GB2501769A - Electrochemical sensor for pH measurement - Google Patents

Abstract

A method of measuring pH of aqueous liquid with little or no buffer present uses an electrochemical sensor which has an electrode comprising a substrate 45, at least one redox active compound 46 on the substrate which is able to undergo a redox reaction involving both electron and proton transfer, such as an aromatic quinone, and a covering layer 49 over the redox active compound. The covering layer allows the passage of hydrogen ions to the redox active compound(s) through the covering layer by exchange of hydrogen atoms along a sequence of groups connected by hydrogen bonds. The covering layer may be formed from a polymeric compound, such as polyvinyl alcohol, or surfactant or lipid compounds. Such an electrochemical sensor may be used for pH measurement in computer controlled equipment for processing an aqueous liquid.

Description

Eleetroehemical Sensor for pH Measurement

Field of the Invention

Embodiments of this invention relate to electrochemical sensors for detecting and monitoring analytes, in particular for determining pH. Fields in which the invention may be utilised include, although are not restricted to, the analysis of water at the Earth's surface and also the analysis of a subterranean fluids which may be in an aquifer or downhole in a hydrocarbon reservoir.

Background of the Invention

There are numerous circumstances in which it is desirable to detect, measure or monitor a constituent of a fluid. One of the commonest requirements is to determine hydrogen ion concentration (generally expressed on the logarithmic p1-I scale) in aqueous fluids which may for example be a water supply, a composition in the course of production or an effluent. The determination of the pH of a solution is one of the most common analytical measurements and can be regarded as the most critical parameter in water chemistry. Merely by way of example, pH measurement is important in the pharmaceutical industiy, the food and beverage industry, the treatment and management of water and waste, chemical and biological research, hydrocarbon production and water supply monitoring. Nearly all water samples will have their pH tested at some stage during their handling as many chemical processes are dependent on ph.

It may also be desired to measure pH of a fluid downhole in a wellbore. The concentrations olsome chemical species, including Ht may change signiticanLly while tripping Lo the surface. The change occurs mainly due to a difference in temperature and pressure between do\vnhole and surface environment. In case of samples taken downhole, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and chemical reaction with the sampling chamber. The value of pH is among the parameters for corrosion and scale assessment. Consequently it is of considerable importance to determine pH downhole.

One approach to pH measurements, both at thc Earth's surface and downhole, employs a solid-state probe utilising redox chemistries at the surface of an electrode. Some redox active compounds (sometimes referred to as redox active species) display a redox potential which is dependent on hydrogen ion concentration in the electrolyte. By monitoring this redox potential electrochemically, pH can be determined. Voltammetry has been used as a desirable and convenient eleetroehemical method for monitoring the oxidation and reduction of a redox active species and it is known to immobilise the redox active species on or in proximity to an electrode.

Prior literature in this field has included W02005/066618 which disclosed a sensor in which two different pH sensitive molecular redox systems and a pH insensitive ferrocene reference were attached to the same substrate. One pH sensitive redox system was anthraquinone (AQ) and the second was either phenanthrenequinone (PAQ) or alternatively was N,N'-diphenyl-p-phcnylencdiaminc (DPPD). W02007/034 131 disclosed a sensor with two redox systems incorporated into a eopolymer. W020 10/001082 disclosed a sensor in which two different pH sensitive molecular rcdox systems were incorporated into a single small molecule which was immobilized on an electrode. W0201 0/111531 described a pH metering device using a working electrode in which a material which is sensitive to hydrogen ions (the analyte) chemically coupled to carbon and immobilised on the working electrode.

In an electrochemical cell, the electromotive force (e.m.f) of the cell is related to the concentration of an ion i by the Nernst equation which takes the form [11 E = E° + (k*T)*log(a) where E is the measured electromotive force (e.m.f) of the cell (all potentials are in volts), a4 corresponds to the activity of the ion 1, and E° is the standard potential (at temperature T) corresponding to the potential in a solution with the activity of ion I equal to one. The slope of a plot of E as a function of log(ai) together with the cell (electrode) constant (E°) may be experimentally determined in a calibration procedure using standard solutions with known activities of ion i. For good quality undamaged electrodes this slope should be very close to the theoretical one, equal to (R*T/F*z), where F is the Faraday constant (2306 I cal/mole), R is the gas constant (1.9872 cal/mole K) and z is the charge of ion 1. At low concentrations the conccntration of an ion is a good approximation to its activity and concentration can be used in the above equation.

