GB2060896A - Silver/silver halide electrode - Google Patents

Abstract

A saturated silver/silver halide electrode for measuring electromotive force, e.g. a reference electrode, comprises a conductor 14 with a metallic silver surface and a solid phase 13 of silver halide such as silver chloride and alkali metal halide such as potassium chloride present in sufficient amount to secure that the silver halide and alkali metal halide solution with which the silver conductor is in contact is saturated even at the maximum operation temperature of the electrode, and in which at least 0.1 mole per cent and preferably 0.5-5 mole per cent of the silver halide is reduced to colloidal free silver which is homogeneously distributed in the solid halide. The electrode shows a remarkably reduced sensitivity towards temperature variations and exposure to light. Preferably the weight ratio of alkali metal halide to silver halide is between 10:1 and 1:10. The electrode with porous plug 12 may contact saturated potassium chloride solution inside a reference electrode or glass or other ion selective electrode. <IMAGE>

Description

SPECIFICATION Electrode The present invention relates to an electrode or electrode component for measuring electromotive force for the determination of ion activity and redox potentials.

British Patent Specification No. 1.281.116 and US patent specification No. 3,676,319 disclose electrodes, half cells or electrode components which comprise a conductor having a surface of metallic silver, a solid phase of a silver halide, and a solid phase of an alkali metal halide. The solide phases of the halides are positioned in such a manner in relative to the conductor that there is communication between the silver surface of the conductor and ions of the halides when the halides are wetted with water, and in order to overcome temperature hysteresis, the amount of the halides is sufficient to secure that there is still solid phase of both halides present at the maximum temperatures at which the electrode or half cell is to be operated.

The silver halide in these known art electrodes is normally silver chloride, and the alkali metal halide is normally potassium chloride. Half cells of this known type are used both in reference electrodes and in ion sensitive electrodes, e.g. hydrogen ion-sensitive electrodes for the determination of pH, that is, both as external and as internal references. The relation between the concepts "electrode" and "half cell" is discussed in the above-mentioned British and US patent specifications. In the present specification, the term "electrode" is used both for the concept "half cell" and for the concept "electrode" proper.

It has been experienced that these known art saturated silver halide electrodes in which solid phase of both silver halide and alkali metal halide halides is present at the maximum operation temperature of the electrode, in spite of their improved properties with respect to reduced temperature hysteresis in comparison with the state of the art at that time, cannot be considered optimally stable in accordance with the standards of today: with the continuing development of improved and more accurate measuring techniques, it becomes necessary to use more stable half cells and reference electrodes which, to a higher degree than previously, are unpolarizable, i.e., are not affected by the current load, and unaffected by exposure to light or by their preceding thermal history.Thus, e.g., a modern pH measuring equipment has a resolving power of 0.001 pH unit, which, for the elelctrometrical pH mesuring equipment, corresponds to 0.060 millivolts. For this reason, it is desirable that the reference elctrode used has a stability of the same order. This has not been possible with the known art electrodes of the saturated silver halide type. Both exposure to light and heat cycling may change the potentials of the known art electrodes up considerably.

The present invention relates to an electrode or electrode component with improved stability properties. The invention provides an electrode or electrode component for measuring electromotive force and comprising a conductor having a surface of metallic silver and a solid phase of a silver halide and alkali metal halide powder which is positioned in such a manner in relation to the silver surface that there is communication between the silver surface and ions of the halides when the powder is wetted with water, the amount of silver halide and alkali metal halide being such that a saturated solution of both halides is ensured at the maximum operation temperature at which the electrode is intended to be used, the said electrode or electrode component being characterized in that at least 0.1 mole per cent of the silver halide is reduced to colloidal free silver which is homogeneously distributed in the solid silver halide phase.

Reference electrodes, half cells and electrode components according to the invention are satisfactorily unpolarizable, and they show markedly improved stability properties and are satisfactory with regard to light sensitivity and temperature hysteresis. In connection with the present invention, an electrode or half cell is considered to have a satisfactorily low light sensitivity and temperature hysteresis when exposure of the electrode to a 1 25 W UV lamp at a distance of 25 cm for 1 hour, or immersion of the electrode in hot water at 90-1 00çC for 10 minutes and subsequent re-establishment of the original temperature, results in an electrode potential change of < 200 microvolts in comparison with the initial value.

