WO2021115616A1

WO2021115616A1 - Clo2 measurement method for acidified sodium chlorite

Info

Publication number: WO2021115616A1
Application number: PCT/EP2019/085085
Authority: WO
Inventors: Daniel Alexander SIEBOLD; Michael Schneider
Original assignee: Ecolab Usa Inc.
Priority date: 2019-12-13
Filing date: 2019-12-13
Publication date: 2021-06-17
Also published as: WO2021115616A9

Abstract

The invention relates to a method for determining the concentration of chlorine dioxide in an aqueous solution using titration.

Description

CIO2 Measurement Method for Acidified Sodium Chlorite The invention relates to a method for determining the concentration of chlorine dioxide in an aqueous solution. Chlorine dioxide (ClO₂) has diverse applications, being used for instance in the electronics industry to clean circuit boards, in the oil industry to treat sulphides, and to bleach textile, candles, and paper. Importantly, chlorine dioxide can also be used in the sterilization of medical and laboratory equipment, surfaces, rooms and tools, and in the disinfection of biological tissue, as chlorine dioxide is highly water-soluble and effective even at low concentrations. Further, chlorine dioxide does not hydrolyse when it enters water; it remains a dissolved gas in solution. It also has a well- documented potent biocidal activity, deactivating even chlorine-resistant pathogens, such as Giardia and Cryptosporidium. To be effective as a disinfectant, chlorine dioxide needs to be applied at a sufficient concentration to achieve the required biocidal activity; whilst, at the same time, a high level of protection for humans and the environment needs to be ensured. To this end, the chlorine dioxide concentration contained in a biocidal product has to be precisely determined. For measuring the concentration of chlorine dioxide in an aqueous solution various issues need to be considered. Chlorine dioxide can decompose into chlorine and oxygen. This decomposition is characterized by a slow induction period followed by a rapid autocatalytic phase that may be explosive if the initial concentration is above a partial pressure of 10.1 kPa. It is therefore dangerous to transport; hence, the chlorine dioxide solution is usually produced on site. This can be achieved either by mixing a two or more components of a product or via an electrochemical device. Some biocidal products continuously generate chlorine dioxide, meaning all concentration measurements are affected by ongoing generation. Thus the process of measuring the concentration needs to be relatively fast. It additionally needs to be considered that chlorine dioxide is a volatile gas and evaporation should be avoided when analysing its concentration. Furthermore, biocidal products containing chlorine dioxide, such as udder care products, can be colorized and/or thickened, which can influence the determination of the chlorine dioxide concentration. This leads to the problem of precisely determining the chlorine dioxide concentration in aqueous solutions, in particular in solutions which are colorized and/or thickened, and in which chlorine dioxide is continuously generated. Mixtures of a two component system that continuously generates chlorine dioxide are often called “activated” which means both components of the product are mixed and continuously generate chlorine dioxide. Chlorine dioxide concentration over time is depending on a lot of factors, for example on the concentration of chlorine dioxide-generating components, on the temperature, the presence of dyes, and on the presence of other ingredients of the formulation. So far, known methods for determining the concentration of chlorine dioxide are: • Spectroscopic o Lovibond / Derivatisation via Tracer o UV-VIS / Direct measurement of ClO2 • Deairation of ClO2 from solution and trap via KI solution • Titration o Amperometric / (Two step) titration with phenylarsenic oxide and iodine o Redox / Titration with thiosulfate and iodine For instance when employing the bubbling-method, air is pushing chlorine dioxide, evaporated from a chlorine dioxide containing solution, towards a wash bottle which holds a trapping solution containing buffered potassium iodide solution. The chlorine dioxide reacts with the potassium iodide whereby iodine is liberated. A sample of the trapping solution can be titrated with thiosulfate for determining the amount of iodine. By noting the amount of thiosulfate used, the initial amount of the trapping solution and the amount of the titrated sample, the concentration of the chlorine dioxide of the solution can be determined. Nevertheless, not all of the chlorine dioxide will evaporate. For that reason, this method is inadequate to determine the precise chlorine dioxide concentration. In contrast, the UV-VIS spectrometer has, for example, the disadvantage that the solutions containing chlorine dioxide have to be colourless. The prior art methods known to us are not optimized for addressing the aforementioned difficulties when determining the concentration of chlorine dioxide in an aqueous solution, especially when analysing a colorized and/or thickened solution. It is therefore an object of this invention to provide by an improved method for determining the concentration of chlorine dioxide in an aqueous solution. This object is solved by the present invention, namely a method for determining the concentration of chlorine dioxide in an aqueous solution comprising the following subsequent steps: a. providing a sample of an aqueous solution, the aqueous solution containing chlorine dioxide; providing an aqueous buffer solution with a pH of from 5 to 9; and providing an oxidizing agent; b. adding the sample of an aqueous solution containing chlorine dioxide to the aqueous buffer solution to obtain a buffered sample, then adding the oxidizing agent to the buffered sample to obtain a titratable aqueous solution; or adding the oxidizing agent to the aqueous buffer solution to obtain a buffered solution containing the oxidizing agent, then adding the sample of an aqueous solution containing chlorine dioxide to the buffered solution containing the oxidizing agent to obtain a titratable aqueous solution; c. titrating any liberated oxidizing agent in the titratable aqueous solution by adding to the titratable aqueous solution a reducing agent; d. detecting the endpoint of titration. In principle, chlorine species can be titrated by adding an oxidizing agent and choosing a specific pH range. It has been found that it is beneficial for the measurement of the concentration of chlorine dioxide to have the sample containing chlorine dioxide buffered to avoid undesired reactions of the chlorine dioxide, of the oxidizing agent or of residual chlorine dioxide-generating compounds, such as chlorite. The following reactions of chlorine species with iodide as an oxidising agent occur at the pH of 0.1 to 9. (pH 7, pH 2, pH < 0.1) (pH 7) (pH 2, pH < 0.1) (pH 2, pH < 0.1)

