METHOD FOR DETERMINING THE TOC CONTENT OF AN AQUEOUS SOLUTION AND CORRESPONDING DEVICE

Description

[0001] The invention relates to a method for determining the TOC content of an aqueous solution after passing through an ion exchange demineralization line, comprising the steps: a) Measuring a first conductivity of the aqueous solution; b) Oxidizing organic carbon contained in the aqueous solution to CO2 and / or H+ and / or HCO3- and / or Na+ + HCO3-; c) Measuring a second conductivity of the aqueous solution; d) Determining the TOC content of the aqueous solution from the two measured conductivities.

[0002] The measurement of total organic carbon (TOC, here synonymous with dissolved organic carbon, DOC) always involves the conversion of organic carbon by oxidation to carbonic acid as a mixture of CO₂ or H⁺ + HCO₃⁻ or Na⁺ + HCO₃⁻. The oxidation to, and the detection of, the resulting carbonic acid can be carried out in various ways. The most accurate technique to date—and also the considerably most expensive—uses a spectroscopic detection of CO₂ in the infrared wavelength range. For this, the addition of analytically pure acid completely converts the carbonic acid produced by the oxidation into the CO₂ form, since HCO₃⁻ is not accessible via spectroscopic detection.

[0003] In contrast, the reverse approach is taken by measuring conductivity. Here, only HCO3-< in conjunction with H+< or Na+< is detectable. However, the application of the law of mass action (LMA) and the Henderson-Hasselbalch equation (HH equation) allows the conversion of the measured conductivity into both the HCO3-< / H+< fraction and the determination of the CO2 fraction, which is not measurable by conductivity, using these equations. The crucial prerequisite for this to work with sufficient accuracy is a very low overall conductivity (with reference to the application according to the invention, essentially due to additional Na+<) of at most approximately 2 µS / cm. This value corresponds to the typical manufacturer's specification for commercial TOC meters using the conductivity method. In practice, significant inaccuracies occur even at much lower conductivities than this, the extent of which depends on the pH value.After an ion exchange demineralization line (DE line), this value is usually above 8, sometimes up to 9 and sometimes even higher.

[0004] The following methods for determining TOC content are known from the prior art. The essential steps consist of oxidation, detection of the resulting carbonic acid, and suppression of zero signals or errors.

[0005] Firstly, catalytic oxidation in an oven is a well-known method. All TOC measurements utilize the oxidation of carbon to CO2. In expensive instruments, this is carried out in an oven heated to up to 1200 °C using catalysts. Under these conditions, the conversion is very complete, but this technique is also the most expensive. Furthermore, wet chemical oxidation is another known method. Wet chemical oxidation is often performed with persulfate. Here, the purity of the chemicals plays a crucial role, and the method also requires significant equipment. Its practical relevance for automated measurement systems is negligible anyway. Finally, oxidation by UV irradiation exists as another method. This is the most cost-effective method, which works by irradiating the sample water with UV light. Low-pressure UV lamps with strong emission lines at 256 nm and 185 nm are suitable for this purpose.This radiation forms OH- radicals in the water, which then carry out the actual oxidation of the carbon to CO2. This technique may not achieve complete oxidation, resulting in less precise measurements, but it is very inexpensive.

[0006] One method for detecting the carbonic acid produced is the spectroscopic one. In principle, CO₂ can be stripped from the sample after any oxidation process and detected using a spectrometer with an NDIR detector. This can achieve detection limits of just a few ppb TOC. While this is the most accurate, it is also the most complex method. Another method for detecting the carbonic acid produced is differential conductivity. Particularly during oxidation with a UV lamp, the conductivity additionally generated by HCO₃⁻ and the associated H⁺ ions is measured against the conductivity before the UV lamp. HCO₃⁻ exists in equilibrium with the electrically non-conductive CO₂, and this equilibrium can, in principle, be calculated. The accuracy of this calculation is very good if no other components besides carbonic acid are present in the water.This method is therefore mostly used in ultrapure water applications, where the conductivity before oxidation is typically less than 1 µS / cm. By calculating the difference with this conductivity before the UV cell, a certain influence of other ions can be compensated for, but the inaccuracy increases significantly with increasing conductivity before the UV cell. The effluent quality of ion exchange demineralization lines often shows values ​​up to approximately 5 µS / cm at the beginning and end of the loading process, which makes this measurement considerably more prone to error, since the strongest influence on the additional conductivity is due to the H+ ions, which are largely neutralized in the slightly alkaline effluent of the demineralization lines.

