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Thick film silver-silver chloride reference electrodes

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1998 Meas. Sci. Technol. 9 1557

(http://iopscience.iop.org/0957-0233/9/9/027)

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Meas. Sci. Technol. 9 (1998) 1557–1565. Printed in the UK PII: S0957-0233(98)90542-2

Thick film silver–silver chloridereference electrodes

A W J Cranny an d J K Atkinson

Thick Film Unit, Department of Mechanical Engineering,University of Southampton, Highfield, Southampton SO17 1BJ, UK

Received 9 January 1998, accepted for publication 14 May 1998

Abstract. The fabrication of prototype thick film silver–silver chlorideelectrochemical reference electrodes is described. Combinations of commerciallyavailable and proprietary thick film pastes have been used in their construction in amulti-layer planar configuration modelled upon the structure of the classic singlejunction silver–silver chloride reference electrode cell. Several variations in thebasic electrode design were fabricated, involving combinations of one of threedifferent paste formulations for the silver–silver chloride layer coupled with one oftwo combinations of paste formulation for the salt containment matrix. The relativeperformances of these different versions of reference electrode were evaluated interms of their chloride ion sensitivity, hydration times required to achieve a stablepotential and usable lifetime. It is shown that, depending on the processingmethodology employed at certain stages in the fabrication of these devices, a largedegree of variation in characteristics can be achieved and therefore exploited in thedesign of reference electrodes suitable for a range of specific applications.

Keywords: reference electrode, ion selective electrode, silver–silver chloride,thick film sensors

1. Introduction

The general increase in the level of new and currentlegislation concerning environmental protection and healthand safety issues in the home and workplace over thelast decade has driven the need for the developmentof new low cost chemical sensors. The scientificcommunity has not been slow to respond, as is evidentby the number of publications on this area and theincreased predominance of time allocated to sessions onchemical sensors at international sensor conferences. Inparticular, the development of electrochemical ion-selectiveelectrodes has witnessed intensive research effort withnovel sensing structures being developed using widelydisparate technologies ranging from fibre optics through tomicromachining.

One particular form of electrochemical sensor thatshould perhaps receive more research effort than itcurrently does is the reference electrode. This is anessential electrode in any electrochemical potentiometricmeasurement system since the response of any other typeof ion-selective electrode is meaningless without a datumagainst which to quantifiably make a comparison. Thereare many classic types of reference electrode and mostgenerally include some ‘wet chemistry’ in their constructionin the form of an electrolyte solution. Implementing suchelectrolyte regions within new sensor designs as an in-linemanufacturing step has often proved to be a stumblingblock and could therefore be a possible reason for the

perceived low degree of research into these sensors. Apossible electronic production technology that may be ableto overcome this problem is thick film printing. Thistechnique has already proved itself commercially viablewith, for example, the mass production of the low costExacTechTM self diagnosis blood glucose test strips thatinclude a printed pseudo-reference electrode marketed byMedisense [1].

In this paper we describe the design, fabrication andfunctional evaluation of reference electrodes producedusing thick film printing techniques. The silver–silverchloride reference electrode was chosen as the traditionalelectrode to mimic since it is well understood. Thick filmversions of this reference electrode structure have beenfabricated on alumina substrates by the sequential printingand firing/curing of patterned layers of both commerciallyavailable and proprietary thick film pastes [2, 3]. Each thickfilm layer processed represents a discreet functional elementof the classic design.