It is preferred that the silver halide in the electrode or electrode component of the invention is silver chloride, and as alkali metal halide, potassium chloride is preferred, as chloride and potassium ions, being of about equal size, have approximately the same rate of diffusion, which means that the liquid junction potentials in the electrodes and half cells of the present kind are minimized.

Already a reduction corresponding to 0.1 mole per cent of the silver halide in the electrode results in a very considerable decrease of the light sensitivity, and at a reduction corresponding to 0.5 mole per cent of the silver in the silver halide, the electrode shows substantially no light sensitivity. The temperature hysteresis of the electrodes decreases already at a reduction of 0.1 mole per cent of the silver in the silver halide, and at a reduction corresponding to 0.5 mole per cent of the silver in the silver halide, a completely acceptable low degree of temperature hysteresis is obtained.

Thus, at a reduction of about 0.5 mole per cent of the silver halide, a satisfactory decrease of both light and temperature sensitivity is obtained for the electrodes, half cells or electrode components of the present invention, and even though it is, of course, possible to reduce 10 mole per cent or even more of the silver halide, it is preferred in accordance with the present invention, to reduce from about 0.5 two 5 mole per cent of the silver halide, as it is desired to have a surplus of solid silver halide in the half cell in order to secure that even at operating temperatures above 1 00 C, the half cell is still saturated with silver halide, and in order to avoid that the silver halide phase gradually disappears due to a diffusion of silver halide from the half cell.

The invention also relates to a process for the preparation of a silver halide mass comprising colloidal free silver homogeneously distributed in the silver halide mass, which process is characterized in that at least 0.1 mole per cent of the silver halide is reduced to free silver which is homogeneously distributed in the solid silver halide phase, using a reducing agent which is mixed homogeneously with the silver halide mass.

For the reduction of silver chloride to free silver, it is known to use various reducing agents such as zinc, hydrogen peroxide, hydroxylamine, hydrazine, beignette salt or phosphorous acid, or to use exposure to light.

However, for the purpose of the present invention, it has been experienced that exposure to UV light does not produce the desired result, as electrodes and half cells containing silver chloride powder which has been exposed to a 1 25 W UV lamp light for two hours were found to be almost more sensitive to light than corresponding electrodes or half cells containing nonreduced silver chloride.

The reduction of silver chloride according to the present invention is preferably carried out by soaking a silver chloride mass, which has referably beem prepared by precipitating silver nitrate with potassium chloride, with the stoichiometrically calculated amount of hydrogen peroxide and then increasing the pH to above 10, e.g. 10-12, by adding a base, suitably a solution of an alkali metal hydroxide such a sodium hydroxide solution, whereby the reducing effect of the hydrogen peroxide on the silver chloride is elicited so that colloidal silver particles homogeneously distributed in the silver chloride mass are formed.

Thus, in this preferred embodiment of the process of the present invention, the reduction is performed in two steps using hydrogen peroxide. In the first step, it is secured that the hydrogen peroxide soaks the silver chloride completely before the reduction. In the second step, the reduction is initiated by increasing the pH by addition of an alkali metal hydroxide. It is hereby ensured that the reduction will proceed uniformly throughout the silver halide mass.

Furthermore, in this preferred embodiment of the process, the only foreign ions added are alkali metal ions which may be removed by the subsequent washing of the reduced silver chloride mass before its use. When this preferred embodiment of the process is performed, a microscopically homogeneous mixture of silver and silver chloride is obtained, whereby the establishment of equilibrium of the reactions which determine the potential will proceed as fast as possible.

Although a certain catalytic decomposition of the hydrogen peroxide takes place, catalyzed by the colloidal reduced silver, such a side-reaction will not have any adverse influence on the homogeneity of the final partially reduced silver chloride, as the decomposition will, of course, take place at the silver particles already formed, whereas the reduction elsewhere in the mixture will proceed freely.

The present invention is explained in greater detailed with reference to the drawing, in which Figure I shows an enlarged cross-section of a part of a half cell in accordance with the present invention, Figure 2 shows a cross-section of a preferred embodiment of a reference electrode in accordance with the present invention, Figure 3 shows a cross-section of an embodiment of a specific ion-sensitive electrode in accordance with the present invention, Figure 4 shows an enlarged cross-section of an electrode component in accordance with the present invention, Figure 5 shows a cross-section of another ion-sensitive electrode in accordance with the present invention comprising an electrode component of the general type shown in Fig. 4, Figure 6 is a graph showing the influence of UV irradiation on known art electrodes and electrodes in accordance with the present invention, respectively, and Figure 7 is a graph showing the influence of 10 minutes' boiling on the electrode potential of known art electrodes and electrodes in accordance with the present invention, respectively.

