Electrochemical detection of monochloramine and / or ammonia

Abstract

Apparatus and method for the determination of the presence or quantity of monochloramine and / or ammonia in a liquid sample, utilising an electrochemical sensor 10 comprising working 11, reference 12 a

Description

Field of invention The present invention relates to an electrochemical method and apparatus for the analysis of a target analyte in a liquid sample and in particular, although not exclusively, to an electrochemical analysis system for the determination of the presence or quantity of monochloramine and / or ammonia in an aqueous sample. Background Monochloramine has found increasing use as a secondary disinfectant in drinking water as it possesses certain advantages over disinfection using only free chlorine (Ch / HOCl / OCl). These advantages are longevity / stability and a lower propensity for formation of harmful disinfection byproducts. Monochloramine is formed from the reaction between free chlorine and ammonia, which are deliberately added to drinking water systems in carefully controlled quantities. Ammonia is also present in natural waters and is toxic to aquatic life. Ammonia may be present in water as aqueous NH3, or as the conjugate acid form NH / . The latter may be considered as a weak acid with a pKa value of 9.25. As stated above, ammonia may be added deliberately to drinking water, but it may also be found in natural waters. Ammonia may enter natural water systems via processes such as the breakdown of organic waste matter and animal waste, in addition to industrial wastewater runoff. Chloramines are formed upon reaction between free chlorine species (Ch / HOCl / OCl) and ammonia species (NH3 / NH4+). The three inorganic chloramines that form are monochloramine, dichloramine and trichloramine, where the prefix relates to the number of chlorine atoms attached to nitrogen. Dichloramine and trichloramine have much lower thresholds for taste and smell in water, and therefore their presence is undesirable. There may also be health concerns with these latter two chloramines. The distribution of these chloramine species is dependent on factors such as chlorine / ammonia ratio, temperature and pH. The generalised equilibria for chloramine formation are shown below in Figure 1. 1. NH / + DCf NHjCI + H-D 2. 2NHXI + H* NHClj * NH / 3. SNHQj + 2Na5 + NH / There is a need to monitor the level of monochloramine and ammonia in water systems using a method that may readily detect these species without interference from free chlorine, dichloramine or tri chloramine. Various methods have been developed as detailed below. Amperometric titration (Standard method 4500-CID) is a method that generally requires a higher level of skill when compared to other methods. A sample is buffered at between pH 6.5 and 7.5 and is titrated with phenylarsine oxide (PAO) until current changes on a microammeter cease. Care must be taken when approaching the end point and the last increment may need to be subtracted to avoid over-titration. The free chlorine concentration is determined from the quantity of titrated PAO. After determining free chlorine, the monochloramine fraction is found by adding a small quantity of potassium iodide solution and continuing titration with PAO. The end point is found in the same way as described above. DPD colourimetric (Standard method 4500-CID) is a method of determining monochloramine concentration in a similar manner to that of free chlorine by addition of diethyl-p-phenylene diamine (DPD). Upon reaction with DPD at the appropriate pH a colour is formed, the intensity of which at 515 nm is proportional to the concentration of free chlorine in the sample. After this point a small crystal of potassium iodide (~0.1 mg) is added and the colour is read again immediately. The amount of potassium iodide added and the time taken for a reading may influence the extent of breakthrough from dichloramine. Phenate (Standard method 45OO-NH3F) is an indophenol based method that utilises the Berthelot reaction to produce a highly coloured compound that may be detected by colourimetry. Due to the high molar absorptivity of indophenol, the method has high sensitivity and is selective for monochloramine. The process is however rather slow. Therefore, a catalyst is typically employed for more rapid colour development. The Berthelot reaction proceeds as a series of steps, described here as: 1. Reaction of ammonia / ammonium species with hypochlorite / hypochlorous acid to form monochloramine. 2. Reaction of monochloramine with phenol to form an imine compound. 