2. Theory

The silver–silver chloride electrode is in essence an ion-selective electrode that exhibits a strong sensitivity to itslocal chloride ion concentration (more strictly, the chlorideion activity). The electrochemical reaction taking place atthis electrode is given by

AgCl(s) + e− � Ag(s) + Cl−(aq). (1)

0957-0233/98/091557+09$19.50 c© 1998 IOP Publishing Ltd 1557

A W J Cranny and J K Atkinson

The steady state or equilibrium potential (E) associatedwith this reaction is related to the activities of the reactantand dissociation products, and can be expressed by theNernst equation [4]

E = E0− 2.303

(RT

nF

)log10[CL−]α (2)

whereE0 is the electrode standard potential (a constant),n

is the number of electrons involved in the electrochemicalreaction,R is the molar gas constant,T is the absolutetemperature,F is the Faraday constant, [Cl−] representsthe concentration of free chloride ions andα is the chlorideion activity coefficient. At low chloride ion concentrations,and assuming ideal solution behaviour, the chloride ionactivity coefficient is unity and so the activity becomesequal to the concentration [5]. Thus the electrode potentialcan be considered to be proportional to the chloride ionconcentration rather than the chloride ion activity. Theterm 2.303(RT/nF ) is temperature dependent and reducesto 59.16 mV at 25◦C, and thus at this temperature,the silver–silver chloride electrode in theory exhibits a59.16 mV decrease in potential for every decade increasein the local chloride ion concentration. In other words,the electrochemical potential generated by the Ag/AgClreference electrode is directly proportional to the chlorideion concentration and, at 25◦C, obeys the relation:

E = E0− 59.16 log[Cl−] mV. (3)

If the silver–silver chloride electrode is located inan environment of constant chloride ion concentration(activity) then the potential generated by the electrode willalso remain constant for a fixed temperature. This isprecisely the function served by the surrounding electrolytein the classic reference electrode design, which is usuallyeither a saturated solution of potassium chloride salt ora solution of this salt at high molarity (e.g. 3.5 Mconcentration). In this classic design, the electrolyte iscontained within the electrode cell body and is not in directcontact with external test solutions in which the electrodeis used. Thus the chemical composition of this internalelectrolyte solution should remain constant and therefore soshould the electrochemically generated electrode potential.Thus it can be appreciated that this type of electrode canbe used to provide an electrochemical reference point ordatum against which other electrochemical measurementsmay be quantifiably performed.

Potassium chloride is generally chosen as the chloridesalt used in the classic reference electrode design becauseboth the chloride ion and the potassium ion have highvalues for their ion mobilities and are approximately ofequal value. This ensures a low-impedance path for ioniccurrent between the silver–silver chloride electrode withinthe reference electrode body and the external test solution,and the high ion mobilities improve the transient responsecharacteristics of the whole reference electrode.

3. Experimental details

3.1. Electrode fabrication

Several different types of reference electrode werefabricated using standard thick film processing techniques

Figure 1. Cross sectional view through a thick film Ag/AgClreference electrode.

[6]. The individual layers that comprised a particularreference electrode were sequentially printed on laserprofiled 2′′ × 2′′ × 0.0025′′ 96% pure alumina substrates(Coors, ADSR96) using a DEK 1202 precision screenprinter and a variety of patterned printing screens withstainless steel meshes of count appropriate for the pasterheologies. Each printed layer was dried in an infrared drier (DEK model 1209) and then, depending on thecomposition of the particular paste, was either fired at hightemperature in a belt furnace (BTU, model VQ41, six-zone,air atmosphere) or thermally cured at lower temperatures ina box oven with fan assisted air circulation (Gallenkamp).Where possible, commercially available thick film pasteswere used and appropriately processed in accordance withthe manufacturers’ specifications.