(pH < 0.1) Equation 1: Chlorine species reactions with iodide at various pH The chlorine dioxide reacts with iodide, at a pH of from 5 to 9, resulting in liberated iodine. The liberated iodine can be titrated, for example with thiosulfate or phenylarsenic oxide. I₂ + 2 Na₂S₂O₃ ⇌ 2 Na⁺ + 2 I- + Na₂S₄O₆ Equation 2: Iodine and thiosulfate reaction I2 + C6H5AsO + 2 H2O ⇌ 2 H⁺ + 2 I- + C6H5AsO(OH)2 Equation 3: Iodine and phenylarsenic oxide reaction These titrations can be done manually using a starch indicator for end point detection. More accurate are amperometric methods; when all of the oxidizing agent is consumed, the potential changes rapidly as the overall potential is based on a logarithmic function.

Equation 4: Nernst Potential Figure 1 shows potential change and inflection point It has been found that the titration of a pure chlorine dioxide solution is not affected by a different order of adding the oxidizing agent. For measuring the concentration of pure chlorine dioxide, it appears to be irrelevant whether first the sample of an aqueous solution containing chlorine dioxide is added to the aqueous buffer solution to obtain a buffered sample, before adding the oxidizing agent to the buffered sample to obtain a titratable aqueous solution; or whether first the oxidizing agent is added to the aqueous buffer solution to obtain a buffered solution containing the oxidizing agent, and subsequently adding the sample of an aqueous solution containing chlorine dioxide to the buffered solution containing the oxidizing agent to obtain a titratable aqueous solution. In the method step (b), the sample of an aqueous solution containing chlorine dioxide is preferably added to the aqueous buffer solution to obtain a buffered sample, then the oxidizing agent may be added to the buffered sample to obtain a titratable aqueous solution. As soon as residual chlorite is present in the aqueous solution containing chlorine dioxide, this particular sequence of addition is advantageous in order to get a precise measurement of the chlorine dioxide concentration. The beneficial effect appears to be strongest when thickened solutions containing chlorine dioxide are analyzed. One possible explanation for this observation is that by adding the thickened sample of an aqueous solution containing chlorine dioxide to the aqueous buffer solution already containing an oxidizing agent, the oxidizing agent reacts quickly with the chlorine dioxide, whilst the chlorine dioxide is not yet fully buffered to the optimal pH. This can result in undesired reactions at low pH, leading to a less accurate determination of the chlorine dioxide concentration. Preferably, the sample in step (a) has a temperature of from 1 °C to 40 °C, more preferably a temperature of from 15 °C to 25 °C, and the titration in step (c) is preferably carried out at a temperature of from 1 °C to 40 °C, more preferably at a temperature of from 15 °C to 25 °C. In the method step (a), the aqueous solution containing chlorine dioxide can be a coloured aqueous solution containing a dye with the specific molar absorption in the spectrum of 320-800 nm. In the method step (a), the aqueous buffer solution preferably has a pH of from 6 to 8, more preferably of from 6.5 to 7.5. The pH range can be important to force the reaction of chlorine dioxide with the oxidation agent and to suppress side reactions with other chlorine species, such as chlorite, with the oxidation agent. At a higher pH, the chlorine dioxide would convert to chlorite, while at lower pH, said undesired reactions of other chlorine species could occur. In the method step (a), the aqueous solution containing chlorine dioxide preferably has a Brookfield viscosity of 0 Pa*s to 3000 Pa*s at a temperature of 20 °C, more preferably a viscosity of 10 Pa*s to 3000 Pa*s at a temperature of 20 °C, even more preferably a viscosity of 100 Pa*s to 3000 Pa*s at a temperature of 20 °C, most preferably a viscosity of 1000 Pa*s to 3000 Pa*s at a temperature of 20 °C, measured using a Spindle 2 or 3 at 12-60 rpm. In the method step (a), the aqueous buffer solution can be diluted before the sample of the aqueous solution containing chlorine dioxide and/or the oxidizing agent is added in step (b). Chlorine dioxide in the aqueous solution containing chlorine dioxide can be generated in various ways. For example, solutions can be used that comprise a chlorine dioxide source. A chlorine dioxide source refers to chlorine dioxide, chlorine dioxide-generating components, and combinations thereof. Chlorine dioxide- generating components refer to at least an oxy-chlorine anion source and an activator of chlorine dioxide generation. Preferably, the activator is an acid source. The components optionally further include a free halogen source. The free halogen source may be chlorine. Alternatively, the activator can be an energy-activatable catalyst. Further, the activator can be a dry or anhydrous polar material. Alternatively, the activator is an aqueous fluid such as water, saliva, mucus, and wound exudate, and/or water vapor. Oxy-chlorine anion sources generally include chlorites and chlorates. The oxy- chlorine anion source may be an alkali metal chlorite salt, an alkaline earth metal chlorite salt, an alkali metal chlorate salt, an alkaline earth metal chlorate salt