3. Reaction of imine with excess phenol to finally yield indophenol. This generalized reaction scheme is depicted below, where intermediate steps are omitted for clarity. However there remains a need for improvements in the apparatus and method to identify and / or quantify monochloramine and / or ammonia concentration in liquid samples. Summary of the Invention It is an objective to provide electrochemical based apparatus and methods for the determination of the presence and / or a quantity of monochloramine and / or ammonia in a liquid sample and in particular an aqueous sample. It is a further specific objective to provide apparatus and method for the effective, convenient and reliable detection of monochloramine and / or ammonia in a liquid sample using an electrochemical potentiostat capable of performing voltametric analysis. It is a further specific objective to provide apparatus and method that is portable and convenient to use at a variety of different locations and in different environments as required. Accordingly, the inventors provide electrochemical apparatus, methods and system based on an electrochemical sensor having a working, reference and counter electrode with the working electrode comprising a formulation that is specifically adapted for monochloramine and / or ammonia detection in a liquid sample. In particular, the present concept is specifically focussed to the electrochemical determination of monochloramine and / or ammonia over a wide analyte concentration range with high accuracy. In particular, the present concept provides a highly sensitive electrochemical analysis system with enhanced precision for target analyte concentration determination relative to existing techniques. The present apparatus and method takes advantage of the Berthelot chemistry and chemical pathways described herein. In particular, the present concept provides a dosed rather than bulk format Berthelot chemical analysis system. In particular, and according to aspects of the present concept, the electrochemical sensor is dosed with a formulation containing constituent components required for the Berthelot reaction in addition to components required for sensor dosing. According to a first aspect of the present concept there is provided an electrochemical sensor to determine the presence or a quantity of monochloramine and / or ammonia in a liquid sample comprising: a substrate; a working, reference and counter electrode provided at the substrate; and a coating provided on the working electrode, the coating comprising a formulation comprising: phenol or a phenol derivative; a complexing agent for phenol retention at the coating; and a coupling agent or a coupling agent precursor compound comprising a transition metal-based complex. Preferably the formulation further comprises any one or a combination of: a wetting agent; an adhesion promoter to promote adhesion of the coating at the substrate; a pH buffer; a coating stabilising agent. Preferably, the wetting agent comprises an alcohol, propan-2-ol or ethanol. Preferably, the complexing agent comprises any one or a combination of alpha-, beta- or gamma cyclodextrin or derivatives. Preferably, the adhesion promoter comprises any one or a combination of carboxymethyl cellulose or cellulose based polymers. Preferably, the pH buffer comprises boric acid and / or the stabilising agent comprises D-Glucitol. Preferably, the transition metal-based complex comprises any one or a combination of: a Mn, Cr or Fe based complex; nitroprusside or sodium nitroprusside; nitritopentacyanoferrate and / or aquopentacyanoferrate. Preferably, the formulation comprises the coupling agent precursor; and wherein the coupling agent is generated in-situ from the transition metal-based complex through the use of a preconditioning potential applied to the sensor. Preferably, the substrate is elongate having a first and second lengthwise ends. Preferably, the working, reference and counter electrodes comprise conductive tracks extending axially on the substrate between the first and second ends and spaced apart from one another in a widthwise direction across the substrate. Preferably, the reference electrode comprises a silver track and wherein the working and counter electrodes each comprise a carbon track. Preferably, a dielectric layer is provided over a region of the working, reference and counter electrodes at a position between the first and second ends of the substrate such that respective lengthwise end regions of the working, reference and counter electrodes are not covered by the dielectric layer and are exposed. Preferably, the conductive tracks of the working, reference and counter electrodes extend generally parallel to one another at the substrate. According to a further aspect of the present concept there is provided an electrochemical sensing kit to determine the presence or quantity of monochloramine comprising: an electrochemical sensor as claimed herein; and a first sample conditioning tablet comprising: an electrolyte salt; and a chlorine scavenger. Preferably, the electrolyte salt comprises any one or a combination of: KNO3, KC1 or Na2SO4; and / or the chlorine scavenger comprises an alkali metal nitrite salt or sodium nitrite. Preferably, the first sample conditioning tablet further comprises: a pH buffer; a pH control agent; and a dissolution agent. Preferably, the pH buffer comprises a weak acid buffer, citric acid or boric acid. Optionally, the pH control agent comprises any one or a combination a weak alkaline, a hydroxide or lithium hydroxide. Optionally, the dissolution agent comprises any one or a combination of an ion-exchange resin, a quaternary