Electrochemical method for detecting and / or quantifying sulfur dioxide in a liquid

Abstract

An electrochemical process for detecting and / or quantifying sulfur dioxide (SO2) in a liquid, the process comprising a step, called the "current variation measurement step," consisting of measuring the change in current produced by the reduction of sulfur dioxide present in its free forms in the liquid at an acidic pH (pHA) less than 1.8, during a potential sweep performed by voltammetry, using a measuring electrode disposed in the liquid and whose active surface in contact with the liquid comprises a metal or a metal alloy. Figure for abbreviation: no figure

Description

Title of the invention: Electrochemical method for detecting and / or quantifying sulfur dioxide in a liquid. Technical field

[0001] The present invention relates to an electrochemical method for detecting and / or quantifying sulfur dioxide in a liquid, and an electrochemical device for detecting and / or quantifying sulfur dioxide in a liquid. Prior art

[0002] Sulphur dioxide is a widely used additive in oenology, as well as in the food industry in general and in the chemical industry.

[0003] In oenology and in the food industry in general, sulfur dioxide, also designated "E220" to "E228" by European regulations, is used mainly as a preservative, for example to prevent the browning of food and drinks, and thus preserve their freshness.

[0004] Indeed, sulfur dioxide has antimicrobial and antioxidant properties.

[0005] Sulfur dioxide can also be used to stop fermentation during the winemaking process.

[0006] For reasons including regulatory ones, it is necessary to have methods and devices for detecting and / or quantifying sulfur dioxide.

[0007] The detection and / or electrochemical quantification of sulfur dioxide, in particular in liquids, especially food liquids such as wine, has been reported on numerous occasions and using a wide variety of sensors or detection techniques.

[0008] However, all of these works very rarely meet the specifications for the quantification of sulfur dioxide, in particular in terms of performance and analytical conditions imposed by the agri-food industry, especially oenologists and the wine industry.

[0009] WO 2018 / 154226 A1 describes an electrochemical method for detecting and / or quantifying sulfur dioxide in its uncomplexed, so-called free forms (free SO2) in an aqueous or hydroalcoholic food liquid, such as wine. This method is based on measuring the change in current produced by the oxidation of free SO2 present in the food liquid during a potential sweep by cyclic voltammetry (also called cyclic voltammetry or CV).

[0010] There remains a need to further improve the performance of existing electrochemical methods and devices for the detection and / or quantification of sulfur dioxide, particularly in terms of sensitivity, selectivity, robustness, reproducibility and ease of implementation. Description of the invention

[0011] The invention aims to meet this need, and thus has as its object, according to a first of its aspects, an electrochemical process for detecting and / or quantifying sulfur dioxide (SO2) in a liquid, the process comprising a step, called "current variation measurement step", consisting of measuring the variation of the current produced by the reduction of sulfur dioxide present in its free forms in the liquid at an acidic pH pHA less than 1.8, during a potential sweep carried out by voltammetry, using a measuring electrode disposed in the liquid and whose active surface in contact with the liquid comprises a metal or a metal alloy.

[0012] The liquid may comprise several phases, including immiscible phases.

[0013] Sulfur dioxide in solution may be present mainly in the following free forms, generally called "sulfites": 1. H2SO3(aq) (also called "sulfurous acid" or "sulfur dioxide in solution" or "molecular sulfur dioxide") which results from the equilibrium SO2(g) + H2O(1) H2SO3(aq), 2. HSO3⁻(aq) (also called the "hydrogen sulfite" or "bisulfite" anion) which results from the equilibrium H₂SO₃(aq) ⇌ H⁺(aq) + HSO₃⁻(aq) with a pKa H₂SO₃(aq) / HSO₃⁻(aq) = 1.8, and 3. SO32'(aq) (also called the "sulfite" anion) which results from the equilibrium HSO3'(aq) H+(aq) + S032'(aq) with a pKa HSO3-(aq) / SO32'(aq) = 7.21.

[0014] In solution, an equilibrium is established between these three free forms. This equilibrium depends on the pH of the solution.