and combinations of such salts. For example, the oxy-chlorine anion source is a metal chlorite. The metal chlorite can be an alkali metal chlorite, such as sodium chlorite and potassium chlorite. Alkaline earth metal chlorites can also be employed. Examples of alkaline earth metal chlorites include barium chlorite, calcium chlorite, and magnesium chlorite. An exemplary metal chlorite is sodium chlorite. For chlorine dioxide generation activated by an acid source, the acid source may include inorganic acid salts, salts comprising the anions of strong acids and cations of weak bases, acids that can liberate protons into solution when contacted with water, organic acids, inorganic acids, and mixtures thereof. In some aspects, the acid source is a particulate solid material which does not react substantially with the metal chlorite during dry storage, however, does react with the metal chlorite to form chlorine dioxide when in the presence of an aqueous medium. The acid source may be water soluble, substantially insoluble in water, or intermediate between the two. Exemplary acid sources are those which produce a pH of below 7, preferably below 6. Exemplary substantially water-soluble, acid-source-forming components include, but are not limited to, water-soluble solid acids such as boric acid, citric acid, tartaric acid, water soluble organic acid anhydrides such as maleic anhydride, and water soluble acid salts such as calcium chloride, magnesium chloride, magnesium nitrate, lithium chloride, magnesium sulfate, aluminium sulfate, sodium acid sulfate (NaHSO4), sodium dihydrogen phosphate (NaH2PO4), potassium acid sulfate (KHSO4), potassium dihydrogen phosphate (KH2PO4), and mixtures thereof. Exemplary acid-source-forming component is sodium acid sulfate (sodium bisulfate). Additional water-soluble, acid-source-forming components will be known to those skilled in the art. Chlorine dioxide-generating components optionally comprise a source of free halogen. Further, the free halogen source may a free chlorine source, and the free halogen is free chlorine. Suitable examples of free halogen source used in the anhydrous compositions include dichloroisocyanuric acid and salts thereof such as trichlorocyanuric acid, salts of hypochlorous acid such as sodium, potassium and calcium hypochlorite, and dibromodimethylhydantoin. For chlorine dioxide generation activated by an energy-activatable catalyst, the energy-activatable catalyst can be selected from the group consisting of a metal oxide, a metal sulfide, and a metal phosphide. Exemplary energy-activatable catalysts include metal oxides selected from the group consisting of titanium dioxide (TiO2); zinc oxide (ZnO); tungsten trioxide (WO3); ruthenium dioxide (RuO2); iridium dioxide (IrO2); tin dioxide (SnO2); strontium titanate (SrTiO3); barium titanate (BaTiO3); tantalum oxide (Ta2O5); calcium titanate (CaTiO3); iron (III) oxide (Fe2O3); molybdenum trioxide (MoO3); niobium pentoxide (NbO5); indium trioxide (In2O3); cadmium oxide (CdO); hafnium oxide (HfO2); zirconium oxide (ZrO2); manganese dioxide (MnO2); copper oxide (Cu2O); vanadium pentoxide (V2O5); chromium trioxide (CrO3); yttrium trioxide (YO3); silver oxide (Ag2O)3 TixZr1-x02, wherein x is between O and 1, and combinations thereof. The energy-activatable catalyst can be selected from the group consisting of titanium oxide, zinc oxide, calcium titanate, zirconium oxide and combinations thereof. The chlorine dioxide can be generated by acidifying chlorite with one or more acids, preferably by acidifying sodium chlorite with one or more acids from the group consisting of citric acid, lactic acid, glycolic acid and acetic acid. The ratio of acid to chlorite may be 1:1 to 100:1, preferably 1:1 to 60:1, more preferably 1:1 to 40:1. However, the chlorine dioxide can be generated electrochemically from sodium chlorite or the chlorine dioxide can be generated photochemically. Preferably, the aqueous buffer solution is based on potassium-di-hydrogenphosphate and di-sodium-hydrogenphosphate in a ratio of from 3:1 to 1:3 by weight, more preferably in a weight ratio of from 2:1 to 1:2.5, even more preferably in a weight ratio of from 1:1 to 1:2.5, and most preferably in a weight ratio of from 1:1.7 to 1:2.3. The aqueous buffer solution preferably has a concentration of from 1 wt.% to 10 wt.%, more preferably of from 1 wt.% to 7 wt.%, potassium-di-hydrogenphosphate and of from 1 wt.% to 30 wt.%, more preferably from 1 wt.% to 17 wt.%, di-sodium- hydrogenphosphate per liter. The oxidizing agent can be a solution containing the oxidizing agent. The solution containing the oxidizing agent can be an aqueous solution. Prior to adding, in method step (b), the oxidizing agent to the buffered sample or to the aqueous buffer solution, the oxidizing agent preferably has a concentration of from 1 to 20 wt.-%, more preferably of from 5 to 15 wt.-%. The oxidizing agent used in the method may be iodide or bromide. Preferably, the oxidizing agent is iodide. The reducing agent can be thiosulfate. Preferably, the reducing agent has a concentration of from 0.1 N to 0.0001 N, more preferably a concentration of from 0.01 N to 0.001 N. Alternatively, the reducing agent can be phenylarsenic oxide. Preferably, the reducing agent has a concentration of less than 0.0056 N.