ammonium resin or 3-[(3-chlorophenyl)sulfonylamino]benzoic acid (such as Amberlite (RTM)). Optionally, the first sample conditioning tablet comprises potassium carbonate, a cellulosic material, a cellulosic derivative or microcrystalline cellulose (such as Avicel (RTM)) and an ionexchange resin or a quaternary ammonium resin. Preferably, to determine the presence or quantity of ammonia species the kit comprises a second sample conditioning table comprising: an electrolyte salt; and a source of chlorine. Preferably, the electrolyte salt comprises any one or a combination of: KNO3, KC1 or Na2SO4; and / or the source of chlorine may be a hypochlorite, typically calcium hypochlorite. Preferably, the second sample conditioning tablet further comprises: a pH control agent; and a dissolution agent. Preferably, the pH control agent comprises any one or a combination a weak alkaline, a hydroxide or lithium hydroxide. Optionally, the dissolution agent comprises any one or a combination of an ion-exchange resin, a quaternary ammonium resin or 3-[(3-chlorophenyl)sulfonylamino]benzoic acid (such as Amberlite (RTM)). Optionally, the second sample conditioning tablet comprises potassium carbonate, a cellulosic material, a cellulosic derivative or microcrystalline cellulose (such as Avicel (RTM)). According to a further aspect of the present concept there is provided an electrochemical method to determine the presence or quantity of monochloramine and / or ammonia in liquid sample comprising: providing an electrochemical sensor having working, reference and counter electrodes on a substrate, the working electrode comprising a coating, the coating comprising a formulation comprising: phenol or a phenol derivative; a complexing agent for phenol retention at the coating; and a coupling agent or a coupling agent precursor compound comprising a transition metal-based complex; at least partially immersing the electrochemical sensor in the liquid sample; and analysing the liquid sample by applying electrochemical voltammetry. Preferably, the electrochemical voltammetry is differential pulse voltammetry. Preferably, the transition metal-based complex comprises nitroprusside or sodium nitroprusside. Preferably, applying a preconditioning potential to the electrochemical sensor prior to applying the electrochemical voltammetry; and generating nitritopentacyanoferrate and / or aquopentacyanoferrate from the nitroprusside or sodium nitroprusside via said preconditioning potential applied to the electrochemical sensor. According to a specific implementation, the chemical reaction pathway may proceed at the sensor according to three distinct phases: 1. in-situ electrochemical generation of aquopentacyanoferrate [Fe(CN)sH2O]3' 2. Berthelot reaction proceeds generating indophenol compound 3. electrochemical analysis of indophenol using voltammetry and optionally differential pulse voltammetry (DPV) Advantageously, the sensor coating at the working electrode comprises a coupling agent precursor compound in the form of nitroprusside. By generating the active coupling agent being nitritopentacyanoferrate and / or aquopentacyanoferrate, the present apparatus and method provides a convenient and effective electrochemical analysis system. Brief description of drawings A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure lisa plan view of an electrochemical sensor comprising a working, reference and counter electrode according to a specific implementation; Figure 2 is a graph of predicted concentration versus pH of speciation of nitroprusside, nitritopentacyanoferrate and aquopentacyanoferrate at different pH values given a known starting concentration of nitroprusside; Figure 3 is a graph of differential pulse voltammogram of a sample containing NH2CI; Figure 4 is a graph of differential pulse voltammograms from samples containing increasing concentrations of NH2CI; Figure 5 is a graph of a calibration curve of NH2CI; Figure 6 is a graph of differential pulse voltammograms from samples containing increasing concentrations of NH4CI; Figure 7 is a graph of a calibration curve of NH4CI; Figure 8 is a flow diagram of a test for monochloramine and ammonia in a liquid sample according to a specific implementation of the present concept. Detailed description of preferred embodiment of the invention Referring to figure 1, sensor 10 comprises an electrically inert plastic substrate 14 having an approximate thickness of around 500 pm. Sensor 10 further comprises carbon electrical contacts 15 provided towards a first end 19 of the generally elongate sensor 10 to provide electrical connection to a potentiostat (not shown). An insulating layer 16 covers conductive tracks 17 to enable sensor 10 to be submerged in a liquid sample. Tracks 17 comprise silver or a silver-based material to provide low resistance current pathways connecting electrical contacts 15 to an array of electrodes provided at a second end 18 of sensor 10. In particular, sensor 10 comprises a carbon counter electrode 13 to allow passage of current between itself and a working electrode 11 and to ensure substantially no current flows into reference