[0015] In the present invention, "electrochemical process for detecting and / or quantifying sulfur dioxide (SO2) in a liquid" means an electrochemical process for detecting and / or quantifying all free forms of sulfur dioxide (SO2) in solution that are present in the liquid at a given pH.

[0016] The step of measuring the variation of the current is implemented at an acidic pH pHA which is less than 1.8, i.e. less than the pKa H2SO3(aq) / HSO3'(aq).

[0017] At such a pH value, the predominant free form of sulfur dioxide in the liquid is H2SO3(aq).

[0018] Indeed, at such a pH value, the proportion of the H2SO3(aq) form is greater than 50%, while the proportion of the HSOs'(aq) form is less than 50%, the proportion of the SOs2'(aq) form being negligible.

[0019] However, in comparison with the forms HSOs'(aq) and SO32'(aq), the form H2SO3(aq) is the one which gives the most intense response in voltammetry.

[0020] Thus, it is particularly advantageous to implement the step of measuring the variation of the current produced by the reduction of sulfur dioxide present in its free forms in the liquid at an acidic pH pHA less than 1.8, so as to obtain H2SO3(aq) as the predominant free form of sulfur dioxide in the liquid and thus increase the sensitivity of the measurement.

[0021] The measurement of the variation of the current produced by the reduction of sulfur dioxide present in its free forms in the liquid is all the more sensitive as the proportion of the form H2SO3(aq) is high in the liquid, compared to the forms HSOs'(aq) and SOs2'(aq).

[0022] Thus, the acidic pHA can be less than or equal to 1.5, preferably less than or equal to 1.4, more preferably less than or equal to 1.3, and even more preferably less than or equal to 1.2.

[0023] For example, the acidic pH pHA is less than or equal to 1.1, or even less than or equal to 1.

[0024] In one embodiment, the acidic pHA is between 0.5 and 1.5, preferably between 0.5 and 1.4, more preferably between 0.5 and 1.3, and even more preferably between 0.5 and 1.2, or even between 0.5 and 1.1.

[0025] For example, the acidic pH pHA is between 0.6 and 1.1, preferably between 0.7 and 1.1, more preferably between 0.8 and 1.1, and even more preferably between 0.9 and 1.1.

[0026] In one embodiment, the acidic pH pHA is approximately equal to 1. Indeed, at such a pH value, the proportion of the H2SO3(aq) form is greater than or equal to 90%, while the proportion of the HSOs'(aq) form is less than or equal to 10%, the proportion of the SOs2'(aq) form being negligible.

[0027] In the process according to the invention, the variation of the current produced by the reduction of sulfur dioxide present in its free forms in the liquid is measured, and not the variation of the current produced by its oxidation.

[0028] This is particularly advantageous insofar as such a measurement by electroreduction makes it possible to overcome certain analytical interfering factors, in particular those present in wine, such as sugars, phenolic acids, flavonoids, etc.

[0029] Indeed, most of these interfering substances are oxidizable compounds but are not or are very difficult to reduce, so that they do not interfere in the measurement by electro-reduction.

[0030] The term "active surface of the measuring electrode" refers to the portion of the measuring electrode that is actually available for faradaic electrochemical reactions. In other words, it is the portion of the measuring electrode across whose surface electrons can be transferred between the measuring electrode and the sulfur dioxide present in its free forms in the liquid.

[0031] In one embodiment, the potential sweep is performed by pulsed differential voltammetry (also called DPV for Differential Pulse Voltammetry in English).

[0032] Compared with other types of voltammetry, such as cyclic voltammetry, the use of pulsed differential voltammetry can increase the sensitivity of the measurement and thus lower the detection limit of the process according to the invention.

[0033] In one embodiment, the potential sweep is carried out in the direction of decreasing potentials, preferably between 0 V and -0.7 V, more preferably between -0.1 V and -0.6 V, even more preferably between -0.1 V and -0.5 V, using an Ag / AgCl, 3M KC1 reference electrode.

[0034] In one embodiment, the potential sweep is carried out at speeds between 1 mV.s 1 and 1000 mV.s *, and preferably between 10 mV.s 1 and 100 mV.s1.

[0035] In one embodiment, the active surface in contact with the liquid of the measuring electrode is made of a metal or a metal alloy.

