The present invention relates to a method for the electrochemical quantification of certain biocides used for controlling microbial growth. More specifically, the method according to the present invention comprises the electrochemical quantification of biocides present in aqueous materials of which the composition is susceptible to change over time. In such cases, it is important to be able to monitor the level of biocide by means of an easily-applicable procedure so that this can be done as often as required and the level of biocide adjusted if appropriate. Examples of such materials are reaction mixtures undergoing polymerisation, in which the nature and proportions of the components change as the reaction progresses. Other examples of such materials are those of which the composition can change gradually upon prolonged storage.

A particular example of a reaction mixture undergoing polymerisation is a latex polymerisation reaction mixture and a particular example of a material of which the composition can change over time as a result of storage is latex.

BACKGROUND OF THE INVENTION

Electrochemistry is a well-explored field of analytical chemistry, and cyclic voltammetry, amongst others, is a commonly utilised electrochemical analytical technique. Cyclic voltammetry (CV) is a method used for determining the kinetics of electrode processes, and is used to investigate electrochemical processes occurring within the system. The current in the electrochemical cell is monitored as the potential of the electrode is changed. The potential applied to the system under investigation is swept between defined potential values, yielding a cyclic voltammogram, which is a plot of current obtained against potential applied. When electrochemical processes occur, a peak is observed on the cyclic voltammogram due to a transfer of charge (electrons) which gives rise to a current flow. The size of the peak so obtained can be related to the concentration of the species under investigation. This allows for a relationship to be built up of current vs. concentration, thus leading to a method of

quantifying the amount of a particular species present in an analyte by virtue of the current value obtained.

Other commonly employed electrochemical techniques that can be employed in the method of the present invention include polarography and chronoamperometry. Each technique gives an electrochemical response and the potential at which this occurs permits identification of the chemical species of interest and the current intensity permits quantification of the species.

However, until now it has not been known to quantify a selected species such as a biocide when it is present in a hostile environment such as an aqueous reaction mixture undergoing polymerisation where the composition is changing as a result of the chemical reactions taking place.

Clearly, incorporating the appropriate quantity of biocide into the reaction mixture is of great importance, so as to provide a sufficient level of protection without causing the reaction mixture to be too toxic or too expensive by the addition of too much biocide. It was therefore one object of the present invention to provide a method of electrochemical analysis which enables the identification and quantification over time of certain biocides in aqueous polymerisation reaction mixtures, and at levels of concentration at which they are used commercially.

There is also the problem of being able to quantify the biocide in a product after the product has been stored for a period of time where the level of biocide may become depleted due to external conditions. Such quantifications are rarely routinely carried out. A method which would enable such quantification is therefore desirable.

A particular example of a material of which the composition can change in this way is latex.

The time latex is held in storage prior to shipment varies significantly from manufacturer to manufacturer and with the type and grade of the latex, hi some cases the latex may be stored for many days or even weeks prior to shipment. During that time there can be changes in the

composition of the latex and this ageing process may make the latex more susceptible to microbiological contamination and may even degrade the biocide that is present. It is therefore important to be able to quantify the amount of biocide present so that it can be adjusted as necessary prior to shipment of the product.

It has surprisingly been found that it is possible to carry out electrochemical identification and quantification of certain biocides in a controlled manner even while the species are in the hostile, dynamic and rapidly-changing chemical environment of an aqueous reaction mixture undergoing polymerisation.

In accordance with the present invention, there is provided a method for the identification and/or quantification of an isothiazolinone-based or pyridine-based biocide in an aqueous material of which the composition is susceptible to change over time, comprising applying an electrical potential to an electrode in the said material, and performing an electrochemical analysis thereon.

As mentioned previously, the material maybe a reaction mixture undergoing polymerisation, for example a reaction mixture undergoing emulsion polymerisation or dispersion polymerisation. A particular example of such a mixture is a latex polymerisation reaction mixture. Alternatively, the composition of the material may be susceptible to change over time as a result of storage. In that case, again, the material may be a latex.

In the method of the invention, the electrode may be immersed directly into the reaction mixture to be assessed, with no sample preparation being necessary. Optionally, a buffer solution, preferably a pH 7.4 phosphate buffer, as a supporting electrolyte, may be added.

The isothiazolinone-based biocide is preferably one of l,2-benzothiazolin-3-one (BIT), 5- chloro-2-methyl-4-isothiazolin-3-one (CMIT), 2-methyl-4-isothiazolin-3-one (MIT) and the pyridine-based biocide is preferably a pyrithione salt or complex. BIT is the most preferred biocide.

Examples of latex polymerisation systems in which the biocides may be successfully quantified are nitrile or substituted (meth)acrylate or acetate latexes, acrylics,

styrene/butadiene polymers, ethylene vinyl acetate polymers, polyvinyl acetate polymers or styrene/butadiene/N-methylol acrylamide polymers.