The Ncrnst equation [1] can be rewritten for pH sensors, i.e. log a(W) as [2] E05 = K-(2.303 RTmInF)*pH where Eo.5 is the half-wave potential of the redox system involved, K is an arbitrary constant, R is the ideal gas constant, m is the number of protons and n is the number of electrons transferred in the redox reaction.

For practical purposes the key point is that observed potential of an electrochemical cell in which a redox active compound undergoes a redox reaction involving electron and proton transfer is proportional to pH if other factors remain constant. Calibration of an electrochemical sensor can be carried out using standard buffer solutions.

An issue with electrochemical sensors (particularly those involving detection mechanisms involving proton transfer) is the ability to make electrochemical measurements when there is no buffer and/or similar species that can facilitate proton transfer reactions. Measurements can be particularly difficult, and error prone, in low ionic strength media, without pH buffcring spccics andlor other species facilitating proton transfers. Measuring the pH of rainwater, and natural waters with very low mineralization, is noted as being particularly difficult.

Merely by way of example, in water industries, such as the management of reservoirs and waste management, the samples being tested or the reservoirs being monitored may not include a buffer solution or the like. Even in non-water industries, there may be occasions when the samples being tested or the fluid being monitored have low amounts of "natural buffers".

A pH scnsor is often tcstcd and calibrated using buffcr solutions which havc stable values of pH. Thc conccntration of buffer in such a solution may bc 0.1 molar or more. It has bccn discovered that cleetrochcmical scnsors utilising an immobilized rcdox compound can give good results when used in a buffered aqueous solution, and yct fail to do so whcn uscd in an unbuffered solution. A number of authors have appreciated this and it has been proposed that thc electrochemistry of quinoncs in unbuffcrcd, ncar neutral solution differs from that obscrved in buffered or strongly acid solution. See for example Quan ct al, J. Am.Chcm.Soc.

vol 129, pages 12847 to 12856 (2007). Quan et al argue that a different mechanism becomes operative in aqueous solution when proton concentration becomes low. Batchelor-McAulcy et al "Voltammetric Rcsponscs of Surthce-Bound and Solution-Phase Anthraquinone Moieties in the Presence of Unbuffered Aqueous Media" J. Phys. Chem. C vol 115 pages 714-718 (2011) attribute this phenomenon of different behaviour in unbuffered solution to depletion ofH' ion concentration in the vicinity of the electrode resulting in a significant local change in pH adjacent to the electrode and thus an erroneous determination ofpl-1 within the bulk solution. In unpublished work we have tried to overcome this by use of a rotating electrode to change the mass transport regime in the vicinity of the electrode, but without appreciable success.

Summary

We have found that anomalous values of pH can be obtained from an electrochemical sensor when there is no buffer and also when buffer is present in the electrolyte at low concentration.

We have also found that anomalous indication of pH by an electrode can be mitigated with a covering over the redox active compounds(s). In a first aspect, the present disclosure provides an electrode comprising a substrate, at least one redox active compound on the substrate which is able, in contact with aqueous solution, to undergo a redox reaction involving both electron and proton transfer and a covering layer over the redox active compound wherein the covering layer allows the passage ofhydrogen ions to the redox active compound.

The covering layer may display selectivity for hydrogen ions and in some embodiments transfer of hydrogen ions through the covering layer may take place by exchange of hydrogen atoms along a sequence of groups connected by hydrogen bonds.

A second aspect ofthe present disclosure provides a method of determining the concentration of an analyte using an electrode as above. The analyte may be hydrogen ions and the method may be applicable to measurement of the pH of aqueous liquids. The method may be applied where the liquid is unbuffered or where it may possibly contain buffer at a concentration up to 0.OlMolar (i.e. does not exceed 10 milliMolar).

