturated solution need not necessarily be transparent and free of particles. Thus, no separation of crystals from the solutions or suspensions is needed to measure the degree of saturation.

In a particularly preferred embodiment, the method according to the invention further comprises the steps of: providing at least one measuring cell, which preferably serves as a crystallization cell or crystallizer, for receiving the solution to be investigated;

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 for the liquid phase of the solution or suspension is substantially permeable and substantially impermeable to the crystalline phase.

In a preferred embodiment, the measuring cell is independent or separated 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, therefore, the entire crystallizer serves as a measuring cell. Thus, the method can be used particularly well for in-line measurements during the crystallization process.

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 periodically modulated light in its polarization into the first incident light beam and the second incident light beam, preferably by means of a beam splitter.

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

Particularly preferably, the saturated solution is additionally admixed with a crystallizate of the solute, wherein the crystallizate preferably contains crystals having a particle size of preferably substantially less than 0.1 mm, more preferably less than 10 μm, most preferably essentially approximately 1 μm. This crystallizate is preferably introduced into the saturation cell or reference cell. Preferably, ultrasound is coupled into the saturated solution at least at times by means of an ultrasound transmitter, which is particularly preferably arranged in the region of the reference cell. This achieves a thorough mixing of the solution with the crystals and preferably a substantially uniform distribution of the crystals in the saturated solution. This results in a change in the composition or the temperature of the solution to a rapid achievement of a new equilibrium.

Another object of the present invention relates to a device for determining a degree of saturation, in particular a supersaturation and / or a supersaturation, of a solution to be investigated or suspension of a dissolved substance 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 at least partially in contact with the solution or suspension to be investigated to form a preferably substantially planar first interface; 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 or suspension of the solute in the solvent to form a preferably substantially planar second interface in contact; at least a first Lichteinstrahlmittel for irradiating at least a first incident light beam of a particular 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 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 determinable 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 one-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.

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 the refractive index of the solution to be investigated or of the saturated solution at at least one wavelength of the first or second incident light beam. In addition, the first and / or second light irradiation means is preferably designed to radiate the first and / or second incident light beam onto the first and second boundary surface, respectively, in such a way that it strikes this interface from the side of the first and second optical element.

Particularly preferably, the first and / or second optical element comprises a first and second incident surface, respectively, through which the first and second incident light beams are substantially perpendicular to each other prior to the total reflection this incident surface is irradiated 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.

The irradiation into the respective optical element preferably takes place via an irradiation surface outside the solution to be investigated or the saturated solution.

In a preferred embodiment, a device further comprises: at least one measuring cell, which is preferably configured as a crystallization cell or as a crystallizer, for receiving the solution or suspension to be investigated; at least one reference cell or saturation cell for receiving the saturated solution or suspension, wherein the saturated solution in the saturation cell is in communication with the solution to be investigated in the measuring cell or in the crystallizer via a filtering membrane in such a way that the filtering membrane for the liquid Phase of the solution or suspension is substantially permeable and is substantially impermeable to the crystalline phase.

Particularly preferably, the light irradiation means comprise at least one laser for generating linearly polarized light, at least one electro-optical modulator for the periodic modulation of 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 and second analyzer and a first and second photodetector, respectively. Particularly preferably, an ultrasonic transmitter for coupling ultrasound into the saturated solution is provided in the region of the reference cell.

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, comprising at least one, preferably at least partially substantially optically transparent, first optical Element, which is preferably substantially formed as a lens and / or prism element, with at least one preferably substantially planar surface which at least partially in contact with the test sample to form a preferably substantially planar first interface 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 at least partially filled with a saturated solution or suspension of the solute in the solvent to form a preferably substantially planar, second interface is in contact 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 a second Lichteinstrahlmittel for irradiating at least one second incident light beam of a certain or determinable second Einstrahlpolarisation such that the second incident light beam is substantially reflected 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 both a composite element in parallel and has 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 one-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 test sample and the second refractive index of the reference sample from the determined difference between the polarization changes.

