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

Abstract

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

Description

[0001] DESCRIPTION

[0002] TITLE: Electrochemical process for the detection and / or quantification of sulfur dioxide in a liquid

[0003] technical field

[0004] 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.

[0005] Prior art

[0006] Sulfur dioxide is a widely used additive in oenology, as well as in the food industry in general and in the chemical industry.

[0007] 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.

[0008] Indeed, sulfur dioxide has antimicrobial and antioxidant properties.

[0009] Sulfur dioxide can also be used to stop fermentation during the winemaking process.

[0010] For regulatory reasons in particular, it is necessary to have methods and devices for detecting and / or quantifying sulfur dioxide.

[0011] The detection and / or electrochemical quantification of sulfur dioxide, particularly in liquids, especially food liquids such as wine, has been reported on numerous occasions and using a wide variety of sensors or detection techniques.

[0012] However, all of this work very rarely meets the specifications for the quantification of sulfur dioxide, particularly in terms of performance and analytical conditions imposed by the agri-food industry, especially oenologists and the wine industry.

[0013] WO 2018 / 154226 A1 describes an electrochemical method for the detection and / or quantification of 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).

[0014] 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.

[0015] Description of the invention

[0016] The invention aims to meet this need, and thus has as its object, according to a first of its aspects, an electrochemical process for the detection and / or quantification of sulfur dioxide (SO2) in a liquid, the process comprising a step, called the "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.

[0017] The liquid may consist of several phases, including phases that are immiscible with each other.

[0018] Sulfur dioxide in solution can be found mainly in the following free forms, generally called "sulfites":

[0019] 1) H2SO3(aq) (also called "sulfurous acid" or "sulfur dioxide in solution" or "molecular sulfur dioxide") which results from the equilibrium SO2(g) + H2O(I) H2SO3(aq),

[0020] 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

[0021] 3) SO3 2 “(aq) (also called the “sulfite” anion) which results from the equilibrium HSO3“(aq) H + (aq) + SO3 2 “(aq) with a pKa HSO3“(aq) / SO3 2 “(aq) = 7.21.

[0022] In solution, an equilibrium is established between these three free forms. This equilibrium depends on the pH of the solution.

[0023] In the present invention, the term "electrochemical method for detecting and / or quantifying sulfur dioxide (SO2) in a liquid" refers to an electrochemical method for detecting and / or quantifying all free forms of sulfur dioxide (SO2) in solution that are present in the liquid at a given pH. The step of measuring the change in current is carried out at an acidic pH. A which is less than 1.8, i.e., less than the pKa H2SO3(aq) / HSO3“(aq).).

[0024] At such a pH value, the predominant free form of sulfur dioxide in the liquid is H2SO3(aq).

[0025] Indeed, at such a pH value, the proportion of the H2SO3(aq) form is greater than 50%, while the proportion of the HSO3“(aq) form is less than 50%, the proportion of the SO3 form 2 “(aq) being negligible.

[0026] However, in comparison to the forms HSO3“(aq) and SO3 2“(aq), the form H2SO3(aq) is the one that gives the most intense response in voltammetry.

[0027] 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. A 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.

[0028] The measurement of the change in current produced by the reduction of sulfur dioxide present in its free forms in the liquid is all the more sensitive when the proportion of the H2SO3(aq) form is high in the liquid, compared to the HSO3“(aq) and SO3 forms. 2 “(aq).

[0029] Thus, the acidic pH Amay 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.

[0030] For example, acidic pH A is less than or equal to 1.1, or even less than or equal to 1.

[0031] In one example implementation, the acidic pH A 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.

[0032] For example, acidic pH A is between 0.6 and 1.1, preferably between 0.7 and 1.1, more preferably between 0.8 and 1.1, and again preferentially between 0.9 and 1.1.

[0033] In one example implementation, the acidic pH Ais 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 HSO3“(aq) form is less than or equal to 10%, the proportion of the SO3 form 2 “(aq) being negligible. 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.

[0034] This is particularly advantageous since such a measurement by electro-reduction makes it possible to overcome certain analytical interfering factors, particularly those present in wine, such as sugars, phenolic acids, flavonoids, etc.

[0035] Indeed, most of these interfering compounds are oxidizable but are not or are very difficult to reduce, so they do not interfere in the measurement by electro-reduction.

[0036] The "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.