Equation 5: Iodine titration with phenylarsenic oxide The detection of the endpoint of the titration may be done via an electrode or optical sensor. Preferably, the detection of the endpoint of the titration is done via a redox electrode. The generated chlorine dioxide preferably has a concentration of less than 3000 ppm, more preferably a concentration of less than 1000 ppm, even more preferably a concentration of less than 500 ppm, and most preferably a concentration of less than 250 ppm. The sample of an aqueous solution containing chlorine dioxide can preferably be added by pouring the sample under a pressure of 0.5 to 2 bar, more preferably under a standard atmospheric pressure at 1 bar. The sample of an aqueous solution containing chlorine dioxide can preferably be taken from an udder care product which continuously generates chlorine dioxide. In one aspect, the method further comprises the step of applying the aqueous solution containing chlorine dioxide to a surface for disinfection. In an alternative aspect, the method further comprises the step of applying the aqueous solution containing chlorine dioxide to another solution for disinfection. The chlorite can be titrated in parallel from the same sample, preferably by using thiosulfate, and preferably by using a second electrode, preferably a second redox- electrode. Meaning, the residual chlorite may be titrated in parallel to the titration of chlorine dioxide from the same aqueous solution containing chlorine dioxide. The residual chlorite can then be calculated from both titrations. Examples: All products are commercially available: Product A (source of lactic acid) Product B (source of sodium chlorite) Product C (source of sodium chlorite) Product D (source of hydrochloric acid) Example 1: The titration of a pure chlorine dioxide solution is not affected by the order of adding potassium iodide. If the sample is prepared like buffer + sample + KI or buffer + KI + sample, no difference is observed. But as soon as residual chlorite is present in the sample the preparation is important to get correct levels of chlorine dioxide. This effect is strongest when thickened product samples are analyzed. This effect was identified when samples thickened with approx.1% Hydroxyethyl Cellulose. To understand if concentration obtained by the claimed method were in the correct magnitude, the UV-VIS spectrometer method was used as comparison. For measurements with the UV-VIS spectrometer, the absorption at 445 nm was used. The analyzed samples were colorless, because the UV-VIS spectrometer cannot be employed with colored samples.