[0036] In one embodiment, the active surface in contact with the liquid of the measuring electrode comprises a transition metal or an alloy of transition metals.

[0037] In one embodiment, the active surface in contact with the liquid of the measuring electrode is made of a transition metal or an alloy of transition metals.

[0038] In one embodiment, the active surface in contact with the liquid of the measuring electrode comprises gold, silver, copper or one of their alloys, preferably gold, copper or one of their alloys, more preferably gold.

[0039] In one embodiment, the active surface in contact with the liquid of the measuring electrode is made of gold, silver, copper or one of their alloys, preferably of gold, copper or one of their alloys, more preferably of gold.

[0040] The measuring electrode may be a conventional electrode, in particular made of gold or copper, in particular flat or cylindrical in shape.

[0041] The measuring electrode can be a flat electrode, in particular with a disc shape, a rectangular shape or a square shape; a spherical electrode; a cylindrical electrode; or a conical electrode.

[0042] In one embodiment, the measuring electrode is a conventional gold electrode of flat shape, in particular with a disc having a diameter of 3 mm.

[0043] The measuring electrode can have an Rtheo between its developed area (Adev) and its apparent area (Aapp) greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20.

[0044] Compared to a conventional measuring electrode, for example in gold or copper, for example of planar, cylindrical, conical or spherical shape, which has a ratio Rtheo of about 1, the measuring electrode having a ratio Rtheo greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20, has a structuring or micro-structuring of its surface, and ultimately an increased specific surface area, while maintaining an acceptable mechanical resistance of the measuring electrode.

[0045] Thus, the use of a measuring electrode having an RtheO ratio greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20, can increase the sensitivity of the measurement and thus lower the detection limit of the process according to the invention.

[0046] The measuring electrode having a theo ratio greater than or equal to 3 may comprise a support covered with a porous sheath, in particular of gold or copper, this sheath having a thickness and pore sizes such that the measuring electrode has an Rtheo ratio between its developed area (Adev) and its apparent area (Aapp) greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20. This porous sheath forms the active surface of the measuring electrode. Such a measuring electrode is said to be "micro-structured porous".

[0047] Alternatively, the measuring electrode comprises a support covered with a rough layer, in particular of gold or copper, consisting of crystallites, in particular of gold or copper, of nano to micrometer dimensions such that the measuring electrode has a ratio RtheO between its developed area (Adev) and its apparent area (Aapp) greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20. This rough layer forms the active surface of the measuring electrode. Such a measuring electrode is said to be "micro-structured rough".

[0048] The apparent area, denoted Aapp, is defined as the macroscopic geometric shape of the measuring electrode. For example, for a flat, rectangular measuring electrode, this area is equal to the product of its length and its width.

[0049] The developed area, denoted Adev, is defined as the maximum exposed area that can interact with the surrounding solution. It corresponds to the actual area taking into account all possible surface structures of the material at the microscopic level (porosity, roughness, etc.).

[0050] The process may include a step, called the "acidification step", consisting of acidifying the liquid so as to obtain the acidic pH pHA, before the step of measuring the variation of the current.

[0051] Such an acidification step is implemented if and only if the liquid to be analyzed has a pH value different from the acid pHA.

[0052] As explained above, such an acidification step can make it possible to obtain H2SO3(aq) as the predominant form of sulfur dioxide present in its free forms in the liquid and thus increase the sensitivity of the measurement.

[0053] The acidification step may include a substep in which an acidic solution is added to the liquid until the acidic pH pHA is obtained.

[0054] The acid solution is preferably an aqueous solution of a strong acid.

[0055] By "strong acid" is meant an acid belonging to an acid-base pair of which the pKa is less than or equal to 1, preferably less than or equal to 0, more preferably less than or equal to -1.

[0056] The acid solution can be chosen from a sulfuric acid solution, a phosphoric acid solution, a nitric acid solution, a hydrochloric acid solution and a mixture thereof, preferably a sulfuric acid solution.

[0057] In the case where the active surface in contact with the liquid of the measuring electrode contains gold, the use of a hydrochloric acid solution as an acid solution should be avoided.

