The present invention relates to a method for determining the quantity (eg concentration) of an oxidant of interest such as chlorine dioxide in an aqueous solution and to an electrochemical sensor for determining the quantity of an oxidant of interest in an aqueous solution.

The treatment of drinking water with chlorine can result in the production of potentially toxic trihalomethanes. Some trihalomethanes are known to be carcinogenic. Thus the use of chlorine dioxide for potable and wastewater disinfection is growing rapidly. Chlorine dioxide is a more effective bacterial and viral disinfectant than chlorine. It is very good at removing and preventing biofilm formation and does not cause an odour nuisance.

The use of chlorine dioxide is carefully regulated because of the toxicity of its active ingredient sodium chlorite. Chlorine dioxide is produced by acidification or the addition of chlorine to sodium chlorite. Chlorite is also a byproduct of the chlorine dioxide disinfection process. In the US, chlorite is regulated under stage 1 of the

Disinfectant/Disinfection Byproduct Rule of the 1996 Safe Drinking Water Act. A maximum contaminant level (MCL) for chlorite has been established at l mgL ^"1 .

Chlorine dioxide is particularly effective against Legionella bacteria. Under the HSE's Approved code of Practice (L8) for Legionella control, there is a requirement to determine the total oxidants present in drinking water. This is the combined chlorine dioxide and chlorite concentration.

The recognised method for chlorite determination is ion chromatography using a conductivity or UV/Vis detector. This is costly and time-consuming and requires specialist equipment and trained personnel on site.

The present invention seeks to improve the determination of an oxidant of interest (in particular chlorine dioxide, chlorine and chlorite) in aqueous solution by deploying measurement at the point of sampling in a method which is rapid, strai htforward and cost-effective.

Thus viewed from a first aspect the present invention provides a method for determining the quantity of an oxidant of interest in an aqueous solution comprising:

(A) applying a voltage through at least one working electrode to a sample of the aqueous solution in the presence or absence of a reductant which undergoes a redox reaction with the oxidant of interest to produce an oxidised reductant, wherein the voltage is effectively constant relative to a reference electrode and is sufficient to cause the oxidant or the oxidised reductant to be reduced at the working electrode;

(B) measuring the electric current generated at the working electrode in step (A); and

(C) relating the electric current to the quantity of the oxidant of interest in the aqueous solution.

The method advantgeously exhibits a substantially linear relationship between current and quantity of oxidant of interest over a useful range.

The aqueous solution may be potable water, recreational water or waste water (eg industrial waste water). Preferred is potable water.

Preferably the aqueous solution is a chlorine dioxide-containing aqueous solution.

The sample of the aqueous solution may be brought into contact with the working electrode by dipping the working electrode into the sample of the aqueous solution or by dosing the sample of the aqueous solution onto the working electrode.

Typically in step (C) the quantity of the oxidant of interest is its concentration.

Preferably the at least one working electrode is a pair of working electrodes. Preferably the (or each) working electrode is a part of a self-supporting electrochemical sensor. Preferably the electrochemical sensor comprises the at least one working electrode, the reference electrode and a counter electrode. The reference electrode and counter electrode may be a combined counter and reference electrode.

The electrochemical sensor may be portable. The electrochemical sensor may be single- use (eg disposable). This advantageously overcomes drawbacks associated with fouling or contamination and calibration drift.

Preferably the oxidant of interest is one or more of the group consisting of chlorine dioxide, chlorine and chlorite, particularly preferably chlorine dioxide.

Preferably the voltage is in the range -0.075 to -0.2V.