[0007] For the suppression of zero signals or errors, spectroscopic methods employ the separation of TIC and TOC in the sample. After oxidation, the previously present HCO₃⁻ / CO₂ (TIC, total inorganic carbon) can no longer be distinguished from the newly generated HCO₃⁻ / CO₂ from the TOC. Therefore, instruments with spectroscopic detection include an acidification stage prior to oxidation, with stripping of the CO₂ released during this process. The stripping air must be CO₂-free, and the acid must be of high purity. When measurements are taken in ultrapure water, the previously present TIC can usually be disregarded. Particularly in the effluent of a deionization line, very little carbonic acid (TIC) is present, as Na⁺ and OH⁻ ions typically dominate. In this application, suppression of TIC present alongside TOC is therefore usually unnecessary.

[0008] In contrast to devices using spectroscopic detection, foreign ions outside the TIC generate an interference signal in the differential conductivity method. For example, Na+ ions and their associated OH- ions produce a noticeable conductivity even before the UV cell, typically ranging from 0.1 to 5 µS / cm. With conductivity increases of 1 to 2 µS / cm due to the UV lamp, it is clear that this influence cannot be neglected. This clearly illustrates the problem circumvented by the inventive method. The situation becomes even more complex because the equations that calculate the parallel but unmeasurable CO2 concentration from the only measurable HCO3- concentration become significantly inaccurate in the presence of OH- ions. Inaccuracies of several orders of magnitude are possible.From publication WO 2009 / 147511 A1, a method for analyzing the purity of water at the outlet of a purification device is known, in which a liquid at the outlet of a filter medium is passed through a resistance measuring cell to determine its specific resistance. Furthermore, a portion of the liquid is exposed to oxidizing agents for a specific number of distinctly different time periods, and the specific resistance of the liquid is determined in each case. The method also includes calculating the amount of organic compounds contained in the purified liquid based on the different measured specific resistances. Similar methods are also known from publications EP O 135 685 A2, US 6,444,474 B1, and DE 10 2019 124 843 A1.

[0009] There is therefore a need to reduce the interfering conductivity contributions without significantly affecting the organic content. This would allow the use of less expensive conductivity measurement techniques without having to resort to the considerably more expensive spectroscopic techniques.

[0010] The objective technical objective of the invention is therefore to improve a method or device for determining the TOC content of an aqueous solution in such a way that the determination of the TOC content is both less prone to error and inexpensive.

[0011] The problem is solved by a method and a device having the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0012] Accordingly, it is provided that before measuring the first conductivity, an exchange of inorganic cations contained in the aqueous solution, such as Na+, K+, etc., for H+ ions takes place, wherein the exchange includes converting the NaOH and / or NaHCO3 fraction of the aqueous solution into H2O and CO2 by means of a cation exchanger in the H+ form.

[0013] The present invention relates to a novel technology for the reliable yet cost-effective measurement of total organic carbon (TOC) content downstream of ion-exchange demineralization lines. The invention achieves this by removing the inorganic cations typically left behind by a demineralization line before measuring the differential conductivity. The invention thus overcomes the problem inherent in the otherwise cost-effective method of measuring differential conductivity using an oxidizing UV lamp: the conductivity upstream of the UV cell can be so high due to cation slippage in the demineralization line that the measurement and calculation methods for determining the carbon dioxide content downstream of the UV lamp yield erroneous TOC values.The inventive method uses a cation exchanger upstream of the UV oxidation cell to solve the problem by converting the interfering NaOH content in the feed water into water. This significantly reduces the error-prone nature of the formulaic evaluation of the conductivity difference while still allowing the use of cost-effective measurement technology. The core of the invention for measuring TOC content after demineralization lines or slightly alkaline and mineral acid-free sample streams thus consists of providing a method or arrangement comprising a cation exchanger, a conductivity measuring cell, a UV lamp in a flow-through cell, and a further conductivity measurement, together with an evaluation method that only needs to account for NaOH and / or NaHCO3 content with minute and, above all, easily calculable corrections.

[0014] The invention thus achieves, in the simplest way, sufficient accuracy or limit of detection for the evaluation of a demineralization line. Furthermore, it is transferable to all applications in which the TOC content is to be measured in a slightly alkaline environment. Therefore, the technology according to the invention represents a very advantageous improvement and further development for a large number of measurement methods in ion exchange demineralization plants and slightly alkaline solutions.