electrode 12. Reference electrode 12 comprises silver or a silver-based material to provide a stable potential from which other potentials are applied. Working electrode 11 comprises carbon augmented / coated with the formulation as detailed in Table 1 below. In particular, the formulation is dosed on to the carbon working electrode 11 and configured specifically to facilitate the Berthelot reaction in addition to possessing physical characteristics such as appropriate wetting. Table 1 details the constituent components, concentration and technical effect. Constituent Concentration Description Phenol Derivative 2.5-10 mM Reaction component Ethanol 10-30% Wetting agent Cyclodextrin 2.5-10 mM Complexing agent for phenol retention Nitroprusside 2.5-10 mM Coupling agent precursor Carboxymethyl cellulose 0.5-1% Polymer for adhesion Boric Acid 50-200 mM Buffer for pH control Sorbitol 10-30 mM Stabilising agent Table 1 - Typical Sensor Dosing Formulation The formulation, as detailed above, was drop-cast onto the screen-printed carbon electrode 11 which was subsequently dried in an oven and sealed in an air-tight package. Cyclodextrins are known to have an affinity for phenol molecules and help prevent loss of phenol during use. Additionally, the stabilising agent is effective to retain moisture and prevent cracks forming in the dosed layer during drying. Working electrode 11 provides -li the working / functioning electrode where reduction and / or oxidation of a target analyte species occurs to provide the resulting electrochemical current that is measured and analysed to determine target species concentration in the liquid sample. According to the present system and concept, it is understood that indophenol is oxidised, resulting in a peak. The electrochemical pathway may involve a first reduction stage (from an initial oxidised state of indophenol) or the indophenol may already be in its reduced state. The present concept is adapted specifically for monochloramine determination and analysis in addition to the determination and analysis of ammonia. Each analyte is analysed by pretreating the liquid sample using a specifically configured pre-electrochemical analysis tablet. The two formulations of the tablets are identified respectively in Tables 2 and 3 below that list the constituent compounds, the mass fractions and associated technical effects. Constituent Mass Fraction (wt%) Description Sodium Sulphate 50-75% Electrolyte Boric Acid 20-30% Buffer Lithium Hydroxide 5-10% pH control Sodium Nitrite 0.5-1.5% Chlorine scavenger Amberlite (RTM) 5-10% Dissolution agent Avicel (RTM) 5-10% Dissolution agent Potassium Carbonate 1-3% Dissolution agent Table 2 - Typical Tablet Formulation for Monochloramine Test Constituent Mass Fraction (wt%) Description Sodium Sulphate 50-75% Electrolyte Lithium Hydroxide 12-17% pH control HTH 1-3% Chlorine source Amberlite (RTM) 5-10% Dissolution agent Avicel (RTM) 5-10% Dissolution agent Potassium Carbonate 1-3% Dissolution agent Table 3 - Typical Tablet Formulation for Ammonia Test The tablets formulated according to Tables 2 and 3, as described, were sealed in air-tight packaging ready for use. The method described herein utilises the Berthelot chemistry, albeit in a dosed rather than bulk format. Sensor 10 and in particular working electrode 11 is dosed with the formulation of Table 1. The reaction proceeds in three distinct phases as detailed below: In-situ, electrochemical Bertelot reaction Electrochemical generation of AqF —► generating indophenol —► detection of indophenol product with DPV Sodium nitroprusside, Na2[Fe(CN)sNO]-2H2O, is added as a catalyst for the Berthelot reaction and allows for more rapid colour development. However that the actual catalyst present in the reaction is considered to be Aquopentacyanoferrate, [Fe(CN)sH2O]3', hereafter abbreviated as AqF. This conclusion is made after observing that the stoichiometry of the reaction between AqF and NH2CI is 1:1. Since AqF is consumed in the reaction it is referred to herein as a "coupling agent \ rather than catalyst. During the reaction, an equilibrium exists between nitroprusside, nitritopentacyanoferrate and AqF in alkaline solution as shown below: Fe(CN)5NCF- + 2OH* ' + H2O = 1.5 x 106 Fe(CN)sNO / + H2O Fe(CN)5OH2 + NO2 = 3 x 10 4 The distribution of these species as a function of pH is shown below in Figure 2, calculated using the equilibrium constants given above. Appreciable ratios of AqF can be seen to exist only at sufficiently high pH (>10). The oxygen reduction reaction (ORR) is a ubiquitous process in the field of electrochemistry, here it is used to facilitate the formation of AqF from NP by increasing the pH in the region surrounding the electrode surface. During the ORR, hydroxide ions are produced at the working electrode surface. This process, coupled with electrochemical reduction of NP from Fe(III) to Fe(II) is understood to encourage formation of AqF without the need for overly high solution pH, which may be detrimental to the electrochemical detection of indophenol. The result of this process therefore is that AqF may be generated from NP in-situ by