refererence electrode agar

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Friday May 28 11:41 AM StyleTag -- Journal: ELECOM (Electrochemistry Communications) Article: 46

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Electrochemistry Communications 1 (1999) 180183

An agar-based silverNsilver chloride reference electrode for usein micro-electrochemistry

Achim Walter Hassel *, Koji Fushimi, Masahiro SeoLaboratory of Interfacial Electrochemistry, Graduate School of Engineering, Hokkaido University, Kita-13 Jo, Nishi-8 Chome, Kita-ku,

Sapporo 060-8628, Japan

Received 26 March 1999; received in revised form 15 April 1999; accepted 19 April 1999

Abstract

A miniaturised AgNAgCl reference electrode is described which can be easily set up. Its electrochemical behaviour was proven by micro-polarisation curves, electrochemical impedance spectroscopy and potential transients. A saturated potassium chloride solution was used whichwas solidified by adding agar. The electrode is as small as 800 mm in diameter and 5 mm in length, with further potential for down-sizing. Amodified version includes an agar salt bridge in the same capillary. After an induction period of 6 h the potential becomes stable within 1 mVfor more than 6 weeks. The electrode shows a slightly different reference potential, which is discussed in terms of the production process.q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Micro-reference electrodes; Micro-electrochemistry; Silversilver chloride; Scanning droplet cell

1. Introduction

The trend towards miniaturisation stops at nothing. Evenelectrochemical equipment is now being down-sized [1,2].

The need of making things smaller results from the grow-ing interests in localised electrochemical phenomena and inscanning techniques that are rapidly gaining in importance.

For some scanning techniques, such as scanning referenceelectrode [3], scanning probe impedance [4], pH micros-copy [5] or scanning droplet cell (SDC) [6,7], a reliablesmall reference electrode is essential or at least desirable.

Although micro-reference electrodes are widely used insensors the adjustment to the above-mentioned techniquesoften fails due to an incompatible geometry, lack of stability,high price or simply the availability.

In this paper we present a facile and inexpensive way toprepare reliable and easy to handle AgNAgCl micro-referenceelectrodes that can be easily adjusted individually. This typeof reference electrode was successfully used in a modifiedversion of the scanning droplet cell [7].

* Corresponding author. Tel.: q81-11-706-6738; fax: q81-11-706-6738;e-mail: [email protected]

2. Experimental

2.1. Chemicals

All solutions were prepared from reagent grade chemicalsand ultrapure water. The latter was prepared from aqua bides-tillata which was redistilled from KMnO4 and finally cleanedwith a Millipore Q filter system. The specific resistance washigher than 16.8 MV cm (298 K). An agar powder with ajelly strength of 400600 g cmy2 from Kanto Chemical Co.Inc., Tokyo, was used. Commercial AgNAgClNsat-KCl ref-erence electrodes (HS-205C, TOA Electronics Ltd., Tokyo)[8] were used for the comparative investigations.

2.2. Preparation

Silver chloride was deposited electrochemically on silverwire (diameter 150 mm, purity 99.9%) in 1 M HCl. Theelectrode was first cleaned and roughened by cycling 10 timesbetween 0.5 V (SHE) and y0.1 V (SHE) with a scan rateof 10 mV sy1. The silver chloride was then deposited bypolarising to 0.1 V (SHE) for 120 s and finally to 0.3 V(SHE) for 600 s. After rinsing with water and drying in airthe wire was ready for use. Commercially available glasscapillaries were formed to the desired shape and size using acapillary puller (PC-10) and a micro-grinder (EG-400,

A.W. Hassel et al. / Electrochemistry Communications 1 (1999) 180183 181


Fig. 1. Sketch of the gel-electrolyte silverNsilver chloride micro-referenceelectrode.

Fig. 2. Potential transient recorded immediately after preparation of themicro-reference electrode. Initially, the potential is 0.9 mV and increaseswithin 20 ks (6 h) to its final value between 1.2 and 1.3 mV. An observationfor 200 ks yields a potential value of Us(1.246"0.016) mV.

Narishige, Japan). The tip diameter of the capillary used inthe electrochemical characterisation was 50 mm.

For preparation of the solidified solutions, 4 wt.% agar wasadded to the saturated KCl solution. The mixture was thenboiled until the agar dissolved. To avoid cooling down orfurther evaporation of water from the electrolyte it was instan-taneously used. The agar electrolyte was then sucked throughthe wider end into the capillary. In order to keep the viscositylow during filling of the capillary it had to be preheatedotherwise an undesired solidification takes place, resulting ina blockage due to a cooling at the inner side of the glass.

After complete cooling and solidification of the agar elec-trolyte, the silver wire, which was mounted on a connector,was pricked into the gel electrolyte. The additional volumetaken by the wire causes a pressure that ensures a good contactbetween electrolyte and electrode. Finally, the gold-platedconnector was embedded in a polymer block (Araldite, Ciba-Geigy) to isolate it from the electrolyte and to increase themechanical stability. After hardening the reference electrodewas ready for use.