In the drawing, identical reference numbers refer to identical elements.

The half cell shown in Fig. 1 has glass walls 11 which define a chamber 1 3 having an open end which is closed with a porous plug 12, e.g. of cotton or glass wool, which constitutes a diaphragm. A conductor 1 4 having a silver surface extends into the chamber 1 3. This conductor may, e.g., be a silver wire or plate or a platinum wire or plate coated with silver. The conductor 14 is surrounded by a mixture 1 5 which contains silver chloride crystals and potassium chloride crystals wherein part of the silver chloride is reduced to free colloidal silver, and a saturated aqueous solution of silver chloride and potassium chloride. A thin wire 1 7 leading through a wall 1 6 is connected with the conductor 14.The free end of the wire 1 7 is protected by tubeformed glass walls 1 8. The wire 1 7 may be of any suitable metal which will conduct an electric current.

In the electrodes or half cells of the invention, as exemplified in the embodiment shown in Fig. 1, the weight ratio between silver halide and alkali metal halide in the powder mixture may vary widely, e.g. from 10:1 to 1:10. Normally, the powder mixture comprising silver halide and alkali metel halide will comprise, in its wetted condition, 10-20 per cent by weight of water, 15-45 per cent by weight of alkali metal halide, preferably potassium chloride, and 40-60 per cent by weight of silver halide, preferably silver chloride, a preferred mixture comprising about 1 5 per cent by weight of water, about 35 per cent by weight of potassium chloride, and about 50 per cent by weight of silver chloride.

Fig. 2 shows a reference electrode according to the present invention. The reference electrode comprises a half cell as described above and as shown in Fig. 1 placed in a glass container 1 9 in the usual manner. The container 1 9 contains a saturated aqueous solution 20 of potassium chloride which serves as a salt bridge electrolyte. A porous plug 21, for example of glass, a ceramic material or asbestos, is placed in an opening in the bottom of the container 1 9 and serves as diaphragm. The potassium chloride solution 20 contains solid potassium chloride crystals 23 which are present in such an amount that they contact the end of the porous plug under all operation temperature conditions. Potassium chloride solution and potassium chloride crystals may be replenished through an orifice 22.

Fig. 3 shows a specific non-sensitive electrode which corresponds to the electrode shown in Fig. 2 with the exception that the bottom of the container 1 9 is a specific ion-sensitive membrane 25. Furthermore, in Fig. 3, the solution 20 in the container 1 9 of Fig. 2 is replaced by a reference electrolyte 24. The specific ion-sensitive membrane 25, e.g. a glass membrane for measuring hydrogen ion activity, permits communication between the reference electrolyte 24 and a liquid sample (not shown in the drawing) when the lower part of the container is immersed therein.

In the electrode component shown in Fig. 4, the conductor 14, at least the surface of which consists of silver, is surrounded by and is in contact with a compressed porous body 26 which comprises silver chloride crystals and potassium chloride crystals, part of the silver chloride being reduced to free colloidal silver. The wire 1 7 is of an inert metal, e.g. platinum or a platinum metal.

The specific ion-sensitive electrode shown in Fig. 5 contains an electrode component of the same general type as the electrode component shown in Fig. 4 which functions as half cell. The silver conductor shown in Fig. 5 is in the form of a network or grid 30. The electrode component comprising this conductor and the surrounding body 26 is mounted in a container 19 of the type shown in Fig. 3. The wire 1 7 consists of an inert metal. In this embodiment, the reference electrode 24 contains crystals 23 of potassium chloride, preferably in such an amount that the exterior of the electrode component is in contct with the crystals at any operation temperature.

Fig. 6 shows the variation of the electrode potential at exposure of various silver-silver chloride electrodes to UV light for a period corresponding to the section from A to B on the time axis (cf. Example 4). Curves a, b and c show the potential for electrodes comprising normal, non-reduced silver chloride and potassium chloride. Curve d shows a short-circuit channel on the printer. Curves e and f show the potential of electrodes where about 0. 1% of the silver chloride is reduced to free silver, and curves g-1 show the potential of electrodes where about 0.5% of the silver chloride is reduced to free silver.