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

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

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

3 shows a schematic representation of an equilibrium curve or saturation curve X ^* = f (Tκ)

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 or suspension to be investigated. In a crystallizer 10, a solution or suspension 12 to be investigated is provided. At least one particular substance is at least partially dissolved in a certain solvent. In the solution 12 to be investigated, a crystallizate of the solute is to be generated by regulation or control of the process conditions. For example, the solute may be NaCl (common salt) or sucrose (sugar) and the Solvent to water act. 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 a saturated solution or suspension 16 of the substance dissolved in the solution to be investigated is in the same solvent. Preferably, the saturated suspension 16 is composed of a liquid phase and a crystallizate of the solute. In this case, the crystals are preferably present 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. 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, by virtue of their large surface area, allow rapid achievement of a state of equilibrium of the saturated solution 16 in the saturation cell 14, i. an occurring supersaturation or undersaturation of the solution in the saturation cell 14 is degraded relatively quickly, so that the solution 16 is substantially 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 14 is in thermal contact with the crystallizer 10 or the saturated solution 16 with the solution 12 to be investigated such that the temperature of the saturated solution 16 substantially corresponds to the temperature of the solution 12 to be investigated. Preferably, the saturation cell 14 has at least one opening 18, which is closed by a filtering membrane 20. The filtering membrane 20 is preferably fastened to the saturation cell 14 by means of a screw cap 22. The filtering membrane 20 and in particular its pore size is preferably adapted to the solution to be examined 12 and the saturated solution 16, that the membrane 20 is permeable to the liquid phase of the solutions, while the crystalline phase of the solutions or suspensions is impermeable. Thereby It is ensured that changes in the material system in the crystallizer 10 during the crystallization process (eg changes in the composition during evaporation crystallization, concentration of impurity or even temperature changes) are also transferred to the saturation cell 14. The crystals introduced into the saturation cell 14 ("bottom body") are preferably suspended and mixed at certain intervals by an ultrasonic transmitter 23.

In a housing 24 of the preferred embodiment, a first optical element 26 is preferably arranged as a hemispherical-shaped lens such that a preferably planar surface of the first optical element 26 at least partially as the first interface 28 with the solution to be examined

12 is in contact. Thus, preferably, a direct transition from the material of the first optical element 26 to the solution 12 to be examined is formed at the first interface 28.

A second optical element 30, which is preferably likewise designed as a hemispherical lens, is preferably arranged in the region of the saturation cell 14 in such a way that it is in contact with the saturated solution 16 with a preferably planar surface, whereby, analogously to FIG first interface 28 forms a second interface 32 between the lens body and the saturated solution 12.

In the preferred embodiment shown, the device comprises a laser 34 which preferably generates linearly polarized light. The linearly polarized light is guided via a first polarization-maintaining optical fiber 36 to an electro-optical modulator 38 (EOM). Preferably, the electro-optic modulator 38 is 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 preferably differs perpendicular to an optical axis of the electro-optical modulator or Pockels cell or the birefringent crystal of 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 through a certain length in the Pockels cell results in a relative phase shift, ie, a difference of 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 light leaving the electro-optical modulator 38 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 passed through a second polarization-maintaining fiber 40 into the housing 24 of the device. There it encounters a beam splitter 42 and is split into a first incident light beam 44 and a second incident light beam 46. Both incident light beams now have the same modulated polarization.

The first incident light beam 44 strikes a preferably spherical surface of the first optical element 26 and enters the first optical element 26. The inlet surface is preferably designed such that the first incident light beam 44 impinges perpendicular to this surface and so that upon entry into the first optical elements 26 is not broken. In the preferred embodiment shown, the first optical element 26 has a refractive index n _? at the wavelength of the laser light, which is greater than the refractive index n _{m of} the solution to be examined 12. The first incident light beam 44 strikes in the first optical element 26 at an angle Θi to the normal direction of the first interface 28 on this first interface 28, wherein the angle of incidence Θi is greater than the critical angle of total reflection. Thus, the first incident light beam 44 is totally reflected in a first outgoing light beam 48.