control of the working electrode potential for a defined amount of time. After in-situ generation of the coupling agent AqF, the Berthelot reaction proceeds whereby indophenol is produced after the reaction between a phenol derivative and monochloramine. The overall reaction is shown in below. OH The rate of this reaction is strongly dependent on pH and any substituent groups present on the phenol molecule. A pH is typically selected such that the phenol is deprotonated, this distribution is governed by the pKa of the phenol species in question. Functional groups in the ortho positions on the benzene ring of phenol may either increase or decrease the rate of this reaction by donating or withdrawing electron density into the aromatic system, respectively. As the Berthelot reaction proceeds indophenol is formed in the dosed layer on the working electrode surface. This species is detected by electrochemical means and in particular using voltammetry and according to the preferred embodiment, Differential Pulse Voltammetry (DPV). This was observed to provide a method offering high sensitivity for the target analytes. When the dosed sensor 10 was tested in a sample solution containing monochloramine with DPV, two peaks were observed. The first of these at about -0.05 V is ascribed to the product of the Berthelot reaction and is proportional to monochloramine concentration. The second peak located at around 0.45 V is due to the oxidation of phenol that is dosed onto the sensor 10. Excess phenol is typically present in the dosing formulation and is therefore not fully depleted in the reaction, resulting in another peak. A voltammogram of this process is shown in Figure 3, where the two peaks can be clearly observed. A calibration curve may be generated by performing tests in samples containing known quantities of monochloramine. The voltammograms for a typical monochloramine calibration are shown in Figure 4, with associated calibration curve data shown in Figure 5. The voltammograms in Figure 4 only focus on the first peak for clarity as the second peak is not proportional to monochloramine concentration. Ammonia may also be detected by the same method, albeit with a preceding chemical reaction that yields monochloramine. The voltammograms for a typical ammonia calibration may be seen in Figure 6, with associated calibration curve data shown in Figure 7. The measurement of monochloramine proceeds as follows: tablet A (detailed in Table 2) primarily containing electrolyte and buffer was added to the sample and dissolved. Sensor 10 was then immersed into the sample and an initial potential applied to generate AqF in-situ. A different potential where no electrochemical reaction occurs was then applied for a longer duration, allowing the Berthelot reaction to proceed. After a pre-determined amount of time, DPV was performed resulting in a peak at around -0.05 V vs. Ag that is proportional to the concentration of monochloramine in sample. The magnitude of this peak was measured after baseline subtraction and is related to monochloramine concentration using a calibration curve. The measurement of total ammonia may be conducted in the same manner as described above albeit with two important distinctions. Tablet B (detailed in Table 3) contains a source of free chlorine and a strong base that imparts a higher pH to the sample once added. This facilitates the first stage of the Berthelot reaction as described herein. The sample solution was then allowed to react for approximately two minutes and the rest of the procedure is conducted as above for monochloramine except that in this instance total ammonia is measured (NH2CI + NH3 / NH4+). The concentration of free ammonia (NHs / NEU*) may be determined by subtraction of monochloramine from total ammonia. The entire process is shown schematically in Figure 8. Referring to figure 8 and Tables 2 and 3: at initial stage 30, tablet A is crushed into the liquid sample to ensure complete dissolution. At stage 31 the sensor is immersed in the sample and a potential applied to the working electrode for a specified time. At stage 32, the potential is applied to the working electrode for a specified time and the chemical reaction is allowed to proceed. At stage 33 the DPV is performed to produce an electrochemical signal. At stage 34, NH2CI concentration is calculated via a calibration curve from baseline-corrected peak magnitude. At initial stage 35, tablet B is crushed into the liquid sample to ensure complete dissolution. At stage 36 the initial reaction is allowed 5 to proceed for about two minutes. At stage 37 the sensor is immersed in the sample and a potential applied to the working electrode for a specified time. At stage 38, the potential is applied to the working electrode for a specified time and the chemical reaction is allowed to proceed. At stage 39, the DPV is performed to produce an electrochemical signal. At stage 40, the total ammonia concentration is calculated via a calibration curve from 10 baseline-corrected peak magnitude. At stage 41 the free ammonia is calculated as the difference between monochloramine and total ammonia.