The basic electrode structure as shown in figure 1 (notto scale) consisted of a strip of silver (ESL, 9912-A),insulated along the majority of its length by a high-temperature firing glass dielectric paste (ESL, 4905-CH)to leave two remote ends exposed. One of theseends formed the interface for the subsequently processed(silver–)silver chloride layers, whilst the other provided asolderable contact for connection to external measurementcircuitry. Three different types of (silver–)silver chloridepastes were evaluated in different versions of the basicreference electrode structure: a commercially availableepoxy polymer Ag/AgCl paste (GEM, C50672R1) andtwo formulations prepared within our laboratories of alow firing temperature glass loaded with either silverchloride powder or a combination of silver and silverchloride powders (henceforth referred to as glassy AgCl andglassy Ag/AgCl respectively). The (silver–)silver chloridelayers of each reference electrode type were overprintedwith a potassium chloride salt containing layer formedfrom either a low melting point glass paste or from athermosetting modified silicone polymer paste. Both ofthese pastes were also produced within our laboratories.Finally, each of the reference electrodes was overprintedwith a polymer insulating paste to provide a degree ofwaterproofing that covered all of the electrode surfaceexcept the solderable end contact and was patterned suchas to leave a small hydration port over a region of theunderlying salt containing layer. The combination of threedifferent (silver–)silver chloride pastes and two differentpotassium chloride pastes meant that in theory six differenttypes of reference electrode could be constructed andevaluated. However, only five different types of electrodewere fabricated due to processing restrictions: it wouldhave proved pointless to try to fire the glassy potassium

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chloride layer over the polymer Ag/AgCl layer since theact of doing so would destroy the latter layer.

3.2. Paste formulation

Both of the glassy (silver–)silver chloride pastes and theglassy potassium chloride paste were produced by mixingappropriate powders into a commercial thick film overglazepaste (Heraeus, IP027) and triple roll milling the resultantmixtures to aid dispersion of the component parts withinthe paste. The glassy AgCl paste was made by loadingthe overglaze to equal proportions by weight with a high-purity silver chloride powder (MCA, biosensor grade). Thesecond glassy Ag/AgCl paste was made by first mixingtogether silver chloride powder (Aldrich, 99%) and silverpowder (Aldrich, 99.9%) in a 2:1 ratio by weight. Thiswas finely ground with an agate pestle and mortar beforemixing the composite powder with the overglaze in equalproportions. The glassy potassium chloride paste was alsomade by mixing finely ground potassium chloride powder(BDH, AnalR) with the overglaze in equal proportions. Allthree of these pastes were fired using a standard thick filmpaste temperature profile with a peak temperature of 500◦C.

The polymer potassium chloride paste was producedby mixing finely ground potassium chloride powder (BDH,AnalR) with a low-temperature thermosetting polymerdielectric paste (ESL, 240SB) to an approximate loadingof 50% by weight. This paste was cured in a box oven(Gallenkamp) at a temperature of 200◦C for 1 h.

3.3. Chloride ion sensitivity

During their fabrication, a number of samples of thedifferent types of reference electrode were removed afterthe printing and processing of the various (silver–)silverchloride layers to be evaluated for their chloride ionsensitivities. These sample electrodes were immersedin ultrapure analytical grade water and their potentialsmeasured with respect to a commercial double junctionAg/AgCl reference electrode (BDH, GelPlas, sat. KCl)using a high input impedance digital multimeter (Keithley,model 2001). The chloride ion concentration of thesolution was varied by the dropwise addition of 100 mMKCl solution and was measured using a Ciba–CorningCheckMate four-terminal conductivity probe and convertedto chloride ion concentration using the tables of Harnedand Owen [7]. The investigation was performed with thesolution temperature maintained within the limits 25±1 ◦C.

3.4. Hydration time

The hydration times of the various types of thick filmreference electrode (arbitrarily defined as the time requiredto exhibit a stable potential) were investigated by measuringthe potentials of a number of sample devices with respect toa commercial double junction Ag/AgCl reference electrode(BDH, GelPlas, sat. KCl) in a solution of deionizedwater at five minute intervals over a period of 20 h.Voltage measurements were performed using a high inputimpedance digital multimeter with data storage capability(Keithley, model 2001). All of the electrodes tested

were fabricated using the same modified silicone polymermaterial for the top layer sealant (ESL, 240SB) and hadthe same size hydration port (approximately 0.4 mm×0.4 mm). An additional investigation into the effect ofthe size of the hydration port on the hydration time ofone particular version of the thick film reference electrode(glassy Ag/AgCl layer and polymer salt matrix) wasalso performed under the same experimental conditionsdescribed above.