[0058] The process can be used to detect and / or quantify free sulfur dioxide (SO2) present in the liquid. By "free sulfur dioxide (SO2) present in the liquid", we mean sulfur dioxide (SO2) in its uncomplexed, or free, forms.

[0059] The process may include a step, called the "alkalinization step", consisting of alkalizing the liquid to a pH greater than or equal to 11, preferably greater than or equal to 12, before the acidification step.

[0060] Such an alkalization step can allow the release of all the sulfur dioxide (SO2) present in combined or complexed form in order to measure the total amount of sulfur dioxide (SO2) present in the environment, i.e. sulfur dioxide (SO2) present in combined or complexed form and sulfur dioxide (SO2) present in free form.

[0061] Indeed, part of the sulfur dioxide in solution, for example in wine, can be combined or complexed, for example with glucose or anthocyanins via weak bonds, or for example with aldehydes via strong bonds.

[0062] Thus, when the process includes the alkalinization step, the process can allow the detection and / or quantification of the total sulfur dioxide (SO2) present in the liquid.

[0063] By "total sulfur dioxide (SO2) present in the liquid", we mean not only sulfur dioxide (SO2) in its uncomplexed so-called free forms, but also sulfur dioxide (SO2) in its complexed forms.

[0064] Thus, the process is particularly advantageous insofar as it can allow the detection and / or quantification not only of free sulfur dioxide (SO2), but also of total sulfur dioxide (SO2) present in the liquid.

[0065] In one embodiment, the alkalinization step consists of alkalizing the liquid to a pH equal to 12.

[0066] The alkalinization step may include a substep ai) in which basic solution is added to the liquid until a pH greater than or equal to 11, preferably greater than or equal to 12, is obtained.

[0067] The basic solution is preferably an aqueous solution of a strong base.

[0068] By "strong base" is meant a base belonging to an acid-base pair whose pKa is greater than or equal to 14.

[0069] The basic solution can be chosen from a solution of sodium hydroxide, potassium hydroxide, calcium hydroxide and a mixture thereof, preferably a solution of sodium hydroxide.

[0070] The alkalinization step may include, after substep aj, a substep a2) in which the added basic solution is left to act in the liquid for a period of between 2 min and 60 min, preferably between 5 min and 30 min, more preferably between 10 min and 20 min, in particular at room temperature, for example at a temperature between 15°C and 30°C, preferably at a temperature between 18°C ​​and 23°C, more preferably at a temperature of about 20°C.

[0071] When the process includes the acidification step or when it includes the alkalinization and then acidification steps, it is necessary to take into account the dilution of the liquid following the addition of an acid / base solution to the liquid, and therefore to calculate the dilution factor and take it into account to calculate the concentration of sulfur dioxide.

[0072] The process may include a step, called the "reduction peak area measurement step", consisting of measuring the area of ​​the reduction peak of sulfur dioxide present in its free forms in the liquid on the voltammeter obtained in the current variation measurement step.

[0073] It is known to measure the height of the peaks on a voltamperogram.

[0074] In this step, measuring the area of ​​the reduction peak of sulfur dioxide present in its free forms in the liquid, rather than its height, can allow the quantification of sulfur dioxide over a wide range of concentrations (i.e. linearity of the calibration curve over a wide range of sulfur dioxide concentrations).

[0075] The liquid may be a liquid in which at least one solid element, in particular a solid food, has been infused and subsequently diluted if necessary.

[0076] The objective of such an infusion is to extract the sulfur dioxide present in the solid element, and therefore that this sulfur dioxide is found in the liquid in which the solid element has been infused, so that it can be detected and / or quantified using the process according to the invention.

[0077] The infusion can be carried out for a period of time ranging from 10 minutes to 24 hours.

[0078] The infusion can be carried out with or without agitation.

[0079] The infusion can be carried out at room temperature, for example at a temperature between 15°C and 30°C, preferably between 18°C ​​and 23°C, more preferably around 20°C.

[0080] The liquid in which the solid element, in particular the solid food, has been infused may be chosen from: - an aqueous solution containing a supporting electrolyte, in particular NaCl, KCl, KNO3 or a phosphate buffer; - an acidic solution, in particular one having an acidic pH (pH A), such as, for example, a sulfuric acid solution; and - a mixed solvent comprising water and one or more organic solvents.