[0015] It may be provided that the Na+ ions in the aqueous solution are exchanged for H+ ions by means of the cation exchanger, which neutralizes excess OH- ions to form water.

[0016] For example, the first conductivity of the aqueous solution can be between ≤ 0.1 and 0.2 µS / cm. In contrast, the second conductivity of the aqueous solution (after the UV cell) can be between 0.5 and 2 µS / cm.

[0017] It may be intended that the aqueous solution after oxidation has concentrations of Na+ and / or OH- of less than 5 µmol / l each.

[0018] Furthermore, it may be provided that the cation exchanger is a strongly acidic or a weakly acidic cation exchanger.

[0019] For example, the oxidation of the organic carbon contained in the aqueous solution can be carried out by means of UV irradiation, whereby the UV irradiation can be carried out using at least one low-pressure UV lamp.

[0020] It may be possible to subtract the intrinsic conductivity of H₂O from the measurements of the first and second conductivities of the aqueous solution. Since this subtracts the intrinsic conductivity of water, which is 0.055 µS / cm (calculated with 10⁻⁷ mol / L H⁺ and 10⁻⁷ mol / L OH⁻ in pH-neutral water), typical conductivity components of 0.05 to 0.15 µS / cm remain for the interfering NaOH. These values ​​are then so small that they only produce false readings of 5 to 10 ppb TOC (with measured values ​​of, for example, 100 to 200 ppb TOC).

[0021] The procedure may further include: measuring the pH of the aqueous solution after oxidizing the organic carbon contained in the aqueous solution; calculating a theoretical pH from the measured second conductivity; averaging the measured pH and the calculated theoretical pH; and using the averaged pH to calculate the conductivity difference. By including the pH value, with its redundant capability of calculating the H⁺ concentration, the accuracy of the procedure can be further increased. To achieve even higher accuracy, pH measurements can also be added at both measurement positions, before and after oxidation, analogous to the above description.

[0022] Overall, this achieves a detection limit of around 10 ppb, which is perfectly adequate for evaluating a demineralization plant. The TOC output of such plants is usually between 20 and 300 ppb, which corresponds to the target measurement range of the method according to the invention. "Poor" plants usually show TOC values ​​above 200 ppb, so alarms in this measurement range have sufficient resolution. "Good" plants, on the other hand, operate with TOC values ​​below 100 ppb. After the interfering influence of NaOH is eliminated, the standard formulas for converting conductivity to HCO₃⁻ and, via the Henderson-Hasselbalch equation, the concentration of the corresponding CO₂ can then be calculated without having to accept significant inaccuracies.

[0023] The invention further relates to a device for determining the TOC content of an aqueous solution after it has passed through an ion exchange demineralization line, comprising at least one measuring device for measuring the conductivity of the aqueous solution, which has a first measuring position for measuring a first conductivity and a second measuring position for measuring a second conductivity of the aqueous solution, wherein the first measuring position is arranged upstream of the second measuring position, and with an oxidation device arranged between the measuring positions for oxidizing organic carbon contained in the aqueous solution to CO₂ and / or H⁺ and / or HCO₃⁻ and / or Na⁺ + HCO₃⁻, as well as with an evaluation unit for calculating the difference between the first and second measured conductivity and for determining the TOC content of the aqueous solution from the calculated conductivity difference.the device further comprises a cation exchanger in H+ form arranged upstream of the first measuring position for exchanging inorganic cations contained in the aqueous solution, such as Na+, K+, etc., for H+ ions.

[0024] It may be specified that the cation exchanger is a strongly acidic or a weakly acidic cation exchanger.

[0025] It is conceivable that the oxidation device includes at least one low-pressure UV lamp.

[0026] The device may further include a device arranged downstream of the oxidation device for measuring the pH value of the aqueous solution, wherein the evaluation unit may further be configured to calculate a theoretical pH value from the measured second conductivity, to form an average of the measured pH value and the calculated theoretical pH value, and to use the averaged pH value for calculating the conductivity difference.

[0027] Furthermore, the device may include an ion exchange demineralization line arranged upstream of the exchange device.