In order to prepare a combined reference electrode/saltbridge device, it was important first to suck the agar contain-ing KNO3 solution into the capillary and rapidly change to aKCl solution during sucking. In this manner, it becomes pos-sible to avoid a chloride contamination of the tip and a block-age of the capillary by a premature solidification. Thesubsequent processing was the same as described above.

2.3. Electronics

An Advantest R6452A multimeter was used at a samplingrate of 100 mHz to capture the data of the potential transients.The micro-polarisation curves and the impedance spectrawere recorded using a Solartron Schlumberger 1287 Electro-chemical Interface and a 1255 B Frequency Response Ana-lyser. All electronic devices were connected to a personalcomputer through the GPIB bus. The perturbation signal usedin the impedance measurement was 1 mV rms.

3. Results and discussion

Fig. 1 shows a sketch of the gel-electrolyte-based sil-verNsilver chloride micro-reference electrode. The diameterof the reference electrode is given by the capillary, whichwas usually 800 mm. Reference electrodes with outer diam-eters of 1200, 1000 or 500 mm were successfully constructedas well. For use with the SDC a length of 1015 mm has beenfound to be suitable, but even shorter versions of 5 mm havebeen realised. The combined reference electrode/salt bridgelooks essentially the same. The only difference is the fillingof the capillary. It goes without saying that the silver wiremust not extend into the salt bridge agar.

Further miniaturisation of the reference electrode or ref-erence electrode/salt bridge can be easily achieved usingsmaller capillaries and thinner silver wires. No effort was

made in this direction, since the described size proved usefulfor the SDC.

In order to characterise the reference electrode, potentialtransients were measured against a commercial, macroscopicAgNAgClNsat-KCl reference electrode. Both reference elec-trodes were immersed into a saturated KCl solution and thepotential was recorded for a few days. Fig. 2 shows such atransient where the potential was recorded immediately afterpreparation of the micro-reference electrode. In the verybeginning the potential is 0.9 mV and increases within 20 ks(5.6 h) to its final value between 1.2 and 1.3 mV. An obser-vation for 200 ks gives a potential value of Us

A.W. Hassel et al. / Electrochemistry Communications 1 (1999) 180183182


Fig. 3. Impedance spectra of two different reference electrode combinations.The commercial electrode was measured between two commercial referenceelectrodes. The micro-electrode was measured between a commercial and amicro-reference electrode. The impedance, calculated from the micro-polar-isation curve (Fig. 4), is also given.

Fig. 4. Micro-polarisation curve of a commercial reference combined witha micro-reference electrode. The open-circuit potential was EOPs2.705 mVafter 1 ks. Cycling was done with dU/dts1 mV sy1 between EOPy10 mVand EOPq10 mV. The dashed line drawn between the turning points has aslope of 11.4 mS. See text for details.

(1.246"0.016) mV. The potential time behaviour showssmall peaks of a few mV height, which appear on averageevery 10 ks. The shape of these peaks offers a sudden increaseand delayed relaxation. Most probably these peaks must beattributed to changes in the temperature of the laboratory,caused by the regulating system of the air-conditioning sys-tem. Since the volume of the micro-reference electrode ismuch smaller than that of the commercial referenceelectrode,its heat capacity is accordingly smaller. Hence, it will followchanges in temperature more rapidly than the macroscopicelectrode. These spikes could not be observed in additionalexperiments where both reference electrodes were stored ina plastic box. This box prevents temperature changes by iso-lating the electrodes from the ambient air convection.

In general, the stability of the potential is excellent. Longtime transients showed that the potential never deviated morethan 1 mV from this early average value for 6 weeks afterconstruction of the reference electrode.

Beside the excellent potential stability for a single refer-ence electrode, the potential usually differed slightly fromzero. Most of the micro-reference electrodes were found tohave a potential being 1 to 3 mV more positive than thecommercial reference electrode.

The following considerations should be made. Since someamount of agar powder as solidifying agent is used, the con-centration of water in the electrolyte decreases accordingly.If silver chloride is insoluble in the added agent, the concen-tration of silver ions should decrease with increasing agarconcentration. This should result in a small negativepotential,according to the Nernst equation:

RTEsE q ln c (1)0 OxFwhere R is the gas constant, T the temperature and F theFaraday constant. The concentration-dependent term isdirectly expressed by the concentration cOx, of the oxidisedspecies (Agq) since the concentration of the reduced species(Ags) remains constant. On the other hand, an interactionbetween the ions and the jelly must be taken into considera-tion. Probably, the activity of the ions is changed as a resultof an interaction between electrolyte and agar, resulting in asmall potential difference.