A: absorbance [a.u.] ε: molar absorption coefficient ε =0.0009135 (L·mg⁻¹·cm⁻¹) l: path length (= 1 cm) The sample used for determination was a 3.4% Citric Acid solution, thickened with 1% HEC and adjusted with KOH to pH 2.1. The sample was mixed with Product B in a ratio of 40:1. After mixing, the samples were stored at 20 °C and let stand for 15 min before further analysis. In parallel, a pure chlorine dioxide solution made from Product C + Product D was analyzed. Table 1: Titration vs. UV-VIS at thickened sample

The experimental data demonstrates that the UV-Vis and Titration measurements of the stock solution are in line (263 ppm vs 264 ppm), whereas the titration of the thickened sample leads to different results when changing the order of iodide addition (789 ppm vs 101 ppm). However, when adding the iodide after buffering the sample, the concentration (101 ppm) is close to the measured concentration of the UV-VIS measurement (71 ppm). Difference can still occur due to ongoing reaction and interference of thickener with UV-VIS. A possible explanation for those differences is that by adding the thickened sample to the Buffer/KI solution, the iodide reacts quickly with the sample whilst the sample is dissolving and yet not fully buffered. This can force the chlorine dioxide to react according to a different redox potential or even some sodium chlorite can contribute to the generation of iodine due to the undesired low pH. To illustrate this, thickened samples have been titrated without buffer, leading to values far above 1500 ppm, as all chlorine species are titrated. Example 2: Activation and storing of samples Activated samples of the aqueous solution containing chlorine dioxide need to be stored in brown glass vessel without degassing cap and stored at the desired temperature (e.g.20 °C, monitored and constant) till use. The size of the vessel should be so small, such the solution nearly fills all space within, leaving only a low headspace above the liquid, which still allows to thoroughly mix thickened products by shaking. Each sample can be used only for one measuring point. One sample is needed per product and passed time, e.g. for one product and 5 time points, 5 separate samples from the same batches stored at same conditions are required. Example 3: Selectivity Other substances contributing to the detection of iodine can be excluded either from theory, having no redox potential big enough, or by titrating the pure raw material in the same way as chlorine dioxide solutions. To exclude the influence of other ingredients on the chlorine dioxide measurements, Product A was added to a known solution of chlorine dioxide (made of a mixture of Product C + Product D to gain an approx.180 ppm chlorine dioxide solution and referred to as “stock solution”). The obtained solution (Product A + stock solution) and the stock solution are titrated for determining the concentration of chlorine dioxide. Table 2: Influence of Product A on the chlorine dioxide recovery

Product A does not influence the reading of chlorine dioxide. Example 4: Repeatability To show the basic and correct function of the method, a few samples of pure chlorine dioxide solution were titrated. This solution was made from Product C and Product B to gain an approx.180 ppm chlorine dioxide solution. Table 3: Samples of Stock solutions titrated at pH 7 within a short period of time