[0081] The solid element can be chosen from: - dried fruits, such as for example dried apricots, peaches, raisins, plums, figs, bananas, apples, pears or prunes; - confectionery; - dried or processed mushrooms; - dried vegetables, especially dried white vegetables; - processed vegetables, in particular processed white vegetables, especially frozen or deep-frozen; - delicatessen products; - crustaceans, such as shrimp; - cereals; - potato products, including dried potatoes, peeled potatoes and processed potatoes, including frozen or deep-frozen potatoes; and - pickles in a jar.

[0082] The liquid may be a food liquid, preferably a beverage, more preferably a fruit-based beverage, in particular fermented or not, in particular sparkling or not.

[0083] The food liquid can be chosen from: - wine, especially red, white or rosé, including sparkling or still wine; - fruit juices, including citrus fruits, apples, pineapples, lemons, limes; - concentrates based on fruit juice or crushed fruit; - malted beverages, such as beer, including alcoholic and non-alcoholic versions; - cider, perry, including sparkling or still; - chouchen; - vinegar, especially food or household vinegar; and - one of their mixtures.

[0084] Preferably, the food liquid is wine.

[0085] The method may include several successive steps of measuring the variation of the current, each step of measuring the variation of the current being carried out in a different liquid.

[0086] Such a process can be implemented in an automated or semi-automated manner, for example by using a sample changer.

[0087] The process may include a pretreatment and activation step of the measuring electrode.

[0088] The pretreatment and activation step of the measuring electrode may include: - a substep of introducing the measuring electrode into an acidic pretreatment and activation solution, then - a potential scanning substep, during cyclic voltammetry, carried out between -0.3 V and +1.5 V, using an Ag / AgCl, 3M KC1 reference electrode, without measuring the current variation.

[0089] Such a pretreatment and activation step of the measuring electrode is particularly advantageous insofar as it can make it possible to do away with the mechanical polishing step which is usually required to pretreat and activate the measuring electrodes.

[0090] Moreover, such a pretreatment and activation step of the measuring electrode can improve the stability of the response over time.

[0091] The acid pretreatment and activation solution preferably has a pH less than or equal to 0.5, preferably less than or equal to 0.3, more preferably less than or equal to 0.2, even more preferably less than or equal to 0.1, or even less than or equal to 0.

[0092] The acid pretreatment and activation solution can be chosen from a sulfuric acid solution, a phosphoric acid solution, a nitric acid solution, an acetic acid solution and a mixture thereof, preferably a sulfuric acid solution.

[0093] For example, the acid pretreatment and activation solution is a 0.5 M sulfuric acid solution.

[0094] The potential sweep substep can be implemented at potential sweep speeds between 10 mV.s 1 and 200 mV.s4, preferably between 50 mV.s 1 and 150 mV.s4, more preferably between 80 mV.s 1 and 120 mV.s4, even more preferably between 90 mV.s 1 and 110 mV.s4, or even between 95 mV.s 1 and 105 mV.s4.

[0095] In one embodiment, the potential sweep substep is implemented at a potential sweep rate of approximately 100 mV.s 4.

[0096] The potential scanning substep can be implemented by performing a number of cyclic voltammetry cycles between 1 and 10 cycles.

[0097] For example, the number of cyclic voltammetry cycles is between 2 and 10 cycles, in particular equal to 5 cycles.

[0098] The pretreatment and activation step of the measuring electrode may further include a substep of storing the measuring electrode in a basic solution, in particular a basic detergent, before the substep of introducing the measuring electrode into an acidic pretreatment and activation solution.

[0099] Preferably, the basic solution has a pH greater than or equal to 12.

[0100] For example, the basic solution is the TFD4 from FRANKLAB, the CETEXALT from AEXALT, or the RBS T 115 from CARL ROTH.

[0101] The pretreatment and activation step of the measuring electrode can be implemented before the current variation measurement step.

[0102] The pretreatment and activation step of the measuring electrode can be implemented after the current variation measurement step, for example between two successive current variation measurement steps.