[0028] Further properties, advantages and features of the invention can be seen in the following description of preferred embodiments of the invention with reference to the accompanying drawings, which show: Fig. 1 a flowchart of a method of catalytic oxidation with spectroscopic detection; Fig. 2 a flowchart of a method of differential conductivity via UV lamp; Fig. 3 a flowchart of an embodiment of the method according to the invention; Fig. 4 a table with expected values ​​for the additional total carbon dioxide after oxidation by a UV cell; Fig. 5 a diagram showing the relationship between conductivity and pH value in three different ranges; Fig. 6 a diagram showing the extension of the evaluable conductivity range to < 1 µS / cm; Fig. 7 a diagram showing a non-recognizable relationship between conductivity and pH value without the use of a cation exchanger; Fig. 8 a diagram showing the clear relationship between conductivity and pH value with the use of a cation exchanger; Fig.Fig. 9 A diagram comparing a conductivity measured after UV oxidation without the use of a cation exchanger with conductivities measured by commercial measuring instruments; Fig. 10 A diagram comparing a conductivity measured by the method according to the invention with conductivities measured by commercial measuring instruments.

[0029] Fig. 1 Figure 1 shows a flowchart illustrating a complex, state-of-the-art process combining TIC stripping, catalytic oxidation, and spectroscopic detection. The sample first undergoes an acidification stage with stripping of the CO₂ released during this process. The stripping air must be free of CO₂, and the acid must be of high purity. The sample then passes through a catalyst-heated furnace at 1200 °C. Finally, the CO₂ content is determined using an IR spectrometer, and the TOC content is calculated from this.

[0030] Fig. 2 Figure 1 shows a flowchart for the simplest method, known from the prior art, for determining the TOC content, namely by determining the differential conductivity of aqueous solution 1 with intermediate oxidation using a UV lamp. First, a first conductivity LF1 of the sample is determined at a first measuring position C1 of a measuring device 4. The sample then passes through an oxidation unit 3, in which the organic carbon contained in aqueous solution 1 is oxidized to CO₂, H⁺, and HCO₃⁻ using a UV lamp. A second conductivity LF2 of the sample is then measured at a second measuring position C2 of the measuring device 4. Finally, the TOC content of the sample is determined from the difference between the measured conductivities LF1 and LF2.

[0031] Fig. 3 Figure 1 shows a flow diagram of an embodiment of the process according to the invention. The aqueous solution 1, in particular a slightly alkaline sample water, which may be, for example, the effluent from a strongly basic anion exchanger (SBA) of an ion exchange demineralization line and which at this point still contains undesirable proportions of NaOH and / or NaHCO₃, flows through a cation exchanger 2 in hydrogen form. According to the invention, this can be either a strongly acidic (SAC) or a weakly acidic (WAC) cation exchanger. Through the cation exchanger 2, all inorganic cations, such as Na⁺, K⁺, etc., are exchanged for H⁺ ions. These, in turn, neutralize the OH⁻ ions contained in the slightly alkaline effluents from demineralization lines.The fact that the released H+ ions are removed from the ion exchange equilibrium by neutralization allows for almost complete uptake of the Na+ ions with very low residual values ​​≤ 1 µmol / l. This neutralization also enables the use of both SAC and WAC resin types. The aqueous solution then proceeds as described in [reference missing]. Fig. 2 Two consecutive conductivity measurements are shown at measuring positions C1 and C2 of a measuring device 4, with oxidation occurring between measurements using a UV lamp by an oxidation device 3. In the illustrated embodiment, a pH measurement is also performed at a third measuring position Q2 at the level of the second measuring position C2 using a pH measuring device 6. Finally, the TOC content is determined in an evaluation unit 5 from the measured conductivities LF1 and LF2 and the measured pH value, as described below. The typically measured residual conductivity LF1 at measuring position C1 is approximately 0.06 to 0.2 µS / cm, primarily 0.07 to 0.15 µS / cm, which, compared to measured values ​​of 0.5 to 2 µS / cm using the prior art method of UV oxidation without prior cation exchange, represents only a minor interference.The test measurements showed that the pH values ​​measured in the aqueous solution after UV oxidation correspond relatively well with the pH values ​​theoretically calculated from the conductivity, within the achievable pH calibration accuracy of ±0.05. This correlation allows for further correction calculations by averaging the two pH determination methods to increase the accuracy of the H+ concentration calculation. The pH measurement can thus be used to further improve the determination of the conductivity difference, although the result is already very accurate even without the additional pH measurement.