Concentration of buffer is the total concentration of partially dissociated acid, base and!or salt which provides the stabilisation of pH. The method and!or the use of a sensor may be carried out to measure the pH of an aqueous liquid which contains buffer at a concentration of at least irn6 molar (0.001 mM) or possibly at least 5 x 106 molar (0.005 mM), or at least iO-5 molar or at least 1 o-molar. The concentration of buffer may perhaps be no more than 5 x 10 molar (5 mM) or even no more than 1 mM.

Because measurement can be made when buffer is at a low concentration, measurement can be performed on aqueous liquids where a small concentration of buffer may be present as a consequence of the origin of the liquid, for example measurement may be carried out on biological samples and natural products containing small concentrations oforganic acids which arc not fully ionised and provide some buffering of pH.

It is envisaged that the aqueous liquid may have a pH which is within two or three units of neutral. Thus the liquid may be mildly acidic from pH 4 or pH 5 up to pH 7 or mildly basic from pH 7 up to pH 9 or pH 10. The aqueous liquid may be liquid flowing within or sampled from equipment for processing the liquid and it may be a foodstuff or other material for human or animal consumption or an ingredient of such foodstuff or material. The aqueous liquid maybe one phase of a composition which is an emulsion, and it may be the continuous phase or a discontinuous phase of an emulsion.

Measuremcnt of pH by the stated method can be carricd out without measuring the buffer concentration. It is advantageous that the method can be employed when buffer concentration in the aqueous liquid is not known or is a parameter which cannot be controlled, without fear of an anomalous result because the concentration of buffer is low.

In a further aspect, the present disclosure provides apparatus to determine pH of water or other aqueous solution. Such apparatus may comprise an electrochemical sensor comprising a redox active compound immobilized to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, means to apply potential to the electrode and observe current flow, and a programmable computer connected and configured to receive current and/or voltage data from the sensor, wherein (as already mentioned above) wherein the electrode has a covering layer over the redox active compound and this covering layer allows the passage ofhydrogen ions to the redox active compound.

The covering layer may display selectivity for hydrogen ions and in some embodiments transfer of hydrogen ions through the covering layer may take place by exchange of hydrogen atoms along a sequence of groups connected by hydrogen bonds.

Such apparatus may be incorporated in equipment to process aqueous liquid, for instance process plant for water treatment, or to manufacture a pharmaceutical or a food product, and the computer which receives data from the sensor may be a computer which monitors or controls operation of that equipoment. Thus this disclosure also provides equipment for processing water or other aqueous liquid, including: a programmable computer operatively connected to control or monitor operation of the equipment, an electrochemical sensor comprising a redox active compound immobilized to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, wherein the electrode has a covering layer over the redox active compound and the covering layer allows thc passage of hydrogcn ions to the redox active compound, which may take place by cxchange of hydrogcn atoms along a sequcnce ofgroups connected by hydrogen bonds, and means to apply potential to the electrode and observe current flow; wherein the computer is connected and configured to receive current and/or voltage data from the sensor.

The means to apply potential to the electrode and observe current flow may be means to apply variable potential to the electrode with the redox-active compound immobilized thereon and then to determine the applied potential at a maximum current for redox reaction of the compound.

The electrochemical sensor may be positioned in the equipment so as to be exposed to liquid flowing within the equipment, or taken from it as a sample, possibly by automated sampling under control of the computer. A programmable computer may monitor the proper operation of equipment and give a readout to a human operator, or the computer may itself control operation of the equipment.

The liquid whose pH is measured by such apparatus and equipment may be unbuffered, or may contain buffer in a concentration up to or above 0.! molar. Incorporating an clcctrochemical sensor as defined mitigates the risk of anomalous determinations of pH in the event that the buffer concentration is low.

An electrochemical sensor may also comprise a second redox active compound as a reference, immobilized to the same or another electrode, where the oxidation and reduction of the reference redox active compound is substantially insensitive to pH.