3.5. Electrode lifetime

The lifetimes of the various different types of thickfilm reference electrode fabricated were investigated byroutinely measuring the potentials of a number of sampledevices with respect to a commercial double junctionAg/AgCl reference electrode (BDH, GelPlas, sat. KCl) intap water using a high input impedance digital multimeter.Over the duration of this investigation, there was no directcontrol of the temperature of the test solution though it wasnoted that ambient temperature within the laboratory variedby no more than±3 ◦C about a nominal value of 21◦C.Throughout the course of the investigation, the referenceelectrodes under test remained in the tap water solutionwhich was topped up when necessary so as to leave theelectrodes immersed to a constant depth.

The effect of the material composition of the top sealantlayer on the lifetime of thick film reference electrodes wasalso studied under the same conditions using a numberof sample reference electrodes that had been fabricatedwith different pastes as their final sealant coating layer butwere otherwise identical in their construction (glassy AgCllayer and polymer salt matrix). Four commercial sealantmaterials were evaluated: an acrylic polymer (GEM,D60308R1); a UV curing polymer (Acheson, Electrodag425SS); a plate etch resist (RonaScreen, 550C-MTA); anda modified thermosetting silicone polymer (ESL, 240SB).Each device had the same size hydration port patterned intothe sealant layer as an open square above the underlyingpolymer salt layer, of approximate dimensions 0.4 mm×0.4 mm.

4. Results

4.1. Chloride ion sensitivity

Figure 2 shows the chloride ion sensitivities of thethree different types of (silver–)silver chloride paste overthree decades of chloride ion concentration, correspondingto solution conductivities ranging from 11µS toapproximately 14 mS. All three pastes exhibit nearperfect Nernstian behaviour with slope factors close tothe theoretical value of−59.16 mV/log[Cl−] at 25◦C.By extrapolating the individual data sets it is possibleto obtain values for the individual electrode standardpotentials,E0, and these were found to be quite similarfor each paste type evaluated. Overall, the performanceof the two home-made glassy pastes tracked the responseof the commercial polymer silver–silver chloride pasteextremely well (correlation>0.99) giving confidence in thefunctionality of these paste compositions. The results aresummarized in table 1.

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Figure 2. Chloride ion response of various thick film (silver–)silver chloride pastes.

Table 1. Summary of measured parameters for various thick film reference electrodes.

Cl ion Standard Hydration PotentialAg/AgCl sensitivity potential KCl salt time drift Lifetime Shelf lifePaste mV/log[Cl−] (mV) matrix (h) (mV day−1)a (days)b (months)c

Polymer −61.24 −9.94 Polymer 6 6.8 25 >6Ag/AgClGlassy AgCl −60.74 −9.99 Polymer 10 4.4 40 >12

Glassy 5 6.0 14 >12Glassy −59.99 −5.88 Polymer 4 — — >12Ag/AgCl

Glassy 1 4.2 20 >12

a Measured over that phase in the lifetime characteristics when the salt loss is constant (see figures 6and 7).b The period during which the salt loss (and hence the drift) are constant.c Dry storage.

4.2. Hydration time

The effect of hydration on the developed potentials of thevarious types of reference electrode fabricated are shownin figures 3 and 4, by composition of the salt containmentmatrix. Figure 3 shows typical responses of samplereference electrodes constructed using either of the threedifferent formulations of (silver–)silver chloride paste, butall having in common the polymer salt containment paste astheir respective electrolyte layers. Clearly, all three types ofreference electrode hydrate at different rates, with all threeeventually exhibiting a stable potential within 10 h. Thereference electrodes fabricated with the glassy AgCl layertake the longest time on average to stabilize and exhibit anoisy signal during the first 10 h. It is believed that this isprobably due to the lower concentration of free silver ionsin this paste, which ultimately results in a higher impedancedevice, when compared to the other two types which hadexcess silver included in their formulation.