Claims

1. An electrochemical sensor to determine the presence or a quantity ofmonochloramine and / or ammonia in a liquid sample comprising:a substrate;a working, reference and counter electrode provided at the substrate; anda coating provided on the working electrode, the coating comprising a formulation comprising:• phenol or a phenol derivative;• a complexing agent for phenol retention at the coating; and• a coupling agent or a coupling agent precursor compound comprising a transition metal-based complex.

2. The sensor as claimed in claim 1 wherein the formulation further comprises any one or a combination of:• a wetting agent;• an adhesion promoter to promote adhesion of the coating at the substrate;• pH buffer;• a coating stabilising agent.

3. The sensor as claimed in claim 2 comprising a wetting agent wherein the wetting agent comprises an alcohol, propan-2-ol or ethanol.

4. The sensor as claimed in claim 1 or 2 wherein the complexing agent comprises any one or a combination of alpha-, beta- or gamma cyclodextrin or derivatives.

5. The sensor as claimed in claim 2 comprising an adhesion promoter wherein the adhesion promoter comprises any one or a combination of carboxymethyl cellulose or cellulose based polymers.

6. The sensor as claimed in claim 2 comprising a pH buffer wherein the pH buffer comprises boric acid and / or the stabilising agent comprises D-Glucitol.

7. The sensor as claimed in any preceding claim wherein the transition metal-based complex comprises any one or a combination of:• a Mn, Cr or Fe based complex;• nitroprusside or sodium nitroprusside;• nitritopentacyanoferrate; and / or• aquopentacy anoferrate.

8. The sensor as claimed in any preceding claim wherein:• the formulation comprises the coupling agent precursor; and• wherein the coupling agent is generated in-situ from the transition metalbased complex through the use of a preconditioning potential applied to the sensor.