Polymerisation reaction mixtures are particularly hostile chemical environments for the electrochemical assay of biocides due to the constantly and rapidly changing dynamics of the reactions and the constantly changing proportions of reactive intermediates such as initiators, chain transfer agents, scavengers, chain terminators, monomers and polymer products present in the system. It is therefore particularly surprising that successful quantification of the biocides can be carried out under such conditions.

It is also envisaged that more than one redox or other electrochemically-active chemical biocide may be quantified at the same time by the method of the present invention, provided that such biocides do not possess identical electrochemical potentials.

A range of working electrodes may be used according to the present invention, such as planar gold electrodes, platinum electrodes, carbon or screen-printed carbon-based electrodes, or again sonochemically-fabricated micro-electrode arrays formed on carbon, gold, platinum or other suitable conductive surfaces. A particular example is a polyaniline-coated carbon ink electrode. Which electrode it is preferred to employ for any given analysis is dependent upon the particular chemical species being quantified and / or the chemistry of the substrate. For example a planar carbon electrode is particularly suitable for determining BIT, e.g. in nitrile polymer emulsions, and a polyaniline-coated carbon ink electrode is especially suitable for determining CMIT or MIT in vinyl acetate/ethylene copolymers. One of the preferred electrodes suitable for use in accordance with the present invention is described in WO 96/33403; alternatively as described above - planar carbon, gold or other conductive surface electrodes may be employed.

Owing to the greater viscosities of certain polymers, such as ethylene vinyl acetate latex, in order to obtain satisfactory data it may be necessary to dilute the sample with a supporting electrolyte such as a pH 7.4 phosphate buffer. Depending upon the actual viscosity of the polymer, polymer: electrolyte dilution ratios of between 1:10 and 10:1, preferably 1 :2 to 2:1, may be employed.

When the electrochemical analysis is performed, the quantification of the biocide may be achieved by calculation of the differentiation of the current response of the biocide from a baseline, wherein the baseline is defined as the response obtained from the same reaction mixture not containing the biocide.

The invention will be further illustrated with reference to the following Examples which are intended to be illustrative only and do not in any way limit the scope of the invention.

Brief Description of the Drawings

Figure 1 shows a cyclic voltammogram exhibiting a strong oxidative response obtained during the measurement of BIT in nitrile polymer emulsions in Example 1.

Figure 2 shows a graph of current against the concentration of BIT obtained using the data of Example 1.

Figure 3 shows a graph of current response against the concentration of BIT obtained during the measurement of BIT in an acrylic latex in Example 1.

Figures 4 and 5 are cyclic voltammograms for CMIT and MIT respectively in vinylacetate/ethylene copolymer emulsions

Example 1: The Measurement of l,2-Benzisothiazolin-3-one (BIT) in Latex Polymer Emulsions.

Summary:

Electrochemical procedures have been used to directly determine BIT in test samples of different grades of nitrile polymer emulsions ^a without sample pre-treatment.

1. Equipment:

Uniscan Potentiostat (e.g. PG 580) and Microarray planar carbon electrodes with data logging on a personal computer. Software used: UiEChem for instrument operation and Microsoft Excel for data processing.

2. Samples

• Polymer Emulsions of butadiene/acrylonitrile/methacrylic acid latices (nitrile polymer emulsions).

■ Sample A was produced without the addition of biocide.

■ Sample solutions of Proxel BD20 were prepared in undosed polymer emulsion (sample A) over a range of concentrations (e.g. 0.025 to 0.1 % w/w equivalent to 50 to 200ppm BIT).

■ Three test samples (B, C & D) of two grades of nitrile polymer emulsions produced with typical dose levels of BIT (approximately lOOppm). The grades differ slightly in their process conditions, in one polymerisation is initiated by redox couple in the other by sodium persulphate alone.

Analyte: Proxel BD20 ^b ^D - a 20% aqueous dispersion of BIT

3. Experimental

Two experimental procedures have been used:

• Cyclic voltammetry. Voltammetric sweep was in the cathodic (reductive) direction first and then anodic. Voltage ranges +/- 1.0V (vs. silver/silver chloride) and with a voltammetric scan rate of 5OmVs ^"1 • Chronoamperometry. Steady state potential utilised was dependant on the peak oxidation current observed and for BIT was typically approx +0.6V

The electrode was immersed directly into the sample solutions to be assessed, no sample preparation was necessary. This is a key feature as the determination of components such as BIT in complex chemical mixtures such as polymer emulsions usually requires a sample

preparation step to remove as much as possible of other components, in particular the polymers which can interfere with the determination, prior to analysis.

4. Results

1. The cyclic voltammograms obtained by the procedure described above display clear evidence of an oxidative peak at around +0.6V which has been found to be specific to

BIT, see Figure 1.

2. In the chronoamperometric experiments the current responses obtained for the Proxel BD20 dosed polymer emulsions displayed a current deviation (an increase) relative to the response obtained for the undosed polymer emulsion (sample A). This current increase can be related back to the concentration of BIT and was found to be linear over the range quoted, see Figure 2.