Brief Description of the Drawings

Fig 1 shows the result of square wave voltammetry of PAQ in buffered and unbuffered solutions, and is discussed in Comparative Example 1; Fig 2 shows voltages at the current peaks in Fig I plotted against pH; Fig. 3 shows voltages at current peaks obtained in Comparative Example 2 plotted against minus log buffer concentration; Fig 4 is a cross section of an electrode with material deposited thereon, as used in the

Examples;

Fig 5 show the results of voltammetry in Example 1; Fig 6 shows a plot of potential against pH, obtained in Example 2; Fig 7 shows another possible electrode construction; Fig. 8 is a diagrammatic illustration of a cable-suspended tool for testing water; and Fig 9 is a diagrammatic view ofa flow line with means for taking samples and measuring the pH of the samples.

Detailed description

An electrode embodying the present invention has a substrate. This may be a conductive substrate and it may be metallic or may be a conductive form of carbon. Forms of carbon which have been used in electrodes include glassy carbon, carbon fibres, carbon black, various forms of graphite, carbon paste and carbon epoxy. One ffirther form of carbon, which has seen a large expansion in its use in the field of electrochemistry since its discovery in 1991 is the carbon nanotube (CNT). The structure of CNTs approximates to rolled-up sheets of graphite and can be formed as either single or multi-walled tubes. Single-\valled carbon nanotubes (SWCNTs) constitute a single, hollo\v graphite tube. Multi-walled carbon nanotubes (MWCNTs) on the other hand consist of several concentric tubes fitted one inside thc other. if the conductivc carbon is in a particulate form, it may be immobilized on another material, which may itself be a form of carbon or may be another material.

An insulating substrate may be used, if a conductive pathway to the redox active material is provided., possibly through conductive material mixed with the redox active compound so that a conductive mixture is deposited on an insulating substrate.

A considerable number of compounds arc known which undergo redox reaction involving the transfer of both electrons and protons. Redox active compounds which have been proposed for use in pH sensors include aromatic quinones, which have been mentioned in various documents including W02005/0666 IS and which undergo a two electron two proton redox reaction. Aromatic nitroso compounds which undergo a one electron one proton reaction have also been proposed, as for instance exemplified in W02010/106404. Quinones used as redox active compounds in embodiments of this invention may have condensed aromatic ring sytems, as for example naphthoquinone, anthraquinone and phenanthrenequinone (also referred to as phenanthraquinone). The latter two are illustrated below: OH 0 A;N c'\ _\\ JL T Ti i *C?&.2 r IF IF i ±2e+2H OH Anthraquinone HOQH -_) / C> ThZ(+ZB () j I Phenanthrenequinone I A redox active compound may be deposited on a conductive subsfrate by evaporation of a solution, or may be immobilised by chemical attachment, in particular by chemical attachment to carbon. This is refewed to as "dcrivitising" the carbon. A versatile method for derivitising carbon is the chemical reduction of a redox active compound covalently attached to a diazonium group, using hypophosphorous acid as the reducing agent. Derivitisation of carbon may also be canied out using a very strong base to convert a precursor to a reactive carbene which then ibrms covalent bonds to a carbon surftce, as described in W02010/106404.

In ilirther embodiments of the present invention, a redox active compound which is sensitive to the analyte concentration/pH may be screen printed onto a substrate which may be an insulating material. The redox active species may be combined with a binding material, which may be a conductive binding material such as a graphite-containing ink, and then screen printed onto the substrate.

In the present invention the redox active compound(s) on the substrate are covered with a layer ofmatcrial which allows hydrogen atoms to pass through the covering layer to the redox active compound.

The material may allow translbr of hydrogen ions through the material by exchange of hydrogen atoms along a sequence of groups in the material which are connected together by hydrogen bonds. A mechanism for the translbr of hydrogen atoms through water by transfer of hydrogen atoms from one more water molecule to another was proposed as early as 1806 by Grotthuss. Such a mcchanism was also suggested by Naglc and Morowitz in "Molecular mechanisms for proton transport in membranes" Proc. Nail. Acad. Sci. USA Vo175 pp298- 302 (1978) as a mechanism for proton transfer along a chain of organic molecules with hydroxy groujs linked by hydrogen bonds in the microbiology context of a transmembrane protein providing a pathway thr transfer of hydrogcn ions through a biological membrane.