Figure 4 shows the hydration responses for prototypereference electrodes fabricated with the glassy salt

containment layer. These versions appear to hydratefaster than their counterparts where the electrolyte saltis contained within a polymer matrix, with both typesexhibiting stable electrode potentials within 5 h. This maybe due to the mobility of the chloride ion being higher inthe glassy paste than it is in the polymer paste.

The effect of the size of the hydration port on thehydration time of thick film reference electrodes is shownin figure 5. Each of the sample electrodes investigatedis of the same construction (glassy Ag/AgCl layer andpolymer salt matrix) differing only in the dimensionsof the hydration port patterned within the top sealantlayer (silicone polymer) and positioned directly over theunderlying salt matrix, which ranged from 1.0 mm2 to12.0 mm2. The results clearly show that as the size ofthe hydration port is increased the rate of salt loss is alsoincreased, as indicated by the increase in the electrodepotential with respect to a commercial single junctionAg/AgCl reference electrode. This is not unexpected sincea larger hydration port simply exposes a greater area of theunderlying salt matrix to the external test solution.

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Thick film Ag–AgCl reference electrodes

Figure 3. Hydration response of various thick film reference electrodes with different formulations for the (silver–)silverchloride layer but all having the potassium chloride salt bound within a polymer matrix.

Figure 4. Hydration response of various thick film reference electrodes with different formulations for the (silver–)silverchloride layer but all having the potassium chloride salt bound within a glassy matrix.

4.3. Electrode lifetime

The results of lifetime studies on each of the various typesof thick film reference electrode fabricated are shown infigures 6 and 7, by composition of the salt containmentmatrix. All of the devices exhibit a similar responsepattern: an initial near linear increase in potential withtime followed by a more stable period when the changein potential with time is less dramatic. The initial periodwhere the potential steadily increases with time can beattributed to salt loss from the salt containment layer eitherdirectly through the hydration port or through the overlyingtop sealant layer. As salt is lost from the electrode, thelocal chloride ion concentration at the silver–silver chlorideinterface decreases and thus the electrochemical potentialrises in accordance with equation (3). The second phase

in the response, when the electrode potential appears morestable, corresponds to a period when the rate of salt lossbecomes negligible. At this time the potential developedis due to the residual chloride ion concentration retainedwithin the salt matrix and local to the silver–silver chlorideinterface, in accordance with the Nernst equation. If thisresidual concentration of chloride ions is low, then theelectrode will be more susceptible to variations in thechloride ion concentration of external test solutions. Inuse, this will ultimately limit the stability of the electrodepotential and thus define the accuracy of the use of thiselectrode as an electrochemical reference system.

The results suggest that the rate of salt loss appearsto be greater in reference electrodes fabricated with aglassy rather than a polymer salt containment matrix, whereidentical silver chloride layers are used. This would seem

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A W J Cranny and J K Atkinson

Figure 5. The hydration response of various thick film reference electrodes as a function of hydration port size, shown in thecurves in mm2.

Figure 6. Potential drift in the response of thick film reference electrodes with a polymer salt matrix, as a function of thecomposition of the Ag/AgCl layer formulation.

to support the hypothesis proposed previously that themobility of the chloride ion is greater in the glassy matrixthan it is in the polymer matrix. A comparative measure ofthe electrode potential drift can be derived for each devicefrom this initial salt loss period, and accordingly valueshave been determined and are summarized in table 1. Here,the potential drift has been arbitrarily defined as that rateof potential change over the first 21 days of the electrodelifetime. The results also show that the stable phase in theprototype reference electrode response is reached quickerfor those electrodes fabricated with the glassy salt matrix.

Salt loss from the thick film reference electrodes isunfortunately not constrained to controlled leaching fromthe hydration port, which would otherwise permit the designof electrode structures with known salt loss rates (and hencepotential drifts) based on hydration port dimensions. Salts

may also be lost to a lesser or greater extent through the topsealant layer either as a result of ion migration or simplybecause this layer does not provide sufficient waterproofing,and this is evident from the results shown in figure 8. Ofthe four commercial sealant materials tested, the modifiedsilicone polymer supplied by ESL exhibited the lowestsalt loss rate as indicated by the lower drift in electrodepotential.