9. The sensor as claimed in any preceding claim wherein the substrate is elongate having first and second lengthwise ends.

10. The sensor as claimed in claim 9 wherein the working, reference and counter electrodes comprise conductive tracks extending axially on the substrate between the first and second ends and spaced apart from one another in a widthwise direction across the substrate.

11. The sensor as claimed in claim 10 wherein the reference electrode comprises a silver track and wherein the working and counter electrodes each comprise a carbon track.

12. The sensor as claimed in claims 10 or 11 further comprising a dielectric layer provided over a region of the working, reference and counter electrodes at a position between the first and second ends of the substrate such that respective lengthwise end regions of the working, reference and counter electrodes are not covered by the dielectric layer and are exposed.

13. The sensor as claimed in any one of claims 10 to 12 wherein the conductive tracks of the working, reference and counter electrodes extend generally parallel to one another at the substrate.

14. An electrochemical sensing kit to determine the presence or quantity of monochloramine comprising:an electrochemical sensor as claimed in any preceding claim; anda first sample conditioning tablet comprising:• an electrolyte salt; and• a chlorine scavenger.

15. The kit as claimed in claim 14 wherein:• the electrolyte salt comprises any one or a combination of: KNO3, KC1 or Na2SO4; and / or• the chlorine scavenger comprises an alkali metal nitrite salt or sodium nitrite.

16. The kit as claimed in claim 14 or 15 wherein the first sample conditioning tablet further comprises:• a pH buffer;• a pH control agent; and• a dissolution agent.

17. The kit as claimed in claim 16 wherein:• the pH buffer comprises a weak acid buffer, citric acid or boric acid• the pH control agent comprises any one or a combination a weak alkaline, a hydroxide or lithium hydroxide; and / or• the dissolution agent comprises any one or a combination of an ion-exchange resin, a quaternary ammonium resin or 3-[(3-chlorophenyl)sulfonylamino]benzoic acid.

18. The kit as claimed in claims 14 to 17 further comprising: a second sample conditioning tablet comprising:• an electrolyte salt; and• a source of chlorine.

19. The kit as claimed in claim 18 wherein:• the electrolyte salt comprises any one or a combination of: KNO3, KC1 or Na2SO4; and / or• the source of chlorine comprises a hypochlorite or calcium hypochlorite.

20. The kit as claimed in claim 18 or 19 wherein the second sample conditioning tablet further comprises:• a pH control agent; and• a dissolution agent.

21. The kit as claimed in claim 20 wherein:• the pH control agent comprises any one or a combination a weak alkaline, a hydroxide or lithium hydroxide; and / or• the dissolution agent comprises any one or a combination of an ion-exchange resin, a quaternary ammonium resin or 3-[(3-chlorophenyl)sulfonylamino]benzoic acid.

22. An electrochemical method to determine the presence or quantity of monochloramine and / or ammonia in liquid sample comprising:providing an electrochemical sensor having working, reference and counter electrodes on a substrate, the working electrode comprising a coating, the coating comprising a formulation comprising:• phenol or a phenol derivative;• a complexing agent for phenol retention at the coating; and• a coupling agent or a coupling agent precursor compound comprising a transition metal-based complex;at least partially immersing the electrochemical sensor in the liquid sample; and analysing the liquid sample by applying electrochemical voltammetry.

23. The method as claimed in claim 22 wherein the electrochemical voltammetry is differential pulse voltammetry.

24. The method as claimed in claims 22 or 23 wherein the transition metal-based5 complex comprises nitroprusside or sodium nitroprusside.

25. The method as claimed in claim 24 further comprising:• applying a preconditioning potential to the electrochemical sensor prior to applying the electrochemical voltammetry; and10 • generating nitritopentacyanoferrate and / or aquopentacyanoferrate from thenitroprusside or sodium nitroprusside via said preconditioning potential applied to the electrochemical sensor.

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