As is explained by Nagle and Morowitz, the transfer of hydrogen ions by this mechanism entails a chain of transfers of hydrogen atoms from one group to an adjacent group with covalent bonds being formed in place of hydrogen bonds and formation of hydrogen bonds between atoms previously connected by covalent bonds, as shown below: H.

H H

LI V

A A

The covering layer may comprise one or more compounds incorporating a least one group which is able to participate in hydrogen bonding. Such groups contain both a hydrogen and an oxygen or nitrogen atom, the common examples being hydroxyl, amino and amido groups.

The concentration and/or positioning of such groups may provide pathways for hydrogen atom transfer from one such group to another. The chain of connected groups may include water molecules included within the covering layer and hydrogen bonded to organic molecules. This coveting layer of material over the redox active compounds may have propcrtics of sclcctivity, because hydrogcn jots transfer through it by an exchange process whcrcas other atoms cannot do so.

The molecules of the covering layer may also fbrm hydrogen bonds to the redox active compound under the covering layer and this may have the effect of reducing the activation energy for proton transfer to fbrm intermediates which have a transient existence in the oxidation or reduction reaction.

This cover layer may be formed from one or more water-insoluble compounds which maybe organic compounds. The covering layer may also be formed from organic compounds having some water solubility. Such compounds may or may not be polymeric. One possibility is polyvinyl alcohol, which is normally made by hydrolysis ofpolyvinyl acetate and has the theoretical foimula [-CHr-CHOH-]n But if hydrolysis is incomplete, the polymer will be a copolymer containing both [-CHr-CHOH-] and [-CHr-CHOAc-] Polyvinyl alcohol forms a film when an aqueous solution of it is evaporated. It remains water soluble, but dissolution when exposed to water is fairly slow.

Another category of materials which may be used for a covering layer are material with a polar portion attached to a non-polar portion. Such materials may be surfactants, and these may be nonionic surfactants with low water solubility These may have a hydrophobic alkyl or alkenyl group as the non-polar portion, and may be ethoxylated alcohols with an HLB value of 10 or less. Another category of materials with a polar head group and a hydrophobic tail is lipids which are naturally occurring materials with hydroxyl and/or phosphate groups in the polar head and one or more alkyl or alkenyl groups in the tail. Lipids with phosphate in the head are generally termed phospholipids.

In some embodiments, a polymer coating which is permeable to water may be applied on top ofthe covering layer already mentioned. A permeable polymer coating may prevent or reduce loss of a somcwhat water soluble covcr layer from the electrode and it may also prevent loss of redox active compound(s). A possible material for a water-permeable polymer layer is a polysulphone.

The invention will now be ftirther explained with reference to the following examples: Comparative Example 1 For this example the test electrode had phenanthraquinone (PAQ) deposited on it by evaporation of a solution of PAQ in dichloromethane. A pH insensitive electrode was prepared in the same way, using ferrocene as the redox compound. This electrode and the tcst cleetrode were electrically connected. Fig I shows as continuous curves the oxidative responses obtained by square wave voltammetry in pH 4, pH 7 and pH 9 buffers. The voltages at oxidative peak currents were plotted against pH as shown as Fig 2. The data points obtained in buffer solutions lie on an obvious straight line \vhich serves as a calibration for measuring the pH of other solutions.

Fig 1 also shows (as a dotted line) the voltammetric response when the electrnlyte was unbuffered 0.1 molar sodium chloride solution at pH 7. The oxidative peak current was at an anomalous low voltage, erroneously indicating a pH above 10. This anomalous data point is shown circled in Fig 2. This anomaly is also observed with anthraquinone (AQ) and other redox active molecules and has been reported by the Batchelor McAuley et al paper mentioned earlier.

Comparative Example 2 For this example the test electrode had anthraquinone (AQ) deposited on it in the manner described above. Voltarnmctry was calTicd out in aqueous solutions containing buffer at low concentration. Three buffers were used: A phosphate buffer contained Na2HPO4 and KH2PO4 in proportions to buffer the solution to pH7.0 as determined using a glass electrode. The molar concentration of buffer was the total molar concentration of all phosphate ions. A phthalate buffer contained potassium hydrogen phthalate with pH adjusted by addition of hydrochloric acid to pH4.0 as determined using a glass electrode. Buffer concentration was the total concentration of phthalate. A borate buffer contained boric acid and sodium tetraborate in proportions to buffer at pH9.0 as determined using a glass electrode. Buffer concentrations was the total molar concentration of all borate ions.