[0103] The invention also relates, according to another aspect, to an electrochemical device for detecting and / or quantifying sulfur dioxide (SO2) in a liquid, particularly for implementing the process as defined above, comprising: - a measuring electrode whose active surface comprises a metal or a metal alloy, - a reference electrode, - a counter electrode, - a potentiostat, - a first container containing an acidic solution, and - a second container holding the liquid, in particular wine.

[0104] By "potentiostat" is meant any electronic device which makes it possible to impose a variation of potential and to measure a current.

[0105] In one embodiment, the reference electrode is an Ag / AgCl, 3M KC1 reference electrode.

[0106] In one embodiment, the counter electrode is a counter electrode made of platinum or vitreous carbon.

[0107] In a first variant, the liquid contained in the second container is a liquid which has the acidic pH pHA.

[0108] This liquid which has the acidic pH pHA can be a liquid, in particular wine, which has undergone an acidification step as described above, or an alkalization step followed by an acidification step as described above.

[0109] In this first variant, the first container allows the pretreatment and activation step of the measuring electrode to be implemented as described above by introducing the measuring electrode into the acidic solution contained in the first container.

[0110] In this first variant, the second compartment allows the step of measuring the variation of the current to be implemented by introducing the measuring electrode into the liquid contained in this second container.

[0111] In a second variant, the liquid contained in the second container is a liquid which has a pH different from the acidic pHA.

[0112] This liquid, which has a pH different from the acidic pHA, may be a liquid that has undergone an alkalinization step as described above.

[0113] For example, the liquid contained in the second container is wine which has not undergone any treatment after bottling (i.e. native wine), or which has undergone only an alkalization step as described above.

[0114] In this second variant, the acidic solution contained in the first container allows for the implementation not only of the pretreatment and activation step of the measuring electrode as described above by introducing the electrode of measurement in the acid solution contained in the first container, but also the acidification step as described above by adding the liquid contained in the second container to the acid solution contained in the first container, or conversely, by adding the acid solution contained in the first container to the liquid contained in the second container.

[0115] In this second variant, the step of measuring the variation of the current is implemented by introducing the measuring electrode into the mixture comprising the liquid and the acid solution. Brief description of the figures

[0116] [Fig-1] [Fig.1] represents voltammeterograms of standardized additions of dioxide sulfur content (mg / L) obtained by cyclic voltammetry (Figures IA and IC) and pulsed differential voltammetry (Figures IB and 1D) in different wine samples;

[0117] [Fig.2] [Fig.2] represents the quantification of the electrochemical response by pulsed differential voltammetry in a white wine acidified to pH = 1 by the addition of sulfuric acid H2SO4 as a function of measured additions of sulfur dioxide (mg / L), for small variations in concentrations (Figure 2A) and larger variations in concentrations (Figure 2B);

[0118] [Fig.3] [Fig.3] represents voltammeterograms of standardized additions of dioxide sulfur (mg / L) in different wine samples (red wine, white wine, sulfite-free red wine, rosé wine, sparkling wine and sweet wine) by pulsed differential voltammetry after acidification of the samples to pH = 1 by adding sulfuric acid H2SO4; and

[0119] [Fig.4] [Fig.4] represents volt-amperograms obtained by the method of pulsed differential voltammetry of the free quantity of sulfur dioxide (in its uncomplexed so-called free forms) in a wine sample (after acidification to pH = 1 by adding sulfuric acid H2SO4) or of the total quantity of sulfur dioxide (in its free and complexed forms) obtained following a pretreatment of the sample (alkalinization to pH = 12 by adding a NaOH solution for 10 min, then acidification to pH = 1 by adding sulfuric acid H2SO4). Examples