These results show that the method is repeatable. Example 5: Recovery Six samples of Product A were activated and stored at 20 °C for 15 min. Afterwards, the first 3 samples were measured for their chlorine dioxide concentration. The next three samples were measured in the same way except for the fact that a specific amount of chlorine dioxide is added prior the titration. 1. Activate samples and store at 20 °C for 15min 2. Briefly before measuring the 15 min samples, control the concentration of the chlorine dioxide solution 3. Measure 3 samples of the above mention activated product 4. Measure 3 samples of above activated product and add x amount of chlorine dilution solution The solution for titration was done in the following way: 75-100 g Buffer solution + 400-425 g deionized Water + X g of activated sample (recorded for calculation) + 5-10 g Iodide solution (10 wt.-%) Activated sample needs to be poured into the buffer solution, because using a pipette might force chlorine dioxide to evaporate and could give wrong results. Samples need to be dissolved by stirring and the iodide solution is added once sample (and stock solution) has been dissolved. The buffer solution is based on potassium-di- hydrogenphosphate and di-sodium-hydrogenphosphate. Table 4: Recovery experiment

Equation 6: Recovery calculation for activated chlorine dioxide solution Example 6: Linearity A chlorine dioxide made of Product C + Product D has been determined. From that solution several dilutions were made (gravimetric) and titrated again. The average chlorine dioxide result versus the percentage of the diluted solution should have a linear correlation. Table 5: Linearity of chlorine dioxide detection with diluting Stock solution

Figure 2 shows linearity of titration / regression based on avg. chlorine dioxide Example 7: Concentration measurement of chlorine dioxide 7.1 Solution for chlorine dioxide The solution for titration is done in the following way: 75-100 g Buffer solution + 400-425 g deionized Water + X g of activated sample (recorded for calculation) + 5-10 g Iodide solution (10 wt.-%) The activated sample was poured into the buffer solution as using a pipette might force ClO2 to evaporate. The sample was dissolved by stirring and the iodide solution was added once the sample has been dissolved. The buffer solution was based on potassium-di-hydrogenphosphate and di-sodium-hydrogenphosphate. 7.2 Solution for residual NaClO2 including ClO2 400-500 g deionized Water + about 20 g of sulfuric acid (25%) + 5-10 g Iodide solution (10 wt.-%) + X g of sample (recorded for calculation) 7.3 Titrator For the titration every device that is capable to automatically dose and measure the titrant volume will work. Here the following setup are used: • Titrando 906 • Dosino 800 • Tiamo Software V 2.5 • Pt-Titrode • Stirrer 801 7.4 Detection ranges with thiosulfate As we have ranges of 0-1000 ppm of chlorine dioxide, different concentrated thiosulfate solutions are required to get optimal results.

All those ranges show how the analyzed sample weight and the concentration of thiosulfate solution depend on the chlorine dioxide level. 7.5 Detection ranges with phenylarsenic oxide The same measurements as conducted for the determination of different concentrated thiosulfate solutions are performed for phenylarsenic oxide. Table 6: Detection range with phenylarsenic oxide in relation to ppm ClO2, sample size and titration volume

Example 8: Basic equations The following summarizes the equations at pH 7 when using thiosulfate or when using phenylarsenic oxide.

Equation 7: Calculations for chlorine dioxide and chlorite at pH 7 and when using thiosulfate

Equation 8: Calculations for chlorine dioxide and chlorite at pH 7 and when using phenylarsenic oxide

Equation 9: Calculations for chlorine dioxide and chlorite at pH 2 and when using thiosulfate Example 8: Comparison between the deairation-method vs. titration method When performing the bubble-method, a known amount of chlorine dioxide containing solution put into a bottle with a stir bar and the solution is continuously stirred. Air is then pushing the evaporated chlorine dioxide towards a wash bottle with a buffered potassium iodide solution. Chlorine dioxide will liberate iodine and a sample of the trapping solution is titrated with thiosulfate. By noting the product amount, the initial amount of the trapping solution and the sample titrated, the chlorine dioxide of the solution can be determined. However, not all chlorine dioxide will evaporate from the solution. Table 7: Comparison of titration and deairation

Claims

Claims 1. A method for determining the concentration of chlorine dioxide in an aqueous solution comprising the following subsequent steps: (a) providing a sample of an aqueous solution, the aqueous solution containing chlorine dioxide; providing an aqueous buffer solution with a pH of from 5 to 9; and providing an oxidizing agent; (b) adding the sample of an aqueous solution containing chlorine dioxide to the aqueous buffer solution to obtain a buffered sample, then adding the oxidizing agent to the buffered sample to obtain a titratable aqueous solution; or adding the oxidizing agent to the aqueous buffer solution to obtain a buffered solution containing the oxidizing agent, then adding the sample of an aqueous solution containing chlorine dioxide to the buffered solution containing the oxidizing agent to obtain a titratable aqueous solution; (c) titrating any liberated oxidizing agent in the titratable aqueous solution by adding to the titratable aqueous solution a reducing agent; (d) detecting the endpoint of titration.