Electrochemical impedance spectra were performed tocharacterise the micro-reference electrode in the frequencydomain. Since the impedance of a single electrode cannot bedetermined directly, an indirect way has be chosen. First, acombination of two identical commercial AgNAgClNsat-KClreference electrodes was investigated. Then, the combinationof a micro-reference electrode with one of these commercialelectrodes was investigated in the same way. The results ofthese experiments are shown in Fig. 3. As expected, the impe-dance is significantly smaller for the combination of twocommercial electrodes. Both spectra have a similar shape. Ingeneral, the impedance shows only a very weak dependenceon frequency. No significant change is observed for frequen-cies above 10 Hz. Even in the lower frequency region, where

the strongest changes were found, the phase shift did notexceed y258. The overall change is a factor of three for achange in frequency of over seven orders of magnitude.

This measurement also proves the applicability of themicro-reference electrode for use in localised impedancemeasurements. No artefacts caused by the reference electrodemust be expected since no strong deviations from an ideal,completely frequency-independent behaviour were found.

Fig. 4 shows a micro-polarisation curve obtained from thecombination of a micro-reference electrode combined with acommercial-type one. This combination was chosen again toallow a direct comparison with the impedance data discussedabove.

First, the open-circuit potential was averaged over 1 ks tobe EOPs2.705 mV. The potential was then cycled with asweep rate of 1 mV sy1 between EOPy10 mV and EOPq10mV. The dashed line in Fig. 4, drawn between the turningpoints, has a slope I/ U of 11.4 mS. The reciprocal sloped dhas a value of 87.6 kV and represents the inner resistance ofthe reference electrode. In principle, this micro-polarisation

A.W. Hassel et al. / Electrochemistry Communications 1 (1999) 180183 183


curve can be interpreted in terms of an impedance measure-ment. The main difference is the triangular instead of sinu-soidal perturbation signal. The corresponding frequency canbe calculated from the perturbation amplitude and the sweeprate to be 1/40 ss0.025 Hz. The impedance taken from themicro-polarisation curve is plotted in Fig. 3 to allow a directcomparison. It should be noted that the perturbation ampli-tude is higher than for the impedance measurement.

According to [9] the exchange current can be calculatedfrom this slope using Eq. (2):

RT dII s (2)0 F dU

A value of 290 nA is calculated for this reference electrode.The micro-polarisation curve shows a hysteresis. The max-

imum deviation is found near the open-circuit potential,where it typically reaches 25% of the maximum polarisation.However, after some time it relaxes to its former open-circuitpotential again. This time depends on the height and time ofthe perturbation polarisation. Usually it relaxes within thetime scale shown in Fig. 1 for a freshly prepared electrode. Itshould be stressed that this deviation from the initial open-circuit potential is due to an unusual treatment which is notexpected for the reference electrode during normal use.

The results from the micro-polarisation curve demonstratethe stability and reversibility of the micro-reference electrodeeven under a considerably high current flow.

4. Conclusions

In this work a modified type of reference electrode is intro-duced. The basic idea is to use a solidified electrolyte. This

simplifies the production and handling of small referenceelectrodes. The principle can be used for different kinds ofreference electrodes. However, in this work a AgNAgClNsat-KCl was employed. Different electrodes were realised withdiameters of 5001200 mm and lengths of 515 mm, withfurther potential for down-sizing. The micro-reference elec-trodes were characterised in the time and frequency domains.Generally, the potential stability is excellent and the innerresistance is 10 times higher than a commercial electrode.

Acknowledgements

A.W.H. is indebted to the Japan Society for the Promotionof Science and the Alexander von HumboldtStiftung for apostdoctoral fellowship. The financial support of the Mon-busho through a Grant-in-Aid for Scientific Research (No.10555236) is gratefully acknowledged.

References

[1] J.W. Schultze (Ed.), Special Issue on Electrochemical MicrosystemsTechnology: Electrochim. Acta 42 (1997).

[2] T. Osaka (Ed.), Special Issue on Electrochemical Applications ofMicrotechnology, Electrochim. Acta, in preparation.

[3] H.S. Isaacs, G. Kissel, J. Electrochem. Soc. 119 (1972) 1629.[4] H.S. Isaacs, M.W. Kendig, Corrosion (Houston) 36 (1980) 269.[5] E. Klusmann, J.W. Schultze, Electrochim. Acta 42 (1997) 3123.[6] A.W. Hassel, M.M. Lohrengel, Electrochim. Acta 42 (1997) 3327.[7] A.W. Hassel, M. Seo, Proc. 8th Int. Conf. Passiv., 1999, in press.[8] G.J. Janz, in: D.J.G. Ives, G.J. Janz (Eds.), SilverSilver Halide Elec-

trodes in Reference Electrodes, Theory and Practice, Academic Press,New York, 1961, pp. 179230.

[9] C.S. Brossia, R.G. Kelly, Electrochim. Acta 41 (1996) 2579.

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