[0037] The term "voltammetry potential sweep" refers to any electrochemical measurement technique that involves applying a controlled and gradual variation of the electrical potential between a measuring electrode and a reference electrode, while simultaneously recording the evolution of the electric current resulting from the redox reactions occurring at the measuring electrode. These reactions depend on the electroactive species present in the solution. This sweep can be continuous, incremental, linear, or incorporate various forms of potential pulses.

[0038] Thus, potential scanning can be carried out using one of the following techniques: linear sweep voltammetry (LSV), cyclic voltammetry (CV), differential pulse voltammetry (DPV), normal pulse voltammetry (NPV), square wave voltammetry (SWV).

[0039] Preferably, the potential sweep is performed by DPV, SWV or NPV. Even more preferably, the potential sweep is performed by pulsed differential voltammetry (also called DPV for Differential Pulse Voltammetry).

[0040] 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.

[0041] In one example embodiment, the potential sweep is carried out in the direction of decreasing potentials, preferentially between 0 V and -0.7 V, more preferentially between -0.1 V and -0.6 V, even more preferentially between -0.1 V and -0.5 V, using an Ag / AgCl, 3M KCl reference electrode.

[0042] In one example implementation, the potential sweep is performed at speeds between 1 mV.s -1 and 1000 mV.s -1 , and preferably between 10 mV.s' 1 and 100 mV.s -1 .

[0043] In one example embodiment, the active surface in contact with the liquid of the measuring electrode is made of a metal or a metal alloy.

[0044] In one example embodiment, the active surface in contact with the liquid of the measuring electrode comprises a transition metal or an alloy of transition metals.

[0045] In one example 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.

[0046] In one example 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.

[0047] In one example 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 gold, copper or one of their alloys, more preferably gold.

[0048] The measuring electrode can be a conventional electrode, in particular made of gold or copper, and in particular of flat or cylindrical shape.

[0049] Preferably, the measuring electrode is made of gold.

[0050] The inventors highlighted that, compared with other types of measuring electrodes such as platinum (Pt) or vitreous carbon (GC) electrodes, the gold measuring electrode offered the following advantages:

[0051] It eliminates the problems of interference with atmospheric oxygen.

[0052] It exhibits a greater affinity for sulfur molecules, including SO2. This allows for a more defined and intense sulfite reduction peak. This clear distinction enables more precise and reliable detection and quantification of free SO2. The achievement of these technical effects is demonstrated by the results obtained in Figures 6 and 7. Note that in each of Figures 7A, B, and C, the gray curve is obtained at the wine's pH, and the black curve is obtained at pH = 1.

[0053] The measuring electrode can be a flat electrode, including a disc shape, a rectangular shape or a square shape; a spherical electrode; a cylindrical electrode; or a conical electrode.

[0054] In one example of an embodiment, the measuring electrode is a conventional flat-shaped gold electrode, in particular with a disc having a diameter of 3 mm.

[0055] The measuring electrode may have an R t héo between its developed area (A dev ) and its apparent area (A app ) greater than or equal to 3, preferably between 3 and 100, more preferably between 3 and 20.

[0056] Compared to a conventional measuring electrode, for example made of gold or copper, for example of a flat, 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.

[0057] Thus, the use of a measuring electrode having a ratio Rtheo 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.

[0058] The measuring electrode having a theo-value 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 a theo-value ratio between its developed area (A dev ) and its apparent area (A app ) 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".

[0059] 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 (A dev ) and its apparent area (A app ) 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".

[0060] The apparent area, denoted A app , is defined as the macroscopic geometric area 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.

[0061] The developed area, denoted Ad ev, 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.).

[0062] The process may include a step, known as the "acidification step", which consists of acidifying the liquid to obtain the acidic pH pH A , before the step of measuring the variation in current.

[0063] Such an acidification step is implemented if and only if the liquid to be analyzed has a pH value different from the acidic pH. A .

[0064] 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.

[0065] The acidification step may include a substep in which an acidic solution is added to the liquid until the desired acidic pH is reached. A .

[0066] The acidic solution is preferably an aqueous solution of a strong acid.

[0067] By "strong acid" we mean an acid belonging to an acid-base pair whose pKa is less than or equal to 1, preferably less than or equal to 0, more preferably less than or equal to -1.

[0068] The acid solution may 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.

[0069] 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.

[0070] The process can be used to detect and / or quantify free sulfur dioxide (SO2) present in the liquid. "Free sulfur dioxide (SO2) present in the liquid" refers to sulfur dioxide (SO2) in its uncomplexed, or free, forms. The process may include a step, called the "alkalinization step," which consists of alkalizing the liquid to a pH greater than or equal to 11, preferably greater than or equal to 12, before the acidification step.