[0032] The UV lamp of oxidation unit 3 is an 11 W low-pressure lamp with strong emission lines at 254 and 185 nm. These two lines, in particular, generate OH radicals in the water, which then cause the actual oxidation reaction.

[0033] Due to the inventive design of the measuring setup with a cation exchanger, the evaluation formula can be described to a good approximation by stating that the aqueous solution 1, after passing through UV oxidation, is exposed to the in Fig. 3 The measurement positions C2 and Q2 shown contain the following components: HCO3- (with a specific conductivity contribution of approx. 36 (µS / cm) / (mmol / l)) H+ (with a specific conductivity contribution of 336 (µS / cm) / (mmol / l)) Na+ (concentration usually < < 5 µmol / l, specific conductivity 45 (µS / cm) / (mmol / l)) OH- (concentration usually << 5 µmol / l, specific conductivity ≈200 (µS / cm) / (mmol / l)) CO2 (no conductivity contribution)

[0034] The invention may include the following calculation steps: First, the millimolar concentration of Na⁺ and OH⁻ before the UV cell is determined by dividing the conductivity C1 by the sum of the individual conductivities of Na⁺ and OH⁻ (45 + 200) µS / cm / (meq / l). This leaves a conductivity contribution for H⁺ + HCO₃⁻ of (336 + 36) µS / cm / (mmol / l), from which the measured conductivity C2 after the UV cell, reduced by 0.055 µS / cm, can be linearly converted into the millimolar HCO₃⁻ concentration. This millimolar concentration is the same as for H⁺, so the theoretical pH value can be easily calculated. It can then be compared with the measured pH value. In practice, this means that two diverse yet redundant measurement methods for the H⁺ concentration can be used. Finally, the quotient CO2 / HCO3 can be calculated using the Henderson-Hasselbalch equation (HH equation) from the pH value determined from the conductivity, the measured value, or an average of both.The total carbon dioxide is then the sum of HCO₃⁻ (from the conductivity) and CO₂ (from the HH equation). The total carbon dioxide in mmol / L is then converted to the TOC value in µg / L = ppb by multiplying by the molar mass of carbon (12000 µg / mmol).

[0035] This calculation method corresponds to the one in Fig. 4 The values ​​shown in the columns are viewed from right to left. The expected values ​​for the carbonic acid additionally produced by TOC oxidation and the measurable values ​​for conductivity and pH after passing through the UV cell are shown for the target measurement range ends of 10 and 1000 ppb. The smallest and largest measured values ​​for LF H+HCO3 (which corresponds to the conductivity LF2 at measurement position C2 after UV oxidation) listed on the right of this table are indeed of a magnitude that directly corresponds to the measured values ​​in the outflow of a demineralization line. This illustrates the fundamental problem of conductivity-based TOC measurement technology, which can, however, be effectively solved by the method according to the invention.The validity of calculating the non-measurable CO₂ content via the pH value from the measurable HCO₃⁻ content can be verified by checking the agreement between the theoretical pH value (calculated from the conductivity) and the measured pH value (which is shown in the . Fig. 3 (as shown as an optional but not necessary addition according to the invention). This error analysis is described in the following section as proof of the effectiveness of the invention.

[0036] The relationship between conductivity and pH value for acids follows a simple logarithmic law, which is expressed in Fig. 5 This diagram shows various measurement points of pH values ​​measured in a flow through an anion exchanger. The diagram covers a large range of NaOH and acids passing through a demineralization line. In the diagram of the Fig. 5 Three areas are discernible: On the left, represented by triangles, is the range of validity for the TOC calculation without the cation exchanger according to the invention, which is significantly distorted by NaOH. As can be seen, the triangles deviate considerably upwards from the regression line and interfere with the calculation method. In the middle area, indicated by black "+" signs, the lowest conductivity that can still be converted correctly is approximately 3 µS / cm, i.e., in the area of ​​the black "+" signs that do not yet deviate upwards from the line. This value would correspond to approximately 2000 ppb TOC, which is therefore a completely unusable range for the application according to the invention.It is readily apparent, however, that the reduction of conductivity interference components to approximately 0.1 µS / cm, as achieved according to the invention, would extend the linearly evaluable range of the black "+" signs, as defined by the intended measurement method, to well below 1 µS / cm, as illustrated by the aforementioned regression line extended to the left. The right-hand gray portion is no longer relevant to the present problem, as it only indicates the region dominated by mineral acids, in which carbonic acid can therefore no longer be measured. It is evident that a distinction between carbonic acid and mineral acid, which would only be possible via the small difference in the additive component (easily discernible by the slight offset of the two regression lines in the region of approximately 20 µS / cm), is not feasible given the existing measurement inaccuracies.It must therefore be assumed that the sample solution contains no mineral acids, which is generally the case for the processes of deionized water lines. In the diagram of the... Fig. 5 The system was significantly overdriven beyond its capacity limit to determine the data points of the anion exchangers, and in doing so, did not produce these mineral acid effluents as intended.