Fig. 7 shows the change of electrode potential which occurs when electrodes of the same kind as used in Fig. 6 are boiled for 10 minutes at point C. The letter designations on the curves have the same meaning as explained above in relation to Fig. 6.

In the below examples which illustrate the invention, the following materials were used: Silver nitrate, p.A. from E. Merck AG, Darmstadt, Federal Republic of Germany, potassium chloride, p.A. from E. Marck AG, 30% hydrogen peroxide, sodium hydroxide, p.A. from E. Merck AG.

The silver chloride-potassium chloride powder in the known art type electrode components and electrodes used for comparison in the examples was produced as follows: 33.97 g silver nitrate was dissolved in 2 liters deionized water, and 16.4 g potassium chloride was dissolved in 220 ml deionized water. The silver nitrate solution was heated to about 80"C whereafter the potassium chloride solution was added within 5 minutes with vigorous stirring.

The resulting slurry was washed 10 times, each time with about 350 ml deionized water at about 65"C, the water being decanted from the residue each time. After decanting of the last portion of wash water, the residue was dried in a crystallization dish at about 90"C in a vacuum drying oven at oil pump vacuum. The evaporation residue was ground in a mortar, and the resulting silver chloride was mixed with potassium chloride in a weight ratio of 3 parts of silver chloride to 2 parts of potassium chloride, and the mixture was ground in a procelain ball mill for 1 5 minutes. This powder comprises non-reduced silver chloride and is identical to the powder used in the commercially available siiver/silver chloride electrodes.

Example 1.

Preparation of a mixed powder of silver chloride and potassium chloride in accordance with the present invention where about 0.1% of the silver chloride is reduced to free silver: 33.97 g silver nitrate was dissolved in 2 liters deionized water, and 16,4 g potassium chloride was dissolved in 220 ml deionized water. The silver nitrate solution was heated to 80"C, and the potassium chloride solution was added dropwise in the course of 5 minutes under vigorous stirring. The resulting suspension was washed twice with each time about 350 ml deionized water at 65"C, the liquid being decanted from the precipitate each time. About 350 ml ion exchanged water at about 65"C was added to the residue, and about 0.6 ml 30% hydrogen peroxide was added with stirring.The stirring was continued for 5 minutes, whereafter 100 ml sodium hydroxide solution, prepared by dissolving 0.8 g sodium hydroxide in 100 ml deionized water, was added. The suspension was washed 7 times with each time about 350 ml ion exchanged water at about 65"C, the liquid being decanted from the precipitate each- time. The resulting residue was dried in a crystallization dish at about 90"C in a vacuum drying oven at oil pump vacuum for 2 hours. The resulting silver chloride powder was gound in a mortar and mixed with potassium chloride powder in the ratio 3 parts by weight of silver chloride powder to 2 parts by weight of potassium chloride powder, and the mixture was ground in a procelain ball mill for 1 5 minutes.

Example 2.

Preparation of a mixed powder of silver chloride and potassium chloride according to the present invention where about 0.5% of the silver chloride is reduced to free silver: The procedure was analogous to the one described in Example 1, using, however, 1.8 ml 30% hydrogen peroxide and 10 ml sodium hydroxide solution prepared by dissolving 2.4 g sodium hydroxide in 100 ml deionized water.

An analysis of the content of free silver in a partially reduced silver chloride-potassium chloride powder mixture prepared as described in Examples 1 and 2 is performed as follows: 0.5-1 g of the silver chloride-potassium chloride powder mixture to be analyzed was suspended in 75 ml concentrated ammonia and was allowed to stand for 1 hour. The precipitate was separated by filtration and washed with hot deionized water until the filtrate was free of chloride (no precipitate when a silver nitrate solution was added to a few drops of remaining wash liquid which was acidified with nitric acid).

The precipitate was dissolved in about 6 ml hot 4N nitric acid, whereafter the solution was evaporated to a volume of about 1 ml. The solution was transferred quantitatively to a 25 ml graduated flask, and 0.2 g potassium nitrate solution was added up to the mark.

The content of silver ions in the above-prepared solution was measured using a silver ionselective electrode (F1212S from Radiometer A/S, Copenhagen, Denmark) and a reference electrode having a double salt bridge (K711 from Radiometer A/S, Copenhagen, Denmark) by measuring the potential difference by means of a pH-meter (PHM64 from Radiometer A/S, Copenhagen, Denmark). The measuring chain was calibrated with a 10-2M, a 10-3M and a 10-4M silver nitrate solution in 0.2M potassium nitrate solution, and the silver content of the silver chloride-potassium chloride powder mixture was calculated from the silver ion content of the solution.