[0071] Such an alkalinization 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 medium, i.e. the sulfur dioxide (SO2) present in combined or complexed form and the sulfur dioxide (SO2) present in free form.

[0072] Indeed, some 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] In one example implementation, the alkalization step consists of alkalizing the liquid to a pH of 12.

[0077] 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 is obtained, preferably greater than or equal to 12.

[0078] The basic solution is preferably an aqueous solution of a strong base.

[0079] By "strong base" we mean a base belonging to an acid-base pair whose pKa is greater than or equal to 14.

[0080] 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.

[0081] The alkalinization step may include, after substep ai), a substep 32) 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.

[0082] 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 when calculating the concentration of sulfur dioxide.

[0083] 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.

[0084] It is known to measure the height of the peaks on a voltamperogram.

[0085] In this step, measuring the area of ​​the peak reduction 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).

[0086] The liquid can be a liquid in which at least one solid element, including a solid food, has been infused and then diluted if necessary.

[0087] The objective of such an infusion is to extract the sulfur dioxide present in the solid element, and therefore for this sulfur dioxide to be 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.

[0088] The infusion can be carried out for a period of time ranging from 10 minutes to 24 hours.

[0089] The infusion can be carried out with or without stirring.

[0090] The infusion can be carried out 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.

[0091] The liquid in which the solid element, particularly solid food, has been infused can be chosen from:

[0092] - an aqueous solution containing a supporting electrolyte, in particular NaCl, KCl, KNO3 or a phosphate buffer;

[0093] - an acidic solution, in particular one having an acidic pH A such as, for example, a sulfuric acid solution; and

[0094] - a mixed solvent comprising water and one or more organic solvents. The solid component may be chosen from:

[0095] - dried fruits, such as for example dried apricots, peaches, raisins, plums, figs, bananas, apples, pears or prunes;

[0096] - confectionery;

[0097] - dried or processed mushrooms;

[0098] - dried vegetables, especially dried white vegetables;

[0099] - processed vegetables, in particular processed white vegetables, especially frozen or deep-frozen;

[0100] - delicatessen products;

[0101] - crustaceans, such as shrimp;

[0102] - cereals;

[0103] - potato products, including dried potatoes, peeled potatoes and processed potatoes, including frozen or deep-frozen potatoes; and

[0104] - pickles in a jar.

[0105] The liquid can be a food liquid, preferably a drink, more preferably a fruit-based drink, especially fermented or not, especially sparkling or not.

[0106] The liquid food can be chosen from:

[0107] - wine, especially red, white or rosé, including sparkling or still wine;

[0108] - fruit juices, including citrus fruits, apples, pineapples, lemons, limes;

[0109] - concentrates based on fruit juice or crushed fruit;

[0110] - malted beverages, such as beer, including alcoholic and non-alcoholic versions;

[0111] - cider, perry, including sparkling or still;

[0112] - chouchen;

[0113] - vinegar, especially food or household vinegar; and

[0114] - one of their mixtures.

[0115] Preferably, the food liquid is wine.

[0116] The process may involve 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.

[0117] Such a process can be implemented in an automated or semi-automated manner, for example by using a sample changer.

[0118] The process may include a pretreatment and activation step for the measuring electrode. This pretreatment and activation step may include:

[0119] - a sub-step of introducing the measuring electrode into an acidic pretreatment and activation solution, then

[0120] - a potential scanning substep, during cyclic voltammetry, carried out between -0.3 V and +1.5 V, using an Ag / AgCl, 3M KCl reference electrode, without measuring the current variation.

[0121] Such a pretreatment and activation step of the measuring electrode is particularly advantageous as it can eliminate the mechanical polishing step which is usually required to pretreat and activate measuring electrodes.

[0122] Furthermore, such a pretreatment and activation step of the measuring electrode can improve the stability of the response over time.

[0123] The acidic 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.

[0124] 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.

[0125] For example, the acid pretreatment and activation solution is a 0.5 M sulfuric acid solution.

[0126] The potential sweep substep can be implemented at potential sweep speeds between 10 mV.s -1 and 200 mV.s -1 , preferably between 50 mV.s' 1 and 150 mV.s -1, more preferably between 80 mV.s -1 and 120 mV.s -1 , even more preferentially between 90 mV.s -1 and 110 mV.s -1 , or even between 95 mV.s -1 and 105 mV.s -1 .

