E L S E V I E R Talanta 42 (1995) 1685-1690

Talanta

Improvement of the silver/silver chloride reference electrode and its application to pH measurement

Satoshi Ito '~,b,*, Hiromitu Hachiya a, Keiko Baba a, Yasukazu Asano ~, Hiroko W a d a b

*Research Center, DKK Corp., 4-13-14, Kichijofi Kitamachh Musashino, Tokyo 180, Japan bDepartment of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan

Received 16 December 1994; revised 24 April 1995; accepted 24 April 1995

Abstract

When a silver/silver chloride (Ag/AgCI) reference electrode was used continuously in a low conductivity solution or reductive solution, it was often observed that stability of the liquid junction potential was lost, This phenomenon was remarkable with a Ag/AgCI reference electrode compared to a calomel reference electrode. We found that 340 mg 1-~ of silver was dissolved in 3 M potassium chloride (KCI) internal solution as silver complex ions (AgCI; (x- ~)) for x = 2 or 3. However, only 1.93 mg 1-~ of silver chloride (AgCI) can theoretically be dissolved in water. The complex ion that effused into the sample solution through the liquid junction clogged the liquid junction (e.g. porous ceramic) as AgC1, or as metallic silver (Ag) in reducing solution. Therefore, the constant effusion of KCI internal solution was inhibited, and the liquid junction potential became unstable or fluctuating. A new reference electrode was developed, which can eliminate AgCI~ - t x - i) in 3 M KCI internal solution by the use of chelating resins. A combination of this reference electrode with a pH electrode made long-term stable pH measurements possible.

Keywords: pH measurement; silver/silver chloride reference electrode

1. Introduction

The stability of a reference electrode in po- tentiometry is very important for the reliability and accuracy of the obtained data because a potential error of l mV causes about 0.02 pH error in pH measurement and also an approxi- mate 4 or 8% concentration error, respectively, for mono- and divalent ion measurements us- ing ion-selective electrodes.

The calomel electrode introduced by Ost- wald in 1890 has been used as a reference electrode for glass pH electrode measurements,

*Corresponding author. Present address: Research Center, DKK Corp., 4-13-14, Kichijoji Kitamachi, Musashino, Tokyo 180, Japan. Fax: (81)422-52-2042.

0039-9140/95/$09.50 © 1995 Elsevier Science B.V. All rights SSD! 0039-9140(95)01628-7

since their inception [1]. These days, because of the toxicity and environmental concern of mer- cury, the Ag/AgCI reference electrode has been used instead of the calomel reference electrode. An a g a r - a g a r salt bridge is generally not used as a laboratory or industrial reference electrode because of its high maintenance and stability, but alternative ion transfer vehicles such as exudation of KCI have been used [2]. In refer- ence electrodes with ion transfer such as KCI exudation, continuous exudation of KCI is one of the most important parameters for reliable performance [3]. Sometimes, the KCI internal solution is pressurized by compressed air to stabilize the rate of KC! exudation. Thus, 0.1 to several ml per day of KCI internal solution exudes constantly through the liquid junction of the reference electrode. I f the exudation of

reserved

1686 S. lto et al./Talanta 42 (1995) 1685-1690

KCI internal solution from the inside of the breference electrode to the outside (into sample solution) is stopped, the KCI solution on the surface of the liquid junction becomes dilute. The reference electrode loses its normal efficiency and gives a different liquid junction potential, and the liquid junction potential be- comes unstable or fluctuating. However, this restriction of exudation of KCI solution is con- sidered to be caused by particles of dirt and soil in the sample solution. Even when no dirt or soil was present in the sample solution, we occasionally found that plugging of the liquid junction occurred. In an Ag/AgC1 reference electrode, clogging of its liquid junction is thought to be the essential problem. To main- tain the initial stable condition in the reference electrode, it is important that the mechanism of plugging be made clear. Nevell and Walsh [2] noticed that silver effused through the liquid junction from a Ag/AgCI reference electrode, and mercury effused from a calomel reference electrode. They stated only that the toxicity of Ag or Hg might adversely influence some sub- stances in the sample solution. Brezinski pointed out that AgCI clogged the liquid junc- tion in a Ag/AgC1 reference electrode [4]. We found that chelating resins could eliminate Ag ions existing in the KCI inner solution, and developed a long-term stable reference elec- trode with a liquid junction that could be kept clean using chelating resins.