5. Discussion

The observed behaviour of the fabricated prototype thickfilm reference electrodes can be explained with reference tothe known and well reported properties of the classic singlejunction Ag/AgCl reference electrode. In this electrode, the

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Figure 7. Potential drift in the response of thick film reference electrodes with a glassy salt matrix, as a function of thecomposition of the Ag/AgCl layer formulation.

Figure 8. Potential drift in the response of thick film reference electrodes as a function of the composition of the top sealantlayer.

value of the standard potential (E0) depends, among otherthings, on the quality of the silver–silver chloride interface,and the value of the electrode potential (E) is governed bythe concentration of chloride ions in the internal electrolytesolution. Although this electrolyte remains remote fromany external test solutions, when the electrode is in use, byvirtue of its containment within the electrode cell body, saltloss through the porous plug still inevitably occurs. Thuseven the classic Ag/AgCl reference electrode will exhibitsome degree of potential drift and for correct use, this typeof electrode should be routinely calibrated and its internalelectrolyte refreshed.

In a similar manner, the values for the standardpotential and the electrode potential of the thick filmreference electrodes also depend on the same parameters.

The quality of the silver–silver chloride interface onreference electrodes fabricated with each of the differentpaste formulations has been demonstrated by examiningthe chloride ion response (see figure 2). The nearperfect Nernstian behaviour observed gives assurance in thestability of this interface. The magnitude of the electrodepotential developed by each prototype reference electrodeis also governed by the localized chloride ion concentration.However, the chloride ion concentrations in the twodifferent salt containment pastes evaluated are different, andthis explains the differences in the observed stable electrodepotentials of the various types of thick film referenceelectrode fabricated. This is simply because although thetwo pastes were formulated from approximately the sameweight ratios of paste to salt, the densities of the former

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are quite disparate and therefore the actual concentrationof potassium chloride salt in the composite mixtures willbe different. In addition, differences in the stabilizedelectrode potentials were expected between devices thatwere constructed with the same salt matrix paste due tothe nature of the fabrication process: homogeneity in thepaste characteristics cannot be guaranteed and thus evenelectrodes printed within the same batch are expected tohave differences in their nominal salt concentration.

There is a major difference in the design between thethick film reference electrode structures investigated andthe classic Ag/AgCl reference electrode they are attemptingto mimic, which explains the requirement for a hydrationperiod in the former electrodes before they can be reliablyused. The electrolyte layer of the thick film referenceelectrode (i.e. the salt containment matrix) is not a liquidor a gel, as is the case with the classic reference electrodedesign, and therefore presents some degree of restriction onthe free movement of ions through this layer. Additionally,the potassium chloride salt contained within the salt matrixof the thick film reference electrode will have undergonevery little (if any) dissociation prior to use and so thenumber of free chloride ions will initially be small. Asa consequence, the thick film reference electrode requiresa finite period of time to hydrate during which the externaltest solution percolates through the salt containment matrixand, in doing so, presents itself as a medium in whichthe matrix bound salt may dissolve. When the externalsolution reaches the silver–silver chloride interface it beginsto dissolve the chloride containing salt localized in thisregion. Eventually the matrix is sufficiently hydrated thata direct pathway for ionic current is established betweenthe external test solution and the silver–silver chlorideinterface, via the hydration port, and the reference electrodethen exhibits a characteristic potential that is dependenton the concentration of chloride ions local to the silver–silver chloride interface. At this point the electrode may bereliably used in a measurement circuit.

Variations in the hydration time of the differenttypes of thick film reference electrode were expectedprimarily because the two different salt containment pastesinvestigated have different degrees of porosity. In addition,the solubility of the potassium chloride salt in the pastes willhave been different, and possibly there may be differencesin ion mobilities between these two pastes. All of thesefactors can account for the observed differences in thehydration times of the prototype reference electrodes (cffigures 3 and 4).