[0127] In one example embodiment, the potential sweep substep is implemented at a potential sweep rate of approximately 100 mV.s -1 .

[0128] The potential scanning substep can be implemented by performing a number of cyclic voltammetry cycles between 1 and 10 cycles.

[0129] For example, the number of cyclic voltammetry cycles is between 2 and 10 cycles, notably equal to 5 cycles.

[0130] 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. Preferably, the basic solution has a pH greater than or equal to 12.

[0131] For example, the basic solution is the TFD4 from FRANKLAB, the CETEXALT from AEXALT, or the RBS T 115 from CARL ROTH.

[0132] The pretreatment and activation step of the measuring electrode can be implemented before the current variation measurement step.

[0133] 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.

[0134] 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 described above, comprising:

[0135] - a measuring electrode whose active surface comprises a metal or a metal alloy,

[0136] - a reference electrode,

[0137] - a counter electrode,

[0138] - a potentiostat,

[0139] - a first container containing an acidic solution, and

[0140] - a second container holding the liquid, in particular wine.

[0141] The term "potentiostat" refers to any electronic device that can impose a change in potential and measure a current.

[0142] In one example embodiment, the reference electrode is an Ag / AgCl, 3M KCl reference electrode.

[0143] In one example of an embodiment, the counter electrode is a counter electrode made of platinum or vitreous carbon.

[0144] In one variant, the liquid contained in the second container is a liquid that has an acidic pH. A .

[0145] This liquid has an acidic pH A may 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.

[0146] In this first variant, the first container allows for the pretreatment and activation step of the measuring electrode, as described above, by introducing the measuring electrode into the acidic solution contained in the first container. In this first variant, the second compartment allows for the measurement step of the current variation by introducing the measuring electrode into the liquid contained in this second container.

[0147] In a second variant, the liquid in the second container is a liquid with a pH different from the acidic pH. A .

[0148] This liquid has a pH different from acidic pH A may be a liquid that has undergone an alkalization step as described above.

[0149] For example, the liquid in the second container is wine that has not undergone any treatment after bottling (i.e., native wine), or that has only undergone an alkalization step as described above.

[0150] In this second variant, the acid 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 measuring electrode into the acid solution contained in the first container, but also of the acidification step as described above by adding the liquid contained in the second container into 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.

[0151] In this second variant, the step of measuring the variation of the current is implemented by introducing the measuring electrode into the mixture containing the liquid and the acid solution.

[0152] Brief description of the figures

[0153] [Fig. 1] Figure 1 represents voltammetry of standardized additions of sulfur dioxide (mg / L) obtained by cyclic voltammetry (Figures 1A and 1C) and pulsed differential voltammetry (Figures 1B and 1D) in different wine samples;

[0154] [Fig. 2] Figure 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 concentration (Figure 2A) and larger variations in concentration (Figure 2B);

[0155] [Fig. 3] Figure 3 represents voltammetry of measured additions of sulfur dioxide (mg / L) in different wine samples (red wine, white wine, red wine without sulfites, rosé wine, sparkling wine and sweet wine) by pulsed differential voltammetry after acidification of the samples to pH = 1 by the addition of sulfuric acid H2SO4; and [Fig. 4] Figure 4 represents voltammetry obtained by the pulsed differential voltammetry method 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).

[0156] [Fig. 5] Figure 5 compares voltammeterograms obtained by different pulsed voltammetry methods: DPV, SWV and NPV.

[0157] [Fig.6] Figure 6 compares voltammeterograms obtained by the DPV pulsed voltammetry method in the same wine sample with a Gold (Or) electrode and a glassy carbon (GC) electrode.

[0158] [Fig. 7] Figure 7 compares voltammeterograms obtained by the DPV pulsed voltammetry method in the same wine sample, before and after its acidification to pH 1, with a Gold (Or) electrode, a glassy carbon (GC) electrode and a platinum (Pt) electrode.

[0159] Examples

[0160] 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) versus pulsed differential voltammetry (DPV). The CV analyses were performed with the following parameters: in the potential range of 0 to -0.5 V vs. Ag / AgCl, 3M KCl at pH 1 and between 0 and -0.7 V vs. Ag / AgCl, 3M KCl at the pH of the native wine; E-step (potential increment between two points with a linear variation) = 5 mV; potential sweep rate = 50 mV / s -1DPV 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 -1 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 KCl). The results are presented in Table 1 below, based on the standard sulfur dioxide additions illustrated in Figure 1.