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 12 to be examined, the angle of incidence Θi 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. The plane of incidence is defined as the plane defined by the first incident light beam 44 and the first outgoing light beam 48, ie, the plane in which both the first incident light beam 44 and the first outgoing light beam 48 are located. Depending on the angle of incidence Θi and the refractive index difference or the refractive index ratio between the first optical element 26 and the solution 12 to be examined, the perpendicular polarization undergoes a different phase shift or 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:

n, = «, (sinΘ,) ² - sinΘ, tanΘ, tan As a result, the polarization of the first outgoing light beam 48 differs from that of the first incident light beam 44. In particular, the relative phase of the components of light polarized perpendicularly and parallel to the plane of incidence changes. The relative shift in the phase position of the vertical and parallel components of the light depends on the refractive index ratio between the first optical element 26 and the solution 12 to be examined, given a fixed angle of incidence Θi.

After leaving the first optical element 26, the first outgoing light beam strikes a first analyzer 50. Depending on the analyzer, only a portion of the first outgoing light beam 48, that is to say a specific polarization component, can penetrate the first analyzer 50 and then strike a first photodetector 52 The other or the other polarization component (s) of the first outgoing light beam 48 are preferably filtered out by the first analyzer 50. The first photodetector 52 detects the intensity of the portion of the first outgoing light beam 48 impinging there and transmits a signal to an evaluation device 54.

The second incident light beam 46 strikes from the beam splitter 42 in the embodiment shown on a mirror 56 and from there to the second optical element 30, which preferably similar to the first optical element 26 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 that is greater than the refractive index of the saturated solution 16. The second incident light beam 46 strikes the second interface 32 at an incident angle Θ ₂ greater than the critical angle of total reflection. As in the case of the first incident light beam 44, the second incident light beam 46 is also totally reflected at the second interface 32 a second outgoing light beam 58 is created, which after passing through a second Analyzer 60 at least partially encounters a second photodetector 62. Analogous to the first light beam, the relative phase angle of parallel and perpendicularly polarized component next to the second angle of incidence Θ ₂ also depends on the refractive index ratio between the second optical element 30 and the saturated solution 16 for the second incident light beam. The difference Δ _r in the phase shift between vertical and parallel polarization of the second light beam in the total reflection fulfills the equation analogously to the first light beam:

Preferably, the second analyzer 60 and the second photodetector 62 are constructed analogously to the first analyzer 50 and the first photodetector 52. Most preferably, the second optical element 30, the second analyzer 60 and the second photodetector 62 are identical to the first optical element 26, the first analyzer 50 and the first photodetector 52.

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

In the apparatus of Fig. 1, a HeNe laser at a wavelength of 633 nm is preferably used. Preferably, linearly polarized laser light having a plane of polarization which includes an angle of 45 ° with the plane of incidence, ie a polarization that passes through an unchanged passage the electro-optical modulator with a linear polarization of 45 ° to the plane of incidence on the first and second interface meets, on the electro-optical modulator whose optical axis with the polarization of the laser light includes an angle different from 0 ° and 90 °. 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 in opposite directions to the first circular polarization and an elliptical polarization leads back to the initial linear polarization.

Preferably, the analyzers are arranged at an angle of 45 ° to the plane of incidence. In this arrangement, a particularly high measurement signal and thus a particularly good measurement resolution can be achieved. The analyzers thus preferably allow only polarization components of the reflected light beams to strike the photodetectors which have an angle of 45 ° to the plane of incidence. The modulation of the polarization of the incident light beams results in a modulation of the light intensity impinging on the photodetectors. In the preferred embodiment shown, a maximum intensity is achieved, in particular, when the relative phase shift caused by the total reflection exactly abolishes the relative phase shift caused at the electro-optical modulator, ie acts exactly counter to this. As a result, linearly polarized light impinges on the analyzer, the polarization plane being equal to the polarization of the laser light and thus, in the preferred embodiment shown, equal to the polarization direction of the analyzer. In contrast, a minimal signal is obtained when the two relative phase shifts in the total reflection and in the electro-optical modulator add up to a total relative phase shift of π. This creates a perpendicular to the analyzer polarization linear polarization of the reflected light beam and the measured intensity drops to zero.