Square wave voltammetry was carried out in solutions containing these buffers at a variety of concentrations ranging from 0.0001 molar to 0.1 molar, together with potassium chloride where required to make up the electrolyte concentration to 0.1 Molar. The voltages corresponding to peak oxidative current were measured, and the results are set out in the

following table.

minus log Buffer Buffer(molar) conc. phthalate phosphate borate 0.1 I -0.34 -0.51 -0.66 0.01 2 -0.35 -0.52 -0.66 0.005 2.30 -0.36 -0.53 -0.66 0.003 2.52 -0.37 -0.55 -0.66 0.00 I 3 -0.44 -0.70 -0.67 0.0001 4 -0.73 -0.73 -0.73 It can be seen that the values of peak current measured in I O' molar (0. I millimolar) buffer differ from those in 0.1 molar buffer and in the case ofphosphate and phthalate buffers the value at somewhat higher buffer concentrations also differ from the values in 0.1 molar buffer.

Example 1

The end portion of a glassy carbon electrode used in this example is shown in diagrammatic cross section in Fig 4. Tt had a glassy carbon rod 10 in a tubular holder 12 exposing a circular end face 14 which is 3 mm in diameter. Anthraquinone was dissolved in dichioromethane at a concentration of 1 mg/ml and a 20 microlifre droplet of this solution was placed on exposed surface 14 of the carbon electrode. The solution was allowed to evaporate thus depositing anthraquinone on the electrode surface, as indicated diagrammatically at 16.

Polyvinyl alcohol, 80% hydrolysed, was dissolved in water at a concentration of 1 mg/ml and a 20 microlitre droplet was placed on the electrode surface. The water was allowed to evaporate and the electrode was then dried in an oven at 130°C. This procedure deposited a covering layer IS of polyvinyl alcohol over the anthraquinone 16.

The electrode was used as the working electrode for cyclic voltammetry using 0.1M sodium chloride in water as the electrolyte. This electrolyte was at neutral pH and contained no buffer. The voltammetry was carried out using a standard three electrode set up, with a standard calomel electrode as reference and a stainless steel rod as counter electrode. A potentiostat was used to cycle the applied potential over a range and record the current flow.

This experiment was then repeated, with the modification that after applying one drop of the polyvinyl alcohol solution and drying it a second drop was applied in thc samc way so as to increase the thickness of the covering layer. Voltammctry was then carried out as beforc.

In fIjrther repeats, the number of drops ofpolyvinyl alcohol which were applied and dried was progressively increased. The results ofvoltammetry are shown in Fig 5. An electrode with dcposited anthrquinone but no polyvinyl alcohol was also examined in this way and its voltammetric response is indicated by a broken line in FigS.

Without any covering layer of polyvinyl alcohol, the peak of the voltammetric wave was at a potential corresponding to an anomalous value of pH, above the true pH 7. As the number of droplets of polyvinyl alcohol used to form the covering layer was increased the potcntial of the peak current, i.e the peak of the voltammetric wave, progressively shifted towards a higher value as indicated by the arrow in Fig 5, thus corresponding to a less anomalous indication of pH.

Example 2

This examplc used a nonionic surfactant oliow water solubility. This was dodecyl ethoxylate ofthe formula C12 H25 (OCH2CH2)110H where n has an average value of 4. This surfactant was available commercially under the trade name Brij3O.

Anthraquinone was deposited on an electrode surface as in Example 1. Brij3O was dissolved in water at a concentration of 1mg/mi and degassed with a flow of nitrogen to remove any trapped oxygen. A 20 microlitre droplet was placed on the electrode surface. The water was allowed to evaporate under nitrogen and the electrode was then dried in an oven at 130°C.

This procedure deposited a covering layer of Brij3O over the anthraquinone.