[0037] The success of the upstream cation exchanger (pre-CAT) 2 is shown in the diagram in Fig. 6 The diagram shows which different measurement points are determined by varying TOC concentrations measured behind the UV cell or oxidation unit 3 at measurement positions Q2 and C2. The very low conductivity measurement range on the x-axis of the diagram is particularly noteworthy.

[0038] It is very clearly evident how well the theoretical relationship between conductivity and pH value still works even at < 1 µS / cm, and how effectively the influence of NaOH in the original sample is suppressed by the inventive pre-KAT 2. It should be noted that a linear x-axis was even chosen here, which shows deviations even more clearly, but in contrast to Fig. 5 a curved logarithmic adjustment curve is produced. This graphic thus clearly shows that the inventive pre-CAT 2 deviates from the prior art diagram in Fig. 5 In fact, as expected, it extends to much lower conductivities and can thus achieve the desired detection limits for TOC.

[0039] Practical measurements show further significant improvements in the evaluability of the conductivity measurements after the UV cell or the oxidation device 3 due to the cation exchanger 2 arranged in front of the UV cell 3 according to the invention. Fig. 7 displays measurement points in a diagram in the same way as in Fig. 5 or Fig. 6 ...without the use of the upstream cation exchanger 2. The measurements were all taken from the slightly alkaline effluent of a demineralization line, which slips slightly over Na, and after the UV cell at the second measuring position C2. However, the anion exchanger of the demineralization line was not used – as in the Fig. 5 and Fig. 6 - not driven over beyond the acid breakthrough, but was still within the optimal working range before the loading end of the VE line. It is very clear that, according to the correlation from the Fig. 5 and 6The expected line of data points is not at all discernible. In fact, a trend with a positive slope is more likely, indicating that residual NaOH is still present. The pH values ​​are correspondingly high in this area. However, it is evident that the diagram displays a set of data points that cannot be evaluated in this form.

[0040] In a further test run, the cation exchanger according to the invention was then used and the same plot of pH value against conductivity was shown at the measuring positions C2 and Q2, respectively, as before. Fig. 8 This shows that at first glance, it is clear how the points now correspond exactly to the theoretical straight line for carbonic acid, which is illustrated by the black line with the theoretical conductivity contributions for H+ and HCO3-.

[0041] The comparison between the diagrams of Fig. 7 and the Fig. 8 This clearly illustrates once again how the cation exchanger according to the invention makes known theoretical formulas usable for evaluating conductivity measurements without empirical corrections. A significant advantage of the invention is that this TOC measurement requires no calibration with TOC standard solutions. The conductivity measurements can be adjusted using simple means. No further calibration or adjustment is necessary, since, as explained above, everything can be calculated using known theoretical relationships and formulas. In contrast, the analytical instruments used for comparison always require expensive calibration solutions with defined TOC concentrations for regular adjustment.

[0042] A diagram with TOC curves without the use of the cation exchanger according to the invention is shown in Fig. 9 shown. In addition to the UV oxidation cell 3 described here, with conductivity measurement at measuring position C2 without a preceding cation exchanger, two commercial analyzers were also used, which, according to their measuring principle, in Fig. 9 are named. However, both commercial analytical instruments required complex calibration using calibration solutions. It is very clear that while there is a large-scale correspondence between the black curve (without the cation exchanger according to the invention) and the two gray reference curves, there is no correspondence to the finer variations of the reference curves.