The residue resulting from the above ammonia treatment was examined by microscopy, and it was found to consist of silver particles having a diameter of 1-2 fim.

Example 3.

Comparison of light sensitivity and temperature hysteresis of electrodes comprising reduced and non-reduced silver chloride-potassium chloride powder, respectively: The electrodes comprising non-reduced silver chloride correspond to the electrodes disclosed in the above-mentioned British and US patent specifications.

Electrodes of the kind shown in Fig. 2 were prepared in the usual manner using the powder mixtures to be tested. The interior half cell was of the kind shown i Fig. 1 and consisted of a lead glass tube (11, 18) closed in the middle and through which a platinum wire was mounted as conductor. This combination of lead glass and platinum was chosen because the heat expansion coefficients of these two materials are the same.In the upper end of the half cell, the platinum wire was equipped with a silver wire 17, and in the lower end of the half cell, it was connected to a short silver wire 1 4. The lower part 1 3 of the half cell was filed with silver chloride-potassium chloride powder which was thereafter soaked with water, whereby the solution became saturated with potassium chloride and silver chloride, and the bottom of the half cell was closed with a plug 1 2. The diameter of the glass tube 11, 1 8 was 1.45 mm. The half cell contaning the silver chloride-potassium chloride powder to be tested was inserted into the electrode container 19, whereafter potassium chloride and water were added.The electrodes to be tested w.ere placed in a double-walled thermostated glass vessel capable of accomodating 12 electrodes, and measurement was performed against a calomel reference electrode (K4018 from Radiometer A/S, Copenhagen, Denmark) which was connected to the + terminal, and which, via a salt bridge, was in contact with the liquid in the thermostated glass vessel (buffer, pH 7-0.5 M KCI). The measurement was performed using a 1 2 channel printer having a sensitivity of 400 microvolts/cm and full scale 25 cm. Two independent thermostating systems were used, one for the glass vessel with the 1 2 electrodes and one for the calomel electrode.

The test temperature was 25"C. The light sensitivity was determined by exposing the electrodes to a 125 W UV lamp at a distance of 25 cm from the thermostated glass vessel. The potential change was measured on the printer paper for 1 hour's exposure to the UV light, and this potential change was taken as the difference in millivots between the initial potential and the final potential of the electrode. The heat treatment was performed by immersing the electrodes in water at 90-100"C for 10 minutes and thereafter again inserting them in the thermostated glass vessel. After about 10 minutes, the potential change caused by the heat treatment was determined from the printer paper.

A comparison between electrodes comprising non-reduced silver chloride-potassium chloride and electrodes comprising the powders prepared as stated in Examples 1 and 2 was preformed.

At a reduction of more than 0.1 % of the silver chloride, the electrodes became practically insensitive to light, and at the same time the mutual difference between the potentials of the electrodes was small. This appears from Table IA which indicates the potential changes of the electrodes at exposure to light performed after 0, 1, 8, 12, 13, 15, and 74 days.

For electrodes where the silver chloride powder used was reduced to an extent of about 0.5% or more, the temperature hysteresis was less than for the known art type comparison electrodes and for the electrodes containing silver chloride reduced to an extent of about 0.1%. This appears from Table IB where the potential change after heating at 90-100"C for 10 minutes and cooling to 25"C is stated in millivolts, the heat treatment being repeated after 6, 7, 9, 12, 14, 16, and 78 days.

A comparison of the three types of electrodes comprising normal silver chloride, silver chloride reduced to an extent of about 0.1%, and silver chloride reduced to an extent of about 0.5%, respectively, versus the calomel reference electrode K401 8 was performed. At increasing degree of reduction, decreasing potential differences and decreasing mutual deviation between the electrode potentials were observed which appears from Table IC which states the potentials for the three types of electrodes after 80 days, measured against K4018 at 25"C in a buffer at pH 7 to which had been added 1 5 g potassium chloride per 500 ml.

Furthermore, 80 days after the preparation of the electrodes, the potential difference between saturated silver chloride reference electrodes containing silver chloride reduced to an extent of more than about 0.5% and the calomel reference electrode K401 8 was equal to the theoretical potential difference. (For a saturated silver/silver chloride electrode, the standard potential at 25"C is 0.1989 V (Bates, Roger G., Determination of pH, table 10-8, 2nd edition, 1973), and for a saturated calomel electrode, the standard potential at 25"C is 0.2444 V (Bates, Roger, G., Determination of pH, table 10-6, 2nd edition, 1973) including the liquid junction contribution.