Electrodes made as above were used as the working electrode for cyclic voltammetry, using reference electrode, counter electrode and potentiostat as in Example 1. Voltammetry was conducted in three types of buffer solutions and results are shown in Fig 6: in standard IIJPAC buffers (points shown by open squares), in Britton-Robinson buffer with successive KOH additions (points shown as open diamonds) and in 0.1 molar phosphate buffer solutions prepared at various values of pH (shown as grey triangles). The peak of the voltammetric wave was determined for each electrolyte and the results are shown as a graph in Fig 6 as the potential at peak current plotted against pH. The plots obtained by this calibration procedure were consistent for the three types of buffers, indicating the behaviour was repeatable and reliable in various buffered media. Two domains were observed with slopes -0.0427 V/pH and -0.0852 V/pH respectively. According to the Nernst equation, this would correspond to a (3e, 2W) and (35, 4W) process respectively, suggesting the system did not follow the simple Nernstian linear slope of a (25, 2W) transfer.

When voltammetry was carried out with unbuffered 0.1 M potassium chloride solution as electrolyte, the pH determined by means of the calibration plots from the observed potential at peak current (shown as a solid black square in Fig 6), was veiy close to the pH determined using a standard glass electrode (shown as a solid black triangle).

Example 3

Using the same procedure as in Example 2, phenanthrenequinonc (PAQ) was deposited on an electrode by evaporation from solution in dichioromethane, and then a covering layer of lecithin which is a phospholipid was applied over it. A comparative electrode had deposited PAQ but no covering layer. The electrodes was used to carry out voltammetry with unbuffered water having a pH of 7.4 as determined using a standard glass electrode, as electrolyte. The pH values obtained using the electrodes of this example were FAQ alone: 10.3 FAQ with lethecin cover layer: 8.7 Thus the anomalous indication of pH was reduced by the lethecin cover layer.

Fig 7 shows another possible electrode construction embodying this invention. An insulating substrate 45 is used. A conductive paste containing graphite and a pH sensitive redox compound is printed on one area 46 of the insulating substrate 45. A second conductive paste containing a pH insensitive ferrocene compound is printed on an area 47 as a reference electrode and both areas 46,47 are connected together and to a control unit which may be a potentiostat by conductivc tracks 48 on thc substrate. A covering layer indicated by its boundary 49 is applied over the area 46; this consists of one or more materials which allow transfer of hydrogen ions through the material by exchange of hydrogen atoms along a sequence of groups in the material which are connected together by hydrogen bonds.

Optionally the entire substrate with deposited materials theieon is finally covered with a water-permeable polymer.

An application of embodiments of electrochemical sensormaybe in the monitoring of underground bodies of water for the purposes of resource management. One or more sensox may be incorporated in a tool deployed on a cable from the surlice within a monitoring well drilled into an aquifer-either for short duration (as part of a logging operation) or longer tcrm (as part of a monitoring application). Thc dcployment of such a pH sensor within producing wells on a cable may provide information on produecd water quality. Also, the pH scnsor may be dcploycd in ixjcetion wdlls, e.g. when water is injected into an aquifcr for later rctricval, whcrc pH may be used to monitor the quality of thc watcr being injcctcd or rctricvcd.

Fig 8 illustrates a tool for investigating subterranean water. This tool has a cylindrical body 72 which is suspended from a cable 73. A pump 74 is accommodated within the body 72 and can be operated to draw subterranean water into a sampling chamber 76 in which there is a pH sensing electrode 78 such as that shown in Fig 7. The tool also encloses also encloses a unit 62 which is a potentiostat for supplying voltage to the electrode 78, measuring the current which flows and transmitting the results to the surface.

Another application of embodiments of the present invention may be in thc monitoring of water within a well penetrating a hydrocarbon reservoir. One or more sensors, which may for instance be such as shown in Fig 7, may be incorporated into a wireline tool, a measuring while drilling tool or a logging while drilling tool.

While the preceding uses of the electrochemical sensor are in the hydrocarbon and water industries, embodiments of the present invention may provide an electiochemical sensor for pH in research laboratories and in a wide range of industries, including food processing, pharmaceutical, medical, water management and treatment and biochemistry.