2. Experimental

2. I. Reagents

All chemicals used were of analytical reagent grade. Potassium chloride (KCI) and silver chloride (AgC1) were obtained from Wako Chemicals (Osaka, Japan). Pure water was used throughout the work. Cation chelating resin Model CR-10 was purchased from Mitsubisi kasei (Tokyo, Japan). Chelating resin Model MX-8, Z-7 and S-1 were purchased from Miyosi Oil & Fat (Tokyo, Japan). The func- tional groups of each chelating resin of Model CR-10, Model MX-8, Model Z-7 and Model S-I are -N(CH2COOH)2, ) N - C H 2 - C H 2 - COOH, ) N - C S 2 H - S H and -SH, respectively.

2.2. Apparatus A Hitachi Model 208 atomic absorption

spectroscopy (air-acetylene flame) was used

for the determination of silver in 3 M KC! solution. A D K K Model COM-20 potentiome- ter, a D K K Model EL7100 silver ion electrode, a D K K Model EL7020 chloride ion electrode and a D K K Model EL4083 reference electrode were used for silver ion measurements. A D K K Model EL6156 glass electrode was used for pH adjustment throughout this work.

2.3. Measurement o f silver in 3 M KCI solution

In water, the solubility of AgC1 is 0.7 mg 1 - at 0°C, 1.93 mg 1-t at 25°C, 5.4 mg 1-~ at 50°C and 21 mg 1- ~ at 100°C [5]. However, in high concentrations of KCI solution such as 3 M KCI, the solubility of Ag is not found in the literature. Therefore, we measured the total silver concentration for saturated AgCI in 3 M KC1 solution at 25°C and at 80°C.

To keep silver ions in the complex state, sample solutions were prepared in 3 M KCI solution. Two 3 M KCI solutions containing 1% (supersaturated) AgCI were prepared. These were allowed to stand overnight in a water bath at 25°C and 800C, respectively. Sample solutions were prepared by accurately diluting ten-fold each supernatant solution containing 3 M KCI solution. The sample solu- tion was analyzed at 25°C.

Table 1 shows the result of measurements by atomic absorption spectroscopy. At 25°C, the solubility of AgC1 in 3 M KC1 solution is 340 mg 1- ~, although it is 1.93 mg ! - 1 in water. At 80°C, it is 700 mg 1-~ in 3 M KCI solution, and is approximately 13 mg 1 - ~ in water. With an increase in the concentration of KCI, the colour of the flame changed to purple owing to the potassium and the flame was not stable. Although the values of measurements by atomic absorption spectroscopy were not pre-

Table 1 Concentrated of total silver in 3 M KCI solution

Condition

Temperature Chelate (=C) resin =

Concentration of silver

25 None 340 mg 1 - 80 None 700 mg 1 - 25 Added ND (less than 0.2 mgl -~) 80 Added N D (less than 0.2 mg I-J)

Model S-I and Model Z-7 were used effectively.

S. lto et aL/Talanta 42 (1995) 1685-1690 1687

-50

-100

-150

o, o 0 --I~

Silver chloride e l e c t r o d e ~

S o o ~ lver sulfide electrode

-200 , L

0.1 I I0 100 1000

Concentratiom Of S U r e r (mlr/I)

Fig. 1. Relationship between electrode potential and silver ion concentration.

cise, we could estimate the concentration of silver in 3 M KCI solution. The data obtained were sufficiently accurate for the purpose of this experiment.

2.4. Elimination of silver in 3 M KCI solution

Silver in 3 M KC1 solution exists as Ag3CI_,= ix- ') complex ions. We tried to use cation ion exchange resin for the adsorption of Ag + ions, in equilibrium with the chloro-com- plex. A preliminary simple experiment was tried using several types of ion exchange resin. According to a study on adsorption properties towards heavy metal ions [6], we found that chelating resins were effective at eliminating Ag ÷ ions in the presence of high concentra- tions of K ÷ ions.