In a preferred embodiment, step (A) comprises:

(A l ) applying a first voltage through a first working electrode to a sample of the aqueous solution in the presence or absence of a reductant which undergoes a redox reaction with the oxidant of interest to produce an oxidised reductant, wherein the first voltage is effectively constant relative to a reference electrode and is sufficient to cause the oxidant or the oxidised reductant to be reduced at the first working electrode and

(A2) applying a second voltage through a second working electrode to a sample of the aqueous solution in the presence or absence of a reductant which undergoes a redox reaction with the oxidant of interest to produce an oxidised reductant, wherein the second voltage is effectively constant relative to a reference electrode and is sufficient to cause the oxidant or the oxidised reductant to be reduced at the second working electrode, wherein the first voltage is different from the second voltage.

By switching from step (A l ) to step (A2) at different voltages, it is possible to extend the linearity of the measurement over a greater concentration range. For example, the concentration of chlorine dioxide can be measured over a dynamic range <0.02 to 50mg/l. The first working electrode may be connected to a first current amplifier and the second working electrode may be connected to a second current amplifier, wherein the gain on the first amplifier is different from the gain on the second amplifier. By judicious selection of the gain on the first amplifier and the gain on the second amplifier and of the first voltage and second voltage, any tendency towards saturation at the surface of the working electrode can be advantageously overcome.

The reductant may be an iodide such as an alkali metal iodide (eg potassium iodide), N, N-diethyl-p-phenyldiamine (DPD) or tetramethylbenzidine (TMB). A preferred reductant is potassium iodide.

Typically the reductant is a component of a reagent formulation. The reagent formulation may further comprise one or more additives such as a buffer, gelling agent, thickening agent, wetting agent or stabiliser. Typical additives are one or more of the group consisting of sodium phosphate, potassium phthalate, sodium carbonate, disodium EDTA, hydroxylethylcellulose and polyvinylpyrrolidone. The reagent formulation may incorporate an acidic salt (eg sodium hydrogen sulphate) which in use reduces the pH to about 2.

The reagent formulation may take the form of a reagent layer on the surface of the working electrode. A reagent layer advantageously permits the redox reaction between the oxidant of interest and the reductant to occur intimately in situ.

The reagent formulation may be deposited and dried onto the surface of the working electrode to form the reagent layer.

The reagent layer may include a porous matrix. The reagent layer may include a porous matrix impregnated with the reductant. The porous matrix may comprise

polyvinylpyrrolidone and/or hydroxyethylcellulose. The reductant may be impregnated in the porous matrix by printing or microdosing. In a first embodiment of the invention, step (A) comprises:

(Aa) applying a voltage through at least one working electrode to a sample of the aqueous solution in the absence of a reductant which undergoes a redox reaction with the oxidant of interest to produce an oxidised reductant, wherein the voltage is effectively constant relative to a reference electrode and is sufficient to cause the oxidant to be reduced at the working electrode.

The first embodiment is advantageously a "reagent-less" method which is rapid, straightforward and cost-effective to deploy.

In the first embodiment, the oxidant of interest is preferably chlorine dioxide. In this embodiment, glycine may be added to the aqueous solution to remove chlorine (by converting it to chloramine). This avoids interference with the measurement of current by chlorine.

Preferably in the first embodiment, the pH of the sample of aqueous solution is in the range 4 to 9, particularly preferably 7 to 9 (eg about 7).

In a second embodiment of the invention, step (A) comprises:

(Ab) applying a voltage through at least one working electrode to a sample of the aqueous solution in the presence of a reductant which undergoes a redox reaction with the oxidant of interest to produce an oxidised reductant, wherein the voltage is effectively constant relative to a reference electrode and is sufficient to cause the oxidised reductant to be reduced at the working electrode.

Preferably step (Ab) is preceded by:

(AbO) degassing the sample of the aqueous solution to substantially remove chlorine dioxide. Particularly preferably step (AbO) is carried out by agitation (eg deformation, mixing or shaking). For example, step (AbO) may be carried out with a conventional stirrer (eg a milk frother).

In the second embodiment, the oxidant of interest is preferably chlorine. Where the oxidant of interest is chlorine, the sample of aqueous solution is preferably non-acidic, particularly preferably the pH of the sample of the aqueous solution is in the range 7 to 9.