The electrochemical sensor may for instance be positioned in a flow line where it is exposed to a liquid whose pH is to be measured, or may be positioned to be exposed to liquid taken as a sample, for instance taken by an automated sampling procedure.

Fig 9 shows diagrammatically an arrangement for periodically taking samples and determining pH. An aqueous liquid to be sampled flows in line 53 as shown by arrows 55. A sampling tube 57 projects into the flow path. When a sample is to be taken, valve 58 is opened, allowing liquid to flow through the tube 57 into chamber 59. This chamber 59 has a sensor 60 within it for measuring the pH of fluid within the chamber 59. This sensor may be ofthc types shown in Fig 7 and is connected to a potentiostat 62. The line 53 is part of equipment 56 for processing water or other aqueous liquid. This plant is controlled by a programmable computer 63 which also operates the valve 58 when required and a further valve 64 for draining the chamber 59 through tube 65. Connections to the computer 63 are shown by broken lines. The computer may be programmed to maintain stable pH, so that pH measurement forms part of a control system, or it may monitor pH and alert a human supervisor if pH goes out of an acceptable range. The latter might be done as a check on incoming water or other aqueous feedstock, for instance.

Claims (16)

1. CLAIMSI An electrode comprising a substrate, at least one redox active compound on the substrate which is able to undergo a redox reaction involving both electron and proton transfer and a covering layer over the redox active compound wherein the covering layer allows the passage of hydrogen ions to the redox active compound(s).
2. 2. An electrode according to claim 1 wherein the covering layer selectively allows the passage of hydrogen ions to the redox active compound(s).
3. 3. Au electrode according to claim 1 wherein the covering layer enables transfer of hydrogen ions through the covering layer by exchange of hydrogen atoms along a sequence of groups connected by hydrogen bonds.
4. 4. An electrode according to any one of the preceding claims wherein the redox active compound(s) comprise an aromatic quinone.
5. 5. An electrode according to any one of the preceding claims wherein the covering layer comprises a polymer containing groups which form hydrogen bonds.
6. 6. An electrode according to any one of claims I to 4 wherein the covering layer comprises one or more materials which have a polar head group connected to a hydrophobic tail.
7. 7. An electrode according to claim 6 wherein the covering layer comprises one or more surfactants or lipids.
8. 8. A pH measuring apparatus incorporating an electrode according to any one of the preceding claims.
9. 9. A method of determining pH using an electrode according to any one of claims I to 7.
10. 10. A method according to claim 9 of determining the pH of an aqueous solution where the concentration of buffer is negligible.
11. I. Use of an electrode according to any one of claims I to 7 for determining the p1-I of an aqueous solution.
12. 12. Usc of an clcctrodc according to any onc of claims I to 7 for determining the pH of an unbuffered aqueous solution.
13. 13. Apparatus to determine pH of an aqueous liquid, comprising an electrochemical sensor comprising a redox active compound immobilized to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous pha.sc, means to apply potential to the electrode and obsen'e current flow, and a computer connected and configured to receive current and/or voltage data from the sensor, characterized in that the electrode has a covering layer over the redox active compound wherein the covering layer allows the passage of hydrogen ions to the redox active compound(s).
14. 14. Equipment for processing an aqueous liquid including a computer operatively connected to control or monitor operation of the equipment an electrochemical sensor comprising a redox active compound immobilized to an electrode and having at least one functional group convertible elcctrochernically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, and means to apply potential to the electrode and observe current flow; wherein the computer is connected and configured to receive current and!or voltage data from the sensor, characterized in that the electrode has a covering layer over the redox active compound wherein the covering layer allows the passage of hydrogen ions to the redox active compound(s).
15. IS. Apparatus or equipment according to claim 13 or claim 14 wherein the covering layer enables transfer of hydrogen ions through the covering Iaycr by exchangc of hydrogcn atoms along a sequence of groups connected by hydrogen bonds.
16. 16. Apparatus or equipment according to claim 13, claim 14 or claim 15 wherein the means to apply potential to the electrode and observe current flow is means to apply variable potential to the electrode with the redox-active compound immobilized thereon; and determine the applied potential at a maximum current for redox reaction of the compound.