[0043] The in Fig. 10 The depicted set of curves shows, as the black curve, the measurement result for the setup according to the invention with cation exchanger 2 and downstream UV cell 3, followed by a conductivity measurement at measurement position C2. The two gray curves again show the measurement data of the commercial reference devices. Fig. 10 is opposite Fig. 9 At first glance, the very clear improvement in the correspondence of the curves is readily apparent. It is even evident that the deviation of the black curve relative to the gray curves is of a similar order of magnitude to the deviations between the two gray reference curves. This means that, due to the improvement described here, the measuring arrangement according to the invention can compete qualitatively with commercially available measuring devices that are many times more expensive. Reference symbol list

[0044] 1 Aqueous solution 2 Exchange device 3 Oxidation device 4 Measuring device 5 Evaluation unit 6 Device for measuring pH value C1 First measuring position C2 Second measuring position LF1 First conductivity LF2 Second conductivity Q2 Third measuring position

Claims

1. Method for determining the TOC content of an aqueous solution (1) after passing through an ion exchanger complete desalination line, comprising the steps: a) Measuring a first conductivity (LF1) of the aqueous solution (1); b) Oxidizing organic carbon contained in the aqueous solution (1) to CO2 and / or H+ and / or HCO3- and / or Na+ + HCO3-; c) Measuring a second conductivity (LF2) of the aqueous solution (1); d) Determining the TOC content of the aqueous solution from the two measured conductivities (LF1, LF2), characterized in that prior to measuring the first conductivity (LF1), exchanging inorganic cations contained in the aqueous solution, such as Na+, K+, etc., for H+ ions takes place, wherein the exchange includes conversion of the NaOH- and / or NaHCO3 portion of the aqueous solution (1) into H2O and CO2 by means of a cation exchanger (2) in H+ form.

2. Method according to claim 1, wherein the cation exchanger (2) exchanges Na+ in the aqueous solution (1) for H+, which neutralizes excess OH- ions to water.

3. Method according to any one of the preceding claims, wherein the aqueous solution (1) after oxidation has concentrations of Na+ and / or OH- each lower than 5 µmol / l.

4. Method according to any one of claims 1 to 3, wherein the cation exchanger (2) is a strongly acidic or a weakly acidic cation exchanger.

5. Method according to any one of the preceding claims, wherein the oxidation of the organic carbon contained in the aqueous solution (1) is performed by means of UV irradiation.

6. Method according to claim 5, wherein the UV irradiation is performed by at least one low-pressure UV lamp.

7. Method according to any one of the preceding claims, wherein during the measurement of the first and second conductivity (LF1, LF2) of the aqueous solution (1), the intrinsic conductivity of H2O is subtracted.

8. Method according to any one of the preceding claims, additionally comprising measuring the pH value of the aqueous solution (1) after oxidizing the organic carbon contained in the aqueous solution (1); calculating a theoretical pH value from the measured second conductivity (LF2); forming an average from the measured pH value and the calculated theoretical pH value; using the averaged pH value to calculate the conductivity difference.

9. Apparatus for determining the TOC content of an aqueous solution after passing through an ion exchanger complete desalination line, comprising at least one measuring device (4) for measuring the conductivity of the aqueous solution (1), which has a first measuring position (C1) for measuring a first conductivity (LF1) and a second measuring position (C2) for measuring a second conductivity (LF2) of the aqueous solution (1), wherein the first measuring position (C1) is arranged upstream of the second measuring position (C2), and an oxidation device (3) arranged between the measuring positions (C1, C2), which preferably comprises at least one low-pressure UV lamp, for oxidizing organic carbon contained in the aqueous solution (1) to CO2 and / or H+ and / or HCO3~ and / or Na+ + HCO3¯, and an evaluation unit (5) for calculating the difference between the first and second measured conductivities (C1, C2) and for determining the TOC content of the aqueous solution (1) from the calculated conductivity difference, characterized in that the apparatus further comprises a cation exchanger in H+ form (2) arranged upstream of the first measuring position (C1) for exchanging inorganic cations contained in the aqueous solution (1), such as Na+, K+, etc., for H+ ions, preferably the cation exchanger is a strongly acidic or a weakly acidic cation exchanger.

10. Apparatus according to claim 9, further comprising a device (6) arranged downstream of the oxidation device (3) for measuring the pH value of the aqueous solution (1), wherein the evaluation unit (5) is further configured to calculate a theoretical pH value from the measured second conductivity (C2), to form an average from the measured pH value and the calculated theoretical pH value, and to use the averaged pH value for calculating the conductivity difference.

11. Apparatus according to any of claims 9 or 10, further comprising an ion exchange complete desalination stage arranged upstream of the exchange device (2)