In the second embodiment, the oxidant of interest is preferably chlorine and chlorite.

In the second embodiment, the oxidant of interest is preferably chlorite.

Where the oxidant of interest is chlorine and chlorite or chlorite, the sample of aqueous solution is preferably acidic (eg strongly acidic), particularly preferably the pH of the sample of the aqueous solution is less than 5, particularly preferably less than 4, more preferably less than 3, most preferably 2 or less.

Preferably step (Ab) is preceded by:

(AbO l ) acidification of the sample of aqueous solution.

Step (AbO l ) may be carried out with a conventional acid such as HC1.

Where the oxidant of interest is chlorite, step (Ab) preferably comprises:

(Ab l) applying a first voltage through at least one working electrode to a sample of the aqueous solution at a first pH in the range 7 to 9 in the presence of a reductant which undergoes a first redox reaction with chlorine to produce a first oxidised reductant, wherein the first voltage is effectively constant relative to the reference electrode and is sufficient to cause the oxidised reductant to be reduced at the working electrode

(Ab2) applying a second voltage through at least one working electrode to the sample of the aqueous solution at a second pH of less than 7 in the presence of the reductant which undergoes a second redox reaction with chlorite to produce a second oxidised reductant, wherein the second voltage is effectively constant relative to the reference electrode and is sufficient to cause the second oxidised reductant to be reduced at the working electrode

and step (B) comprises

(Bb l ) measuring a first electric current generated at the working electrode by step (Abl)

(Bb2) measuring a second electric current generated at the working electrode by step (Ab2)

and step (C) comprises

(Ca) relating the first electric current and the second electric current to the quantity of chlorite in the aqueous solution.

The first voltage and second voltage may be the same.

Preferably the first pH is in the range 7 to 9

Preferably the second pH is less than 5, particularly preferably less than 4, more preferably less than 3, most preferably 2 or less.

Step (Ca) may comprise subtracting the first electric current from the second electric current.

The at least one working electrode may be pre-calibrated or calibrated in situ.

The present invention further seeks to provide an electrochemical sensor which exploits a specific arrangement of electrodes to achieve a versatile and effective means for carrying out the method defined hereinbefore.

Viewed from a further aspect the present invention provides an electrochemical sensor for determining the quantity of an oxidant of interest in an aqueous solution comprising: an elongate substrate layer having a first end opposite to a second end;

a first, second, third and fourth conductive track deposited axially onto the substrate layer in a parallel mutually spaced apart relationship, wherein the first conductive track constitutes a reference electrode, wherein on the second conductive track near to the second end of the substrate layer is a carbon deposit whereby to constitute a counter electrode and on each of the third and fourth conductive tracks near to the second end of the substrate layer is a carbon deposit whereby to constitute a pair of working electrodes, wherein the first and second conductive tracks are adjacent and flanked by the third and fourth conductive tracks, wherein each of the first, second, third and fourth conductive tracks terminates near to the first end of the substrate layer in an electrical contact; and

a non-conductive layer deposited on the first, second, third and fourth conductive tracks, wherein the non-conductive layer is fabricated to fully expose each electrical contact near to the first end of the substrate layer, to fully expose the carbon deposit on the second conductive track near to the second end of the substrate layer, to fully expose the first conductive track near to the second end of the substrate layer and to partially expose discrete working regions of the carbon deposits of the third and fourth conductive tracks through an array of apertures.

According to the electrochemical sensor of the invention, it is possible to have a different voltage and different current amplifier gain for each working electrode which

advantageously allows a wider dynamic measurement range. Furthermore by deploying the two working electrodes on the flanks of the reference electrode and counter electrode in the specific arrangement of the electrochemical sensor according to the invention, there is minimal lateral diffusion of reagents between the pair of working electrodes and therefore minimal interference.