[0120] The limits of detection (LOD) of sulfur dioxide in a white wine sample were determined by measuring the change in current produced by the reduction of sulfur dioxide present in its free forms in the white wine sample, either at the pH of the native wine (comparative test), or after acidification of the wine to pH = 1 by the addition of sulfuric acid H2SO4 (according to the invention), during a potential sweep performed by cyclic voltammetry (CV) vs. pulsed differential voltammetry (DPV). CV analyses were performed with the following parameters: in the potential range between 0 and -0.5 V vs Ag / AgCl, 3M KCl at pH = 1 and between 0 and -0.7 V vs Ag / AgCl, 3M KCl at native wine pH; "E step" (potential increment between two points according to a linear variation) = 5 mV; potential sweep rate = 50 mV.s'. DPV analyses were performed with the following parameters: "E step" (potential increment between two consecutive pulses) = 5 mV; "E pulse" (pulse amplitude) = 30 mV; "t pulse" (pulse duration) = 50 ms; potential sweep rate = 50 mV.s'. CV and DPV analyses were performed in a three-electrode system consisting of a polycrystalline gold measuring electrode (3 mm diameter), a glassy carbon counter electrode (3 mm diameter) and a reference electrode (Ag / AgCl, 3M KC1).

[0121] The results are presented in Table 1 below, based on the measured additions of sulfur dioxide illustrated in [Fig. 1]. Table 1#: pH of native wine (comparative test) Acidification of wine to pH = 1 (according to the invention) Cyclic Voltammetry (CV) 10 mg / L (see Figure IA) 4 mg / L (see Figure IC) Pulsed Differential Voltammetry (DPV) 5.5 mg / L (see Figure IB) 1.5 mg / L (see Figure 1D)

[0123] It is observed that the limits of detection (LOD) of sulfur dioxide determined after acidification of the wine to pH = 1 (according to the invention) are lower than those determined at the pH of the native wine (comparative test).

[0124] It is also observed that for the same pH of the white wine sample, the use of Pulsed Differential Voltammetry (PDV) makes it possible to obtain lower limits of detection (LOD) of sulfur dioxide than those obtained using Cyclic Voltammetry (CV).

[0125] Figure 2 shows the quantification of the electrochemical response by Pulsed Differential Voltammetry (PDV) in a white wine sample acidified to pH 1 by the addition of sulfuric acid (H₂SO₄) as a function of measured additions of sulfur dioxide (mg / L), for small concentration variations (Figure 2A) and larger concentration variations (Figure 2B). The analyses are performed with the following parameters: E step (potential increment between two consecutive pulses) = 5 mV; E pulse (pulse amplitude) = 30 mV; t pulse (pulse duration) = 50 ms; potential sweep rate = 50 mV.s⁻¹. The analyses are performed in a three-electrode system consisting of a polycrystalline gold measuring electrode (3 mm diameter), a glassy carbon counter electrode (3 mm diameter)

[0126]

[0127]

[0128] diameter) and a reference electrode (Ag / AgCl, 3M KC1). The amplitude values ​​(coulometric charge) are obtained by integrating the peak of reduction of the detected sulfur dioxide. Figure 3 shows voltammetry readings of measured additions of sulfur dioxide (mg / L) in different wine samples, which differ, for example, in their color, sugar content, or gas content (red wine, white wine, sulfite-free red wine, rosé wine, sparkling wine, and sweet wine), obtained by Pulsed Differential Voltammetry (PDV) after acidification of the samples to pH = 1 by adding sulfuric acid (H2SO4). The measurements were performed with the following parameters: "E step" (potential increment between two consecutive pulses) = 5 mV; "E pulse" (pulse amplitude) = 30 mV; "t pulse" (pulse duration) = 50 ms; potential sweep rate = 50 mV / s. The analyses are carried out in a three-electrode system consisting of a polycrystalline gold measuring electrode (3 mm in diameter), a glassy carbon counter electrode (3 mm in diameter) and a reference electrode (Ag / AgCl, 3M KC1). Table 2 below allows comparison of free and total quantities (mg / L) of sulfur dioxide in three types of wine (white, red and rosé), measured by the process according to the invention and by an oenological method (Fourier transform infrared spectroscopy, FTIR, for example with WineScan-FOSS apparatus). It is observed that the free and total quantities (mg / L) of sulfur dioxide determined by the process according to the invention and by an oenological method (Fourier transform infrared spectroscopy, FTIR, for example with WineScan-FOSS equipment) are quite comparable. Table 2#:

[0129] Method: Procedure according to the invention. FTIR (comparative test) Free [SO2] Total [SO2] Free [SO2] Total [SO2] White wine 53 112 50 114 Red wine 36 74 36 81 Rosé wine 33 106 32 102