3. The data obtained for test samples B, C & D was used to calculate the BIT concentration in these samples by simple calculation from the current response and the gradient of the calibration graph given in Figure 2. The results obtained 78ppm, 92ppm & 97ppm are in agreement with values obtained by HPLC.

Notes: a. The case exemplifies nitrile polymer emulsions, however a similar calibration curve was observed for an acrylic latex. See Figure 3.

b. Proxel BD20 was used but other Arch UK Biocide formulations of BIT were found to give similar data when account is taken of the BIT concentration e.g. Proxel XL2 (9.3% BIT in aqueous propylene glycol), Proxel AQ (9.3% BIT in aqueous solution),

Proxel Ultra 10 (9.3% aqueous solution of BIT) & Proxel Ultra 5 (5.3% aqueous solution of BIT).

Example 2: The Measurement of 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT) in Latex Polymers.

Summary:

Electrochemical procedures have been used to directly detect CMIT in test samples of different concentrations of CMIT in a vinyl acetate/ethylene copolymer without sample pre- treatment.

1. Equipment:

Uniscan Potentiostat (e.g. PG 580) and polyaniline coated carbon ink electrodes with data logging on a personal computer. Software used: UiEChem for instrument operation and Microsoft Excel for data processing.

2. Samples

• Vinyl acetate/ethylene copolymer.

^■ Sample A was produced without the addition of biocide.

^■ Sample solutions of a CMIT formulation were prepared in undosed polymer emulsion (sample A) over a range of concentrations.

3. Experimental

• Cyclic voltammetry. Voltammetric sweep was in the cathodic (reductive) direction first and then anodic. Voltage ranges +/- 1.0V (vs. silver/silver chloride) and with a voltammetric scan rate of 5OmVs ^"1

The electrode was immersed directly into the sample solutions to be assessed, no sample preparation was necessary. This is a key feature as the determination of components such as CMIT in complex chemical mixtures such as latex copolymers usually requires a sample preparation step to remove as much as possible of other components, in particular the polymers which can interfere with the determination, prior to analysis.

4. Results

• The cyclic voltammograms obtained by the procedure described above show that an electrochemical reaction occurs for CMIT within the vinyl acetate/ethylene latex copolymer.

• The peak potential at which CMIT undergoes a reduction reaction ( — I-0.02V) is different to that observed for the reduction of the copolymer species alone ( — 0.1 IV). Also the current response observed for the CMIT is very much greater than that seen for the latex species (for both reductive and oxidative reactions). Indeed a current response of approximately 25microAmps is observed for a 1 lOppm concentration.

• It can be seen in Figure 4 that there are distinct changes in current responses observed when differing concentrations of CMIT in the latex copolymer are reviewed voltammetrically. In addition, for each differing concentration, the potential at which the CMIT/MIT undergoes reduction remains consistent (~ 0.02V vs. Ag/ AgCl).

• Indeed, there is an approximate four-fold increase in current response observed for a 120ppm concentration of CMIT (curve D) when compared to the current response for a 30pρm concentration (curve C). Thus a calibration profile can be produced as previously achieved for the BIT system (Example 1).

• Signal reproducibility in terms of concentration is good as there is a similar current response observed for similar CMIT concentrations.

Example 3: The Measurement of 2-methyl-4-isothiazolin-3-one (MIT) in Latex Polymers.

Summary: Electrochemical procedures have been used to directly detect MIT in a test sample of MIT in a vinyl acetate/ethylene copolymer without sample pre-treatment.

1. Equipment:

Uniscan Potentiostat (e.g. PG 580) and polyaniline coated carbon ink electrodes with data logging on a personal computer. Software used: UiEChem for instrument operation and Microsoft Excel for data processing.

2. Samples

• Vinyl acetate/ethylene copolymer.

^■ Sample A was produced without the addition of biocide.

^■ Sample solutions of a MIT formulation were prepared in undosed polymer emulsion (sample A) at a concentration of 250ppm.

3. Experimental

• Cyclic voltammetry. Voltammetric sweep was in the cathodic (reductive) direction first and then anodic. Voltage ranges +/- 1.0V (vs. silver/silver chloride) and with a voltammetric scan rate of 5OmVs ^'1

As in Example 2 the electrode was immersed directly into the sample solutions to be assessed, no sample preparation was necessary.

4. Results

• The cyclic voltammograms obtained by the procedure described above show that an electrochemical reaction occurs for MIT within the vinyl acetate/ethylene latex copolymer.

• The peak potential at which MIT undergoes a reduction reaction (~ - 0.3V) (curves Y, Z) is different to that observed for the reduction of the copolymer species alone (~

-0.1 IV) (curve X). Also the current response observed for the MIT is very much greater than that seen for the latex species (for both reductive and oxidative reactions). Indeed a current response of approximately 12.5microAmps is observed for a 250ppm concentration. • Thus measurement of MIT is feasible using poly(aniline) coated carbon electrodes as the signal response is excellent and the potential at which the MIT undergoes reduction is different to any potentials at which reduction reactions occur within the latex itself.