2.5. Potentiometric measurements of silver in the presence of chelating resin

Silver standard solutions were prepared as follows. Saturated AgCI in 3 M KCI solution at 25"C was used as 340 mg 1 - ' silver standard solution. By ten-fold dilution of this 340 mg 1 - t silver standard solution with 3 M KCI solution, 34 mg 1-i silver standard solution was pre- pared. In a similar manner, 3.4 mg 1 - i and 0.34 mg 1 - z silver standard solutions were prepared. The relationship between potential and silver concentration is shown in Fig. 1. Chloride ion electrodes with AgCI membrane (D) do not respond to silver ion concentration. Silver ion electrodes with Ag2S membrane (<>) show good Nernstian response. Silver ions in 3 M KC!

~du i ion could be measured with a silver ion electrode, but not with a chloride ion electrode.

Chelating resin (10 g was a sufficient amount for eliminating Ag) was added to 100 ml of 3 M KCI solution, the pH of which was adjusted to about 7 with hydrochloric acid or potassium hydroxide. The silver ion electrode potential was initially measured in 100 ml of saturated AgCl in 3 M KCl solution at 25°C. After adding l0 g chelating resin to this solution, the potential was measured periodically every l0 min for 12 h.

2.6. Improvement of the structure of the reference electrode

We developed a double junction reference electrode as shown in Fig. 2. The inner cham- ber is a conventional Ag/AgCI reference elec- trode, in 3 M KCI solution. The outer chamber is filled with some chelating resin and 3 M KCI solution. The inner chamber is closed, but the outer chamber and the inner chamber are con- nected through an inner chamber ceramic junc- tion. The 3 M KCI solution in the outer chamber serves to keep the junction more sta- ble. There is a hole in the outer chamber for pressure balance, and to supply 3 M KC1 solu- tion. The small amount of silver ions in the 3 M KCI solution that diffuses through the inner chamber ceramic junction is adsorbed by chelating resin. The 3 M KCI solution effusing through the outer chamber ceramic junction does not contain any appreciable amount of dissolved silver.

~ Lead wire

Cap

Rubber plug

Hole (for 3M KCI supply) Ag-AgCI electrode

Outer chanber

3M KCl solution

Inner chamber

Inner chamber ceramic junction Chelate resins

Outer chamber ceramic junction

Fig. 2. Structure of long-term stable double junction refer- ence electrode.

1688 S. Ito et al./Talanta 42 (1995) 1685-1690

3. Results and discussion

Two kinds of double junction reference elec- trodes were prepared. One was placed in a water bath at 250C, and the other in a water bath at 80°C. As the inner solution of the outer chamber was gradually reduced, 3 M KC1 solu- tion was supplied to the initial level periodi- cally. Water in the water bath was often exchanged to maintain low conductivity. Once a month, the silver concentration of the inner solution in the outer chamber was measured by atomic absorption spectroscopy. The measure- ment was continued for 6 months.

3. I. Change in silver concentration of the outer chamber solution

Depending upon the chelating resin, different results were obtained. In Model CR-10, the silver concentration in the outer chamber solu- tion gradually rose to 10 mg 1-t at 25°C and to 50 mg 1- ~ at 80°C over 6 months. In Model MX-8, it was below the detection limits (less than 0.2 mg I - ~) at 25°C, but gradually rose to 10 mg 10-J at 80°C. In Model Z-7 and Model S-I, it was not detected (less than 0.2 mg 1-J) at 25°C or 80°C. After 6 months, the rate of effusion of the outer inner solution was mea- sured. In the test reference electrode, in which was observed the existence of dissolved silver in the outer chamber, i.e. in Model CR-10 at 25°C and 80°C and in Model MX-8 at 80°C, the rate of effusion was reduced to one-half or one-quarter of its initial value, and the appear- ance of the outer chamber ceramic junction

0 Model CR-lO

-50

e+ -100

Model MI-8 ^

Model S-I

-150

-200 0 2 4 6 8 10

Time hour

Fig. 3. Eliminating ability of chelate resins.

12

tion ability of chelating resins for Ag + changed in the order Model Z-7 > Model S- 1 > Model MX-8 > Model CR-10. Model Z-7 had the best adsorption power among them.