[0161] Table 1:

[0162] 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).

[0163] It is also observed that for the same pH of the white wine sample, the use of Pulsed Differential Voltammetry (PDV) allows lower limits of detection (LOD) of sulfur dioxide than those obtained using Cyclic Voltammetry (CV).

[0164] 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 were performed with the following parameters: Estep (potential increment between two consecutive pulses) = 5 mV; Epulse (pulse amplitude) = 30 mV; tpulse (pulse duration) = 50 ms; potential sweep rate = 50 mV·s 1The 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), and a reference electrode (Ag / AgCl, 3M KCl). The amplitude values ​​(coulometric charge) are obtained by integrating the peak of sulfur dioxide reduction detected.

[0165] Figure 3 shows voltammetry readings of measured additions of sulfur dioxide (mg / L) to different wine samples, which differ, for example, in 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 1 The analyses are performed in a three-electrode system consisting of a polycrystalline gold measuring electrode (3 mm in diameter), a vitreous carbon counter electrode (3 mm in diameter) and a reference electrode (Ag / AgCl, 3M KCl).

[0166] 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).

[0167] 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.

[0168] Table 2:

[0169] 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 were performed with the following parameters: "E step" (potential increment between two consecutive readings) = 5 mV; "E reading" (reading amplitude) = 30 mV; "t reading" (reading duration) = 50 ms; potential sweep rate = 50 mV / s -1The 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 KCl). Figure 5 compares voltammetry readings obtained by different pulsed voltammetry methods: DPV, SWV, and NPV.

[0170] The results obtained show that these three methods can be used for the detection of free SO2 in a wine sample. The DPV method offers excellent differentiation of the SO2 reduction peak from the baseline, allowing for easy and precise measurement of the corresponding peak area (coulometric loading). This characteristic greatly facilitates the establishment of calibration curves: loading = f([SO2]).

[0171] Therefore, the DPV technique is the preferred method according to the invention, as it offers the best compromise between sensitivity, accuracy and ease of use of the measured electrochemical signal.

[0172] Figure 6 compares voltammeterograms obtained by the DPV pulsed voltammetry method in the same wine sample with a Gold (Or) electrode and a glassy carbon (GC) electrode.

[0173] The results obtained show that the Gold electrode offers a higher SO2 reduction potential (less negative) and higher detection sensitivity (more intense reduction currents), as well as superior selectivity towards dioxygen present in the sample compared to a glassy carbon electrode of the same dimensions.

[0174] Therefore, gold material is the preferred material for the measuring electrode according to the invention.

[0175] Figure 7 compares voltammeterograms obtained by the DPV pulsed voltammetry method in the same wine sample, before and after its acidification to pH 1, with a Gold (Or) electrode, a glassy carbon (GC) electrode and a platinum (Pt) electrode.

[0176] The results obtained show that analysis in acidified wine increases the SO2 reduction potential on each electrode surface, as well as the sensitivity of SO2 detection. The gold electrode offers the best sensitivity for SO2 detection (more intense reduction currents) in wine at its native pH and in acidified wine, as well as high selectivity towards dioxygen present in the sample, compared to glassy carbon and platinum electrodes of the same dimensions.

[0177] Therefore, analysis in an acidified wine at pH < 1 with a gold measuring electrode and by DPV voltammetry constitutes the preferred protocol for measuring free or combined SO2 according to the invention.

Claims

DEMANDS 1. Electrochemical process for the detection and / or quantification of sulfur dioxide (SO2) in a liquid, the process comprising a step, known 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. 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 KCl reference electrode.

4. A method according to any one of the preceding claims, wherein the potential sweep is performed at speeds between 1 mV.s 1 and 1000 mV.s' 1 , and preferably between 10 mV.s -1 and 100 mV.s' 1 .

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 process according to claim 7, comprising 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.

9. A method according to any one of the preceding claims, comprising a step, referred to as 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 voltammeterogram 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. Method according to claim 11, the food liquid being wine.

13. A method according to any one of the preceding claims, comprising a pretreatment and activation step of the measuring electrode.

14. The 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 scanning substep, during cyclic voltammetry, carried out between -0.3 V and 1.5 V, using an Ag / AgCl, 3M KCl reference electrode, without measuring the current variation.

15. An 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 holding the liquid.

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