2. The method according to claim 1, wherein in step (a) the sample has a temperature of from 1 °C to 40 °C, and wherein in step (c) the titration is carried out at a temperature of from 1 °C to 40 °C, preferably at a temperature of from 15 °C to 25 °C.

3. The method according to any of the preceding claims, wherein in step (a) the aqueous solution containing chlorine dioxide is a coloured aqueous solution containing a dye with the specific molar absorption in the spectrum of 320-800 nm.

4. The method according to any of the preceding claims, wherein in step (a) the aqueous buffer solution has a pH of from 6 to 8, preferably of from 6.5 to 7.5.

5. The method according to any of the preceding claims, wherein in step (a) the aqueous solution containing chlorine dioxide has a viscosity of 0 Pa*s to 3000 Pa*s at a temperature of 20 °C, measured using a Spindle 2 or 3 at 12-60 rpm.

6. The method according to any of the preceding claims, wherein in step (a) the aqueous buffer solution is diluted before the sample of the aqueous solution containing chlorine dioxide and/or the oxidizing agent is added in step (b).

7. The method according to any of the preceding claims, wherein the chlorine dioxide is generated by acidifying chlorite with one or more acids, preferably by acidifying sodium chlorite with one or more from the group consisting of citric acid, lactic acid, glycolic acid and acetic acid.

8. The method according to any of the preceding claims 1-6, wherein the chlorine dioxide is generated electrochemically from sodium chlorite.

9. The method according to any of the preceding claims, wherein the aqueous buffer solution is based on potassium-di-hydrogenphosphate and di-sodium- hydrogenphosphate in a ratio of from 3:1 to 1:3 by weight, preferably in a weight ratio of from 1:1.7 to 1:2.3.

10. The method according to claim 9, wherein the aqueous buffer solution has a concentration of from 1 wt.% to 10 wt.% potassium-di-hydrogenphosphate and of from 1 wt.% to 30 wt.% di-sodium-hydrogenphosphate per liter.

11. The method according to any of the preceding claims, wherein the oxidizing agent is a solution of the oxidizing agent.

12. The method according to claim 11, wherein the oxidizing agent has a concentration of from 1 to 20 wt.-%, preferably of from 5 to 15 wt.-% prior adding in step (b) the oxidizing agent to the buffered sample or to the aqueous buffer solution.

13. The method according to any of the preceding claims, wherein the oxidizing agent is iodide or bromide, preferably wherein the oxidizing agent is iodide.

14. The method according to any of the preceding claims, wherein the reducing agent is thiosulfate, preferably having a concentration of from 0.1 N to 0.0001 N, preferably of from 0.01 N to 0.001 N.

15. The method according to the claims 1-13, wherein the reducing agent is phenylarsenic oxide.

16. The method according to any of the preceding claims, wherein the detection of the endpoint is done via an electrode or optical sensor, preferably by a redox electrode.

17. The method according to any of the preceding claims, wherein the generated chlorine dioxide has a concentration of less than 3000 ppm, preferably of less than 1000 ppm, more preferably of less than 500 ppm, and even more preferably of less than 250 ppm.

18. The method according to any of the preceding claims, wherein in step (b) the sample of an aqueous solution containing chlorine dioxide is added by pouring the sample under a pressure of 0.5 to 2 bar, preferably under standard atmospheric pressure at 1 bar.

19. The method according to any of the preceding claims, wherein the sample of an aqueous solution containing chlorine dioxide is taken from an udder care product which is continuously generating chlorine dioxide.

20. The method according to any of the preceding claims, further comprising the step of applying the aqueous solution containing chlorine dioxide to a surface for disinfection.

21. The method according to any of the preceding claims, further comprising the step of applying the aqueous solution containing chlorine dioxide to another solution for disinfection.

22. The method according to any of the preceding claims, wherein residual chlorite is titrated in parallel from the same sample, preferably by using thiosulfate, and preferably by using a second electrode, preferably a second redox-electrode.