The necessary voltage ratios at the electro-optical modulator for reaching a maximum thus depend on the phase shift in the total reflection and differ for the first incident light beam and the second incident light beam as a function of the refractive index ratios at the first and second interface. The two detected intensity signals thus have a phase shift which is a measure of the difference in the relative phase shifts at the first and second interfaces and preferably coincides with the difference in the relative phase shifts. In this case, only additional phase shifts, caused by further reflections on mirrors and beam splitters, must be taken into account, which, however, can be taken into account by appropriate calibrations or calibrations.

Thus, the linearly polarized laser beam is periodically modulated by the electro-optical modulator (EOM) in its polarization and decomposed by means of a beam splitter 42 into a reference beam and a measuring beam. The intensity / _{r of} the reference beam is measured 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. The intensity / _{m of} the measuring beam is measured after the total reflection at the interface 28 between prism 26 and test solution 12 and the passage through the analyzer 50 with the photodetector 52. The measured intensities / _r and A preferably have the following course:

/ _r = i [l-cos (r + Φ _s + Φ _w + Δ _r )]

where r is the phase of the electro-optical modulator and a ² the relative intensity of the two beams to each other, Φ ^ the phase shift by the reflection at the beam splitter 42 and Φ _{M is} the phase shift by the reflection at the mirror 56. The measured intensities I _x and / _t show a phase difference Δ = Δ _r -Δ _t> which represents a measure of the differential refractive index Δn of the solution to be investigated. 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.

FIG. 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 investigated according to the present invention. Instead of lenticular optical elements with substantially spherical entrance and exit surfaces, the second preferred embodiment has a first optical element 26 and a second optical element 30, 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, essentially planar surfaces on which further total reflections of the incident or outgoing light beams take place. In a departure from the first preferred embodiment, in the second embodiment shown, the first 44 and the second incident light beam 46 impinge at the same angle on the first 28 and second boundary surfaces 32, ie, Θ, = 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 polarization on additional mirrors 56, 64 need to 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. When such liquid could be used, for example, water in which preferably substantially no solute is located.

In addition to the examples shown, a number of other embodiments and variations are also possible. While in the embodiments shown, both the beam splitter 42 and the optical elements 26, 30, analyzers 50, 60 and photodetectors 52, 62 housed in the common housing 24, alternatively, the beam guidance 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 26 and second optical element 30 may 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.

Notwithstanding the embodiments shown, other angles for an electro-optical modulator, an incident laser light and / or an analyzer can be used. Also, it is not necessary to use linear polarization analyzers. The measurement of circular polarization would be possible and leads to the goal in an analogous manner. In addition, unlike the preferred embodiments shown, it is not necessary to modulate the polarization of the incident light. It is only necessary that the incident light have perpendicular and parallel polarization components. In this case, however, an absolute measurement of intensities and a comparison of the absolute intensities of measuring and reference signal is required instead of a phase shift of measured intensity oscillations. Thus, the embodiments shown with a periodic modulation of the polarization have the advantage that in the manner of a lock-in technique noise can be filtered out and the sensitivity increases significantly. LIST OF REFERENCE NUMBERS

10 crystallizer or measuring cell or crystallization cell

12 solution to be examined

14 saturation cell or reference cell

16 saturated solution 18 opening

20 filtering membrane

22 screw cap

23 ultrasonic transmitter

24 housing 26 first optical element

28 first interface

30 second optical element

32 second interface

34 laser 36 first fiberglass

38 electro-optical modulator

40 second fiberglass

42 beam splitter

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 device 56 mirrors

58 second reflected or outgoing light beam

60 second analyzer

62 second photodetector

64 mirrors

Claims

claims

1. A method for determining a degree of saturation of a solution or suspension (12) of a substance dissolved in a solvent at a specific or determinable temperature (T), which comprises the following steps:

Forming a first interface (28) between a first optical element (26) and the solution (12) to be examined; - At least a first incident light beam (44) of a specific or determinable 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 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);

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);

Irradiating at least one second incident light beam (46) of a specific or determinable second single-beam polarization onto the second interface (32) in such a way that the second incident light beam (46) is reflected into the second interface (32) into a second one substantially by total reflection Light beam (58) is reflected with a second outgoing polarization, wherein 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);

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 examined (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 examined (12) from the determined refractive index difference.