3.3. Application to pH combination electrode

In Fig. 4, a conventional-type electrode is shown which is the most popular model of a combination pH electrode. It consists of a glass electrode, reference electrode and temperature sensor. To compensate for temperature effects, the glass electrode and reference electrode have similar structures. In addition, an inner junc- tion tube of porous polypropylene resin for improved-type electrodes was added to the in- ner chamber of the pH and reference electrodes for prevention of diffusion of Ag ÷ or AgCI,- c, - I) ions.

changed to brown owing to AgCI. Table 1 shows the concentration of silver in 3 M KCI ~ L~,~,, i . : l q=l l solution after 6 months measured by atomic [ ] absorption spectroscopy. In the case of addi- ] I* - c ~ - tion of Model S-I chelate resin or Model Z-7 ~ ~ . ^ a o , ~ a , , a , chelate resin, silver was not detected. The addi-

tion of these chelate resins was very useful for II I | I1 " IIII I III II the elimination of silver in the KCI solution. IHIHIHII _____I , _ , ~ , , , K c , , - - - - - - - - - ~ l l ~ l H I

lill/ ffq INlil:fll[ll 3.2. Eliminating ability with chelating resins Illll U l /~ ~ - ' ' ~ ' ~ ' ' ° ~ ' ~ ' ' Illlll I IIlll

I kllL I I IIII UIlII Fig. 3 shows the eliminating ability of the II,lllk ,lU,II

chelating resins. The silver ion electrode poten- ~ [ ~ 1 ~ ~ ' ~ - - - - - - ' - - - - ~ [ [ ~ [ I J tial decreased gradually with addition of ~ . . ~ ~ r , ~ , , , ~ . - ~ ~ . . . . ] ~ chelating resins, and became constant after 12 ~ ~ o ~ , , ~ q ~ , ~ - ~ , , , , - - - ~ h. Final potentials showed 187, 30, 2.4, 0.3 mg ~ p a ~ , ~ , ~ 1- ~ Ag + concentration, respectively, against Improved Type Conventional Type

each chelating resin of Model CR-10, Model Fig. 4. Comparison of pH electrode structure in improved-

MX-8, Model S-1 and Model Z-7. The adsorp- type and conventional-type combination electrodes.

S. /to et al./Talanta 42 (1995) 1685-1690 1689

Based on the results mentioned above, we improved our conventional-type combination pH electrode as shown in Fig. 4. A double junction-type reference electrode was fabri- cated. The Ag/AgCI electrode was enclosed in the plastic junction tube, i.e. the inner cham- ber. The Ag/AgC1 electrode was made shorter than in the conventional-type electrode, and was put in the upper part of the improved-type electrode. The distance between the Ag/AgCI electrode and the inner ceramic junction was longer than in the conventional-type electrode. Further, porous polypropylene resin was packed between them. Using this idea, the rate of diffusion of Ag complex ions became slow. As a result, the activation rate of the junction based on Ag was minimized. The silver ab- sorbent, i.e. the chelating resin, was placed in the outer chamber of the reference electrode. Owing to the presence of this chelating resin, silver complex ions were no longer exuded from the outer ceramic junction to the sample solution, and the junction potential became very stable for long periods of time.

3.4. Effect of improved pH combination electrode in field test

When a pH electrode is used in a low con- ductivity solution (less than 0.1 mS cm-2) , abnormal phenomena in pH measurements, such as errors and fluctuation of pH values, sometimes occur. These phenomena are accel- erated when the sample temperature becomes high. Fig. 5 shows an example of measure- ments in low conductivity solution. There is a remarkable difference between the improved- type pH electrode and the conventional-type pH electrode. The data of continuous pH mea- surement using the improved-type pH electrode were in fairly good agreement with the pH from the batch sampling method, within -I-0.05 pH units.

When the pH electrode was used in boiler water containing a reductant such as hydrazine in a power plant, some abnormal phenomena in pH measurements occurred, which are shown in Fig. 6. The data of continuous pH measurement using the improved-type pH elec- trode were again in fairly good agreement with the pH from the batch sampling method, within + 0.02 pH units. In the case of the conventional-type pH electrode, the observed pH values are generally lower than normal. This error often reaches pH - 0 . 3 to - 0 . 7 .