The electrochemical sensor has the advantage that the effective working area of each working electrode defined by the working regions is small which means that the total current passed by each working electrode is small. This makes it possible to operate in aqueous solution without the addition of a supporting electrolyte. Furthermore the array of apertures enhances the rate of mass transport and reduces the likelihood of double- layer capacitance. This leads to improved levels of detection.

The pair of working electrodes may be a first working electrode connected to a first current amplifier and a second working electrode connected to a second current amplifier.

The non-conductive layer may be fabricated by a known deposition or growth technique such as printing (eg screen printing, silk screen printing or thick film printing), casting, spinning, sputtering, lithography, vapour deposition, spray coating or vacuum deposition. Preferably the non-conductive layer is fabricated by screen printing. The non-conductive layer may be composed of a non-conductive ink.

Each conductive track may be fabricated by a known deposition or growth technique such as printing (eg screen printing, silk screen printing or thick film printing), casting, spinning, sputtering, lithography, vapour deposition, spray coating or vacuum deposition. Each conductive track may be composed of an inert metal such as gold, silver or platinum. Each conductive track may be composed of a conductive ink such as silver or silver/silver chloride ink. The conductive ink may be printable.

The substrate layer may be a sheet or strip. The substrate layer is typically composed of an insulating polymer. The substrate layer may be composed of polyester, polycarbonate or polyvinyl chloride.

The carbon deposit on each of the second, third and fourth conductive track may be deposited by known techniques such as printing (eg screen printing, silk screen printing or thick film printing), sputtering, lithography, vapour deposition, spray coating or vacuum deposition. The carbon deposit may be composed of inert carbon such as graphite, glassy carbon or pyrolytic carbon. The array of apertures may be fabricated in the non-conductive layer by a mechanical, chemical or physical removal technique such as ablation (eg photoablation) or etching. The array of apertures may be fabricated in the non-conductive layer by screen printing.

Each aperture may have a substantially regular shape. Typically the apertures are uniformly shaped. Each aperture may be substantially circular or non-circular (eg rectangular or square). Preferably each aperture is substantially circular. The array may adopt any suitable pattern (eg cubic). The array may comprise 10 to 500 apertures, preferably 50 to 200 apertures, more preferably 80 to 120, most preferably about 95 apertures.

Preferably each aperture has a dimension (eg diameter) in the range 50 to 400μηι, particularly preferably in the range 100 to 200μιη.

Typically the electrochemical sensor is interfaced with an instrument (preferably a portable field instrument) in a system which facilitates the electrochemical sensor to be operated amperometrically.

The electrochemical sensor may be integrated in an on-line system. Alternatively the electrochemical sensor may be portable. The electrochemical sensor may be single-use (eg disposable).

The present invention will now be described in a non-limitative sense with reference to the accompanying Figures in which:

Figure 1 illustrates an embodiment of an electrochemical sensor of the invention in (a) plan view and (b) cross sectional view;

Figure 2 is a typical dose-response curve for chlorine dioxide using an electrochemical sensor of the type illustrated in Figure 1 ;

Figure 3 is a typical dose-response curve for chlorine dioxide using an electrochemical sensor of the type illustrated in Figure 1 ; Figure 4 is a typical dose-response curve for chlorite at low range using an

electrochemical sensor of the type illustrated in Figure 1 ; and

Figure 5 is a typical dose-response curve for chlorite at high range using an

electrochemical sensor of the type illustrated in Figure 1.

Example 1 - Measurement of chlorine dioxide

The quantitative measurement of chlorine dioxide in an aqueous sample was carried out with an electrochemical sensor 1 shown in plan view in figure 1 (a) and in cross section in Figure 1 (b). The electrochemical sensor 1 comprises a substrate in the form of a polymeric strip 2 on to which successive layers are deposited progressively by screen printing. A first successive layer is composed of four parallel spaced apart conductive tracks 9 of a highly conductive printable ink such as silver or silver/silver chloride. Each of the conductive tracks 9 terminates near to a first end 1 1 of the strip 2 in an electrical contact 10.