The method of claim 1, wherein in the steps of forming the first (28) and second interfaces (32), the first (26) and second optical elements (30) are at least in the region of the first (28) and second interfaces (32 ) has a refractive index which is greater than the refractive index of the solution (12) or the saturated solution (16) to be examined at at least one wavelength of the first (44) or second incident light beam (46) and wherein the first (44) and second incident light beam (46) is irradiated on the first (28) and second interface (32), respectively, so as to be incident therefrom from the side of the first (26) and second optical elements (30), respectively.

3. The method according to claim 1 or 2, further comprising the steps:

Providing at least one measuring cell (10) for receiving the solution (12) to be examined; - providing at least one reference cell (14) for receiving the saturated solution (16); wherein the saturated solution (16) in the reference cell (14) is contacted with the solution (12) to be tested in the crystallization cell (10) via a filtering membrane (20) such that the filtering membrane (20) is liquid Phase of the solution or suspension is substantially permeable and is substantially impermeable to the crystalline phase.

4. The method according to any 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 periodically modulated light in its polarization into the first incident light beam (44) and the second incident light beam (46).

The method of any of claims 1 to 4, wherein in the step of determining the difference in polarization changes, the first (48) and / or second reflected light beam (58) after passing through a first (50) and second analyzer (60), respectively ) is detected by a first (52) or second photodetector (62).

6. The method according to any one of claims 1 to 5, wherein the saturated solution (16) is additionally mixed with a crystallizate of the solute.

7. The method of claim 6, wherein at least temporarily ultrasound is coupled into the saturated solution.

8. An apparatus for determining a degree of saturation of a solution or suspension (12) of a solute dissolved in a solvent, comprising: - at least one first optical element (26) with at least one

Surface which is at least partially in contact with the solution (12) to be examined to form a first interface (28); at least one second optical element (30) having at least one surface which is at least partially contactable with a saturated solution (16) of the solute dissolved in the solvent to form a second interface (32); at least one first light-emitting means for irradiating at least one first incident light beam (44) of a specific or determinable first Einstrahlpolarisation 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 Com- component and a component perpendicular to the plane of incidence of the first incident light beam (44); at least one second light irradiation means for irradiating at least one second incident light beam (46) of a definable second Einstrahlpolarisation such that the second incident light beam (46) substantially 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); at least one polarization detection device for determining a difference between a first polarization change from the first one-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 examined (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 examined (12) from the determined refractive index difference.

9. The apparatus of claim 8, wherein the first (26) and second optical element (30) at least in the region of the first (28) and second interface (32) has a refractive index at least one wavelength of the first (44) and Second incident light beam (46) greater than the refractive index of the solution to be examined (12) or the saturated solution (16) and the first and second Lichteinstrahlmittel is designed, the first (44) and second incident light beam (46) on to irradiate the first (28) or second interface (32) to meet them from the side of the first (26) or second optical element (30).

10. Apparatus according to claim 9, wherein the first (26) and second optical element (30) a first or second irradiation surface comprises, 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 and second outcoupling surface, through which the first

(48) or second reflected light beam (58) after the total reflection substantially perpendicular to this outcoupling surface from the corresponding optical element (26, 30) exits.

11. Device according to one of claims 8 to 10, further comprising: at least one measuring cell (10) for receiving the solution to be examined (12); at least one reference cell (14) for receiving the saturated solution (16); wherein the saturated solution (16) in the reference cell (14) communicates with the solution (12) to be tested in the measuring cell (10) via a filtering membrane (20) such that the filtering membrane (20) for the liquid phase the solution or suspension is substantially permeable and is substantially impermeable to the crystalline phase.

12. Device according to one of claims 8 to 11, wherein the light irradiation means comprise: at least one laser (34) for generating linearly polarized light, - at least one electro-optical modulator (38) for periodic

Modulation of the polarization of the linearly polarized light and at least one beam splitter (42) for splitting the polarized in its periodically modulated light in the first incident light beam (44) and the second incident light beam (46).

13. Device according to one of claims 8 to 12, wherein the first and / or second polarization detector comprises a first (50) and second analyzer (60) and a first (52) and second photodetector (62).

14. 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 in the saturated solution (16) is provided.