• 8.5

A

. 7.5

6.5 L I

I0 20 30 Passage of days

Fig. 5. Example of measurements in a low conductivity solution. The sample is an industrial supply of water (underground water), the electric conductivity of which is about 0.05 mS cm - 2. Data from the conventional-type pH electrode (©) and improved-type pH electrode (O) are shown, as well as data from a laboratory pH electrode by the batch sampling method (•). © and O are from continuous measurement data, and • are manual analysis data taken once a week.

3.5. Mechanism of deterioration in the Ag/AgCI reference electrode

Although silver chloride is only slightly solu- ble in water, AgCl is more soluble in concen- trated KCI solution than in water. We measured the concentration of silver in 3 M

10

9.5

8.5

~00 000000

0000000000(

I I

10 20 30 Passage of days

Fig. 6. Example of measurements in boiler water contain- ing several mg l - ' hydrazine. The electric conductivity is about 0.01 mS cm - 2. Data from the conventional-type pH electrode (O) and improved-type pH electrode CO) are given, as well as data from a laboratory pH electrode by the batch sampling method (e). O and O are from continuous measurement data, and • are manual analysis data taken once a week.

1690 S. lto et al./Talanta 42 (1995) 1685-1690

KC1 solution, and showed that the solubility was larger by about two orders of magnitude than in water. In 3 M KC1 solution, Ag seems to exist as AgCl,. -~- '-° for x = 2 or 3. The inner solution containing AgC1,. -~-'- ~) in the reference electrode effuses to the sample solu- tion through the liquid junction. At the surface of the liquid junction, the effused solution is then diluted by the sample solution, and the concentration of KCI becomes lower. There- fore, AgCI,7 (,--~) ions that were soluble in the inner solution revert to AgCl, and AgCl is deposited on the surface of the liquid junction. Silver chloride adheres gradually on the mate- rial comprising the liquid junction such as porous ceramic or a glass sleeve. Further, when reductant was present in the sample solution, AgC1 was reduced and changed to metallic Ag. The form of silver under some conditions is shown as follows:

AgCI ~-(x - ~) (in concentrated KC1 solution)

AgC1 Ag )

(in water) (after reduction)

The potential of a liquid junction covered with AgCI will be influenced by the CI- ion concentration or activity; further, one covered with Ag will be influenced by the redox poten- tial, because the functioning of the reference electrode deteriorates on adhesion of AgCI or Ag at the liquid junction. Some examples of this are as shown in Figs. 5 and 6.

We believe there are various mechanisms by which the inner solution containing AgCI,. - ( ' - ~)

is prevented from effusing through the liquid junction. One is that gelation of the inner solution prevents its diffusion or convection, and slows down the movement of AgCI,7 ~-'- ~ from the Ag/AgC1 electrode to the sample solu- tion; another is that diffusion is slowed down by a sufficiently long distance between the Ag/ AgCl electrode and the liquid junction. Al- though Brezinski reported a method for eliminating AgCl by mechanical barriers such as cation-selective membrane [4], it is not desir- able to create membrane potentials between the Ag/AgCl electrode and the liquid junction in a reference electrode. The use of mechanical bar- riers seems to provide incomplete elimination of AgCl, resulting in an unnecessary potential or high electrical resistance. We conclude that this method with chelating resins is one of the most practical, because the operational charac- teristics of the reference electrode are kept sta- ble for long periods of time.

References

[1] A.S. Brown, J. Am. Chem. Soc., 56 (1934) 646. [2] T.G. Nevell and F.C. Walsh, Trans. Inst. Metal Fin-

ish, 70 (1992) 144. [3] A.K. Covington and P.D. Whalley, Anal. Chim.

Acta, 169 0985) 221. [4] D.P. Brezinski, Anal. Chim. Acta, 134 (1982) 247. [5] W.F. Linke, Solubilities: Inorganic and Metal-

Organic Compounds, Vol. l, 4th edn., American Chemical Society, Washington, DC, 1958, p. 59.

[6] A. Lezzi, S. Cobianco and A. Roggero, J. Polym. Sci., Part A: Polym. Chem., 32 0994) 1877.