Disposable reference electrode

Provided for non-commercial research and education use. Not for reproduction, distribution or commercial use! ELSEVIER

SEHlORS ACTllA10IS

B

CHEMICAL

Sensors and Actuators B 24-25 (1995) 276--278

Disposable reference electrode C. Diekmann, C. Dumschat, K. Camm~mn, M. Knoll lnstitut fUr Chemo- Wid Biosensorik, Wilhelm-Klemm-Strusse 8, D-48149 Manster, Germany

Abstract In this paper we present a new low-cost disposable reference electrode. The construction of the electrode is based on a microfibre matrix, e.g., filter paper, which is filled with a redox electrolyte solution. The disposable reference electrode shows a reproducible and stable potential, which is little affected by the composition of the measuring solution. Keywords: Disposable electrode; Reference electrode

1. Introduction

The determination of ion activities with ion-selective electrodes (ISEs) is important for medical diagnostic and environmental analysis. In this application, especially, disposable electrodes have some advantages, e.g., the small probe volume and that no cleaning after contamination with blood or other dirt is necessary. Disposable ion-selective electrodes produced by a complicated multifilm method are available from Kodak [1,2]. Recently we developed a disposable ISE in double matrix membrane technology which can be produced economically [3--6]. These electrodes require a compatible reference electrode for practical use, which is described in this paper.

2. Experimental

2.1. Electrode design The electrode construction is shown in Fig. 1. For a simple construction, filter paper and heat-sealing films were used for the electrode body. A redox pair in an equimolar ratio is used as the internal electrolyte solution so that the electrode potential is nearly dilution independent. For an inert strip conductor a graphite material was used. All materials used are cheap mass products and allow production in a batch process, so that it is possible to produce a great number of lowcost reference electrodes. 0925-4005195/$09.50 © 1995 Elsevier Science S.A. All rights reseNed SSDl 0925-4005(94)01357-N

immersion end

contoclpad

45mrn

Fig. L Schematic view of the disposable reference electrode.

2.2. Electrode preparation In the first step graphite strip conductors (Conductive Carbon Cement, Neubauer, MUnster, Germany) are deposited on a sheet of filter paper (type 5893 Blue Ribbon, Schleicher & Schuell, Dassel, Germany) by using a coinputer-controlled dispersing system (Automove model 402, Asymtek, Carlsbad, CA, USA). The sheet is cut into stripes with a size of 45 mm X4 mm, which are insulated by laminating (Lamipacker LPD3203, Team Codor, Haltern, Germany) each with a perforated heat-sealing film (thickness 125 !-Lm, Team Codor). In this process a 10 mm long pad of the strip conductor is not sealed so that the electrode can later be contacted there. A barrier of a UV-curable epoxy resin (Vitralit" 1900, Panacol-Elosol, Oberursel, Germany) is applied to separate the internal electrolyte solution from the contact pad, thus avoiding a short circuit. Now the electrode is filled through an opening, obtained by cutting a small strip from the encapsulated electrode, by immersing it in a solution of 0.025 M K..[Fe(CN)6], 0.025 M K3 [Fe(CN)6] and 3 M KCl in

C. Diekmann et al. / Sensors and Actuators B 24-25 (1995) 276-278

E [mV]

concentration changes to

10"

277

M KGI

276

10.2 M KGI 272

268

10"

M KGI

f'>'"'---;,,~~----.._~ ..r--..!I

264

80

60

40

20

t [min) 6

Fig. 2. Stability of the potential difference bctwccn the disposable and the commercial reference electrode from 10- to 10- 2 M KCl.

E [mV] 300 295

290 285 260 I

10

15

t [min)

25

20

Fig. 3. Reproducibility and stability of 19 disposable reference electrodes from one lot. Table 1 Total change of the potential difference between the disposable reference electrode and the commercial AgiAgCI reference e1ectrodc in the given concentration alterations Concentration alteration

2

10-'-1010-'-10- 2 10-'_10- 2 pH=1O to

M KCI M NaCI M NH,Cl pH=2

----lfJl'f1:ll!Iraturell

-electrodl!lpolentiall

T[KJ

'10

305 300

Potential change [mY]

295 290

1.7 1.5 1.8

285

260 30

3.4

60

90

120

150

180

t[mln]

Fig. 4. Measurement of the temperature influence on the electrode pOlential in 130 mM NaCI and 5.4 mM KCl. Table 2 Reproducibility of electrode preparation: the potential of 19 electrodes out of one lot in a solution containing 130 mM NaCI and 5.4 mM KCl are listed with the confidence interval at the given times Time [min] 2 5

10 20

Mean value [mY]

Confidence interval [mY] (P-99%)

291.3 291.3 291.2 291.0

2.3 2.5 2.6 2.6

a nitrogen atmosphere for two days. After closing the opening to avoid drying up, the electrode can be stored. The opening has to be uncovered before use.