The lifetimes of the thick film reference electrodesultimately depend on the rate at which salt is lost fromthe confining paste, and the route of this loss may bethrough the hydration port or through the top layer sealant.The results have shown that through correct choice of thetop sealant layer and dimensioning of the hydration port itshould be possible to design an electrode that loses salt ina controlled and defined manner. This would mean that theelectrode potential will also change in a defined mannerwith time, and if the exact relationship is known, thenthe electrode response can be drift compensated and theservicing/calibration interval increased.

It should also be possible, by suitably combiningdifferent materials that comprise the functional layers of thereference electrode, to design electrodes specific to certainapplications. For example, the use of a glassy silver–silverchloride layer in conjunction with a glassy salt paste anda large hydration port in the top sealant (or no top sealantlayer at all) will produce a fast hydrating reference electrodewith limited lifetime. The hydration time of this devicecould be quickened even further by increasing the porosityof the salt containment layer. Methods for achieving thisinclude the addition of small silica beads to the paste or theaddition of carbon powder which ‘burns out’ when the pasteis fired to leave a hollowed structure. A reference electrodeof this type could possibly be used as a one-shot instrumentand thus short operational lifetime would not necessarily bea problem. Alternatively, long-lifetime reference electrodesthat may have a concomitant lower accuracy could also bedesigned for applications where serviceability precludes theuse of more accurate though short-lived devices.

The near perfect Nernstian behaviour exhibited by thedeveloped (silver–)silver chloride pastes would suggest thatelectrode structures fabricated up to this stage only, duringthe processing of a full reference electrode structure, couldbe used in their own right as sensors for the measurementof chloride ion activity. It is also possible to changethe chemical composition of the pastes developed byusing other halides of silver to produce, for example, asilver–silver bromide reference electrode. Naturally, aconcomitant change in the composition of the halide saltused in the salt matrix pastes will also be required. Theseother silver–silver halide reference electrodes may possiblyfind use in other areas (e.g. in systems where the chlorideion activity is widely variable which could otherwise causeinstability in the potential developed by a silver–silverchloride electrode).

6. Conclusions

A method for fabricating silver–silver chloride referenceelectrodes using thick film printing and processingtechniques has been described. The electrodes wereconstructed in a planar configuration with each separateprinted layer representing a specific functional element ofthe classic silver–silver chloride single junction referenceelectrode. This work has perforce required the developmentof new pastes compatible with the enabling technology, inparticular the formulation of (silver–)silver chloride pastesand of pastes containing potassium chloride salt. Resultsfrom experimental evaluation have shown that the chemicalbehaviour of the developed (silver–)silver chloride pastesobeys the Nernst condition (i.e. the developed electrodepotentials exhibit a defined relationship with the localchloride ion activity). Such electrodes could therefore beused as sensors for the direct measurement of chloride ionconcentration. It has also been shown that, dependingon the combinations of pastes used during manufacture,thick film reference electrodes with different properties (e.g.hydration time, stability and lifetime) can be produced.Thus it should prove possible to develop application-specific thick film reference electrodes for use in a widerange of chemical measurements.

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[3] Moss A J, Hewinson J, Walton P, Birch B J, Ball C L,Cranny A, Atkinson J K and Siuda P R 1996Water Quality Sensor ApparatusUS Patent 5483164

[4] Pletcher D 1991A First Course in Electrode Processes(Romsey: The Electrochemical Consultancy, AlresfordPress) ch 1

[5] Bockris J O’M and Reddy A K N 1973ModernElectrochemistryvol 1 (New York: Plenum) ch 3

[6] Holmes P J and Loasby R G 1976Handbook of Thick FilmTechnology(Glasgow: Electrochemical)

[7] Harned H S and Owen B B 1958Physical Chemistry ofElectrolytic Solutions3rd edn (New York: Reinhold) App A

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