A first of the four conductive tracks 9 constitutes a reference electrode 4. On a second of the four conductive tracks 9 near to a second end 1 1 of the strip 2 is deposited carbon to form a counter electrode 5. On a third and fourth of the four conductive tracks 9 near to the second end 1 1 of the strip 2 is deposited carbon to form a pair of working electrodes 6a, 6b. The working electrodes 6a, 6b flank the reference electrode 4 and the counter electrode 5.

Over the top of each electrode 4, 5, 6a, 6b is screen printed a layer of a first and second non-conductive ink 8. During screen printing , a screen used to deposit the first non- conductive ink is such that the electrical contacts 10 and the electrodes 4, 5, 6a, 6b are left exposed. A screen is used to deposit the second non-conductive ink on the carbon deposit of each working electrode 6a, 6b in such a way as to describe an array of apertures 7. Each aperture exposes a discrete working region of the working electrode 6a, 6b which thereby constitutes 95 discrete disc electrodes. The reference electrode 4 and the counter electrode 5 near to the second end 1 1 of the strip are left exposed. The electrochemical sensor 1 was interfaced with a suitable portable field instrument (ChlordioXense, Palintest Ltd) which allowed the electrochemical sensor 1 to be operated amperometrically. The electrochemical sensor 1 was dipped into the sample and a constant voltage of -0.075V (vs reference electrode 5) was applied to the working electrodes 6a, 6b. Chlorine dioxide is reduced at the working electrodes 6a, 6b according to equation (1 )

C10 ₂ + e ^" ► C10 ₂ - (1 )

The measured reduction current is proportional to the concentration of chlorine dioxide. A typical dose-response curve for chlorine dioxide is shown in Figure 2.

Example 2 - Measurement of Chlorine and Chlorite

Chlorine dioxide exists as a gas in aqueous solution. A 100ml sample of the aqueous solution was dispensed into a suitable vessel and vigorously agitated for 5 minutes using a milk "frother" to remove the chlorine dioxide in its entirety and leave behind any chlorine or chlorite.

A reagent containing potassium iodide was added dropwise to the sample at pH 7 using a dropper bottle. The reagent comprised potassium iodide (60%w/v), sodium carbonate (l %w/v) and disodium EDTA (1 .5%w/v). Chlorine in the sample reacted with iodide to produce iodine according to equation (2):

Cl ₂ + 2Γ _► l ₂ + 2C1 ^" (2)

The iodine was reduced at the working electrode 6a, 6b of the electrochemical sensor 1 as described hereinbefore and the current was measured using the field instrument. The measured reduction current is proportional to the concentration of chlorine. A typical dose-response curve for chlorine is shown in Figure 3. W 201

The measurement of chlorite is a continuation of the chlorine test. To determine the quantity of chlorite, the pH of the sample of the aqueous solution was adjusted to around pH 2 by the dropwise addition of hydrochloric acid (2.9M). At pH 2, chlorite reacts with iodide to produce iodine according to equation (3):

C10 ₂ ^" + 4Γ + 4H ^{+ ►} 2 + 2H ₂ 0 + CI ^" (3)

The acidified sample was allowed to react for 2 minutes. The iodine was reduced at the working electrode 6a, 6b of the electrochemical sensor 1 as described hereinbefore and the current was measured using the field instrument. The response is attributable to the presence of chlorine and chlorite. The quantity of chlorite was determined by subtraction of the quantity of chlorine measured according to the protocol described above.

A typical dose response curve for low range chlorite levels (0.05-5mg/l) is shown in Figure 4. By optimising one of the two working electrodes 6a, 6b, it is possible to determine the quantity of chlorite at higher concentrations by varying the applied voltage and the a/d amplifier gain. Figure 5 shows a typical dose response curve for high range chlorite levels (5-50mg/l).