[0130] Figure 4 shows voltammetry readings obtained by pulsed differential voltammetry of the free amount of sulfur dioxide (in its uncomplexed, or free, forms) in a wine sample (after acidification to pH 1 by adding sulfuric acid, H₂SO₄) or of the total amount of sulfur dioxide (in its free and complexed forms) obtained following pretreatment of the sample (alkalinization to pH 12 by adding a NaOH solution for 10 min, then acidification to pH 1 by adding sulfuric acid, H₂SO₄). The measurements are performed with the following parameters: "E step" (potential increment between two consecutive draws) = 5 mV; "E draw" (draft amplitude) = 30 mV; "t draw" (draft duration) = 50 ms; potential sweep speed = 50 mV.s. The analyses are performed in a three-electrode system consisting of a polycrystalline gold electrode (3 mm diameter), a glassy carbon counter electrode (3 mm diameter) and a reference electrode (Ag / AgCl, 3M KC1).

Claims

Demands

1. An electrochemical method for detecting and / or quantifying sulfur dioxide (SO2) in a liquid, the method comprising a step, referred to as the "current variation measurement step", consisting of measuring the change in current produced by the reduction of sulfur dioxide present in its free forms in the liquid at an acidic pH pHA less than 1.8, during a potential sweep carried out by voltammetry, using a measuring electrode disposed in the liquid and whose active surface in contact with the liquid comprises a metal or a metal alloy.

2. Method according to claim 1, wherein the potential sweep is carried out by pulsed differential voltamperometry.

3. A method according to claim 1 or 2, wherein the potential sweep is carried out in the direction of decreasing potentials between 0 V and -0.7 V, preferably between -0.1 V and -0.6 V, more preferably between -0.1 V and -0.5 V, using an Ag / AgCl, 3M KC1 reference electrode.

4. A method according to any one of the preceding claims, wherein the potential sweep is carried out at speeds between 1 mV.s1 and 1000 mV.s1, and preferably between 10 mV.s1 and 100 mV.s1.

5. A method according to any one of the preceding claims, wherein the active surface in contact with the liquid of the measuring electrode comprises a transition metal or an alloy of transition metals.

6. A method according to any one of the preceding claims, wherein the active surface in contact with the liquid of the measuring electrode comprises gold, silver, copper or one of their alloys, preferably gold, copper or one of their alloys, more preferably gold.

7. A method according to any one of the preceding claims, comprising a step, referred to as the "acidification step", consisting of acidifying the liquid so as to obtain the acidic pH pHA, before the step of measuring the variation of the current.

8. A method according to claim 7, comprising a step, referred to as the "alkalinization step", consisting of alkalizing the liquid to a pH greater than or equal to 11, preferably greater than or equal to 12, before the acidification step.

9. A method according to any one of the preceding claims, comprising a step, called the "reduction peak area measurement step", consisting of measuring the area of ​​the reduction peak of sulfur dioxide present in its free forms in the liquid on the voltamgram obtained in the current variation measurement step.

10. A method according to any one of claims 1 to 9, the liquid being a liquid in which at least one solid element, in particular a solid food, has been infused.

11. A method according to any one of claims 1 to 9, the liquid being a food liquid, preferably a beverage.

12.

13. A method according to claim 11, the food liquid being wine. A method according to any one of the preceding claims, comprising a pretreatment step and activation of the measuring electrode.

14. Method according to claim 13, the pretreatment and activation step of the measuring electrode comprising: - a substep of introducing the measuring electrode into an acidic pretreatment and activation solution, and - a potential sweep substep, during cyclic voltammetry, carried out between -0.3 V and 1.5 V, using an Ag / AgCl, 3M KC1 reference electrode, without measuring the current variation.

15. Electrochemical device for detecting and / or quantifying sulfur dioxide (SO2) in a liquid, in particular for carrying out the process according to any one of the preceding claims, comprising: - a measuring electrode whose active surface comprises a metal or a metal alloy, - a reference electrode, - a counter electrode, - a potentiostat, - a first container containing an acidic solution, and - a second container containing the liquid.

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