2.3. Measurements The remarkable feature of a reference electrode is a reproducible and stable potential that is not influenced by the measuring solution. In order to test the electrode characteristics, the following measurements were made by using a microprocessor pH-meter (pH 3000 with multiplex 3000, wrw, Weilheim, Germany) and a commercial reference electrode with a double junction (model 90-02, Orion, Boston, USA), which is filled with 3 M KCI. First we investigated if the electrode potential is affected by the outer electrolyte solution. Known amounts of different ions were added to the measuring solution. In order to investigate the electrode production reproducibility, a batch of 20 electrodes was fabricated.

278

C. Diekmann el al. I Sensors and Aclualors B 24-25 (1995) 276-278

One leaking electrode out of this batch was optically sorted out before measurement. Then it was tested whether the potential of the remaining 19 electrodes was identical and stable in an aqueous solution containing 130 mM NaCI and 5.4 mM KCI. For determining the temperature coefficient, the potential of five disposable electrodes was measured at 283, 298 and 313 K against a commercial reference electrode, which was connected by a salt bridge filled with 3 M KCI. The lifetime of the disposable electrode was evaluated by recording the potential stability in 130 mM NaCl and 5.4 mM KCl versus the Orion electrode.

3. Results and discussion

The investigations of the influence of the measuring solution composition on the reference electrode potential show that the potentials are nearly concentration independent (Table 1). One measurement for KCI is given as example in Fig. 2. In low-concentration solutions the potential shows noise and straggling caused by the low conductivity of the solution, whereas in more concentrated solutions the potential becomes stable. With regard to the potential reproducibility, it can be remarked that the electrode potentials of 19 electrodes from one lot are very similar and stable over 20 min (Table 2, Fig. 3). The confidence intervals are based on P=99% and n=19 measurements. Further, the temperature response was reversible with a value of -1.2 mV K- 1 in the range 283-313 K (Fig. 4). The drift was determined to be 0.2 mV h - J over 56 h, so that the lifetime is longer than two days. This long period is only of theoretical interest, because the electrode is designed to be disposable.

4. Conclusions The results show that the new disposable reference electrode is useful for many practical applications. In combination with a low-cost ion-selective electrode it is possible to determine ion activities in an easy way. The temperature dependence of the potential is a thermodynamic problem that should be reduced by using another redox couple. The lifetime of the electrode is surprisingly high, whereas storage shows complications, because the potential increases by oxidation of ~[Fe(CN)6J with O 2 and the inner electrolyte solution dries up. A better electrode design should prevent this drying up and reduce oxygen access. Further, another redox system that is more stable against oxygen could be used to improve the electrode characteristics. A traditional Ag/AgCl system with KCl solution is also possible for a disposable reference electrode. Acknowledgement We are grateful to the Ministerium fijr Wissenschaft und Forschung, Nordrhein-Westfalen, Germany, for financial support. References [I) c.J. Battaglia, J.C. Chang and D.S. Daniel, US Palent No. 4214 968 (1980). [2] Kodak EkJachem DT System, Kodak Diagnostik GmbH, Braunschweig, Germany, 1992. [3) M. Knoll, Gennan Patent No. DE 4115414 Al (1991). [4] M. Knoll, K. Cammann, C. Dumschat, M. Borchardt and G. Hogg, Micro fiber matrix supported ion-selective PVC-membranes, Sensors and Actualors B, 20 (1994) I. [5] C. Dumschat, M. Borchardt, C. Diekmann, J. Hepke, K. Cammann and M. Knoll, Double matrix membranes for potentiometric cation selective electrodes] Fresenius' Z. Anal. Chern., 348 (1994) 553. [6] M. Borchardt, C. Diekmann, C. Dumschat, K. Cammann and M. Knoll, Disposable sodium electrodes, Talanla, 41 (1994)

1025.