Thus, the theoretical potential difference between the saturated silver/silver chloride electrode and the saturated calomel electrode is - 45,5 millivolts.) The test results show that the electrode potential was highly reproducible, whereas electrodes containing non-reduced silver chloride had a potential difference deviating from 9% from the theoretical value. At a reduction of about 0.1 %, the deviation from the theoretical potential difference was considerably smaller than for electrodes containing non-reduced silver chloride, that is, about 0.8 millivolts, corresponding to about 1.8%.

Table IA Change of potential in mV after exposure to UV light for 1 hour Silver chloride-potassium chloride powder Days Normal 0.1% reduced 0.5% reduced n x s n x s n x s 0 3 -1.2 0.7 2 -0.12 0.06 6 -0.08 0 1 3 -2.1 1.4 2 -0.04 0.06 6 0 0 8 3 -0.6 0.1 2 0 0 6 -0.03 0.07 12 3 -1.6 0.6 2 -0.04 0 6 -0.02 0.02 13 3 -1.6 2.2 2 -0.02 0.03 6 0 0 15 3 -2.2 1.6 2 -0.06 0.03 6 -0.03 0.02 74 3 -1.9 0.8 2 -0.04 0 6 +0.08 0 ---------------------------------------------- N 7 7 7 Mean value 1 # x -1.6 -0.05 -0.01 N n: number of electrodes x: potential difference in mV s: deviation in mV N: number of testing series x: mean value of x Standard deviation between mean value x 0.56 0.04 0.05

1.2 0.04 0.03 95% k. for (-2.1; -1.1) (-0.09; -0.01) (-0.06; 0.04) mean value Table IB Potential change in mV after heating for 10 minutes at 90 - 1000C and cooling to 250 C.

Silver chloride-potassium chloride powder Days Normal 0.1% reduced 0.5% reduced n x s n x s n x s 6 3 2.5 1.2 2 0.6 0.04 6 0.02 0.08 7 3 1.5 0.7 2 0.5 0.03 6 0.03 0.07 9 3 2.1 0.9 2 0.6 0.2 6 0.2 0.06 12 3 2.5 1.1 2 0.6 0.4 6 0.2 0.6 14 3 2.0 0.9 2 0.4 0 6 0.06 0.05 16 3 2.0 1.0 2 0.5 0.08 6 0.07 0.05 78 3 2.6 1.8 2 0.5 0 6 -0.11 0.03 N 7 7 7 Mean value 1 z x 2.2 0.53 0.13 N Deviation between mean value x 0.4 0.08 0.13

1.1 0.17 0.23 95% k. for mean value (1.8; 2,6) (0.46; 0.60) (0.01; 0.25) Table IC Potentials in mV measured against K4018 at 250C in buffer having pH 7 with addition of 15 g KCl per 500 ml.

Silver chloride-potassium chloride powder Days Normal 0.1% reduced 0.5% reduced n x s n x S n x s 80 3 -41.5 1.7 2 -44.7 0.3 6 -45.5 0.2 Example 4.

The potential changes of the electrodes tested in Example 3 after exposure to UV light and heat treatment were examined more closely.

The potential changes after exposure to UV light are shown in Fig. 6. Curves a-c were obtained with electrodes containing non-reduced silver chloride, curve d is a short-circuit channel, curves e and f were obtained with electrodes containing silver chloride reduced 0.1%, and curves g - 1 were obtained with electrodes containing silver chloride reduced about 0.5% or more.

It appears from Fig. 6 that the potential of electrodes containing non-reduced silver chloride increases drastically when exposure to UV light starts at point A, and a steep potential drop is observed when the exposure to UV light stops at point B. After each of these events, longlasting transient phenomena were observed.

The electrodes contaning silver chloride reduced about 0.1 % (curves e and f) showed only little change, and for the electrodes containing silver chloride reduced about 0.5%, the potential was unaffected.

The results of boiling the same electrodes as above for 10 minutes appears from Fig. 7 where the boiling of the electrodes was performed at point C. It should be noted that the potential of the non-reduced electrodes decreased after boiling, whereafter it increased and had still not reached stability after 3 hours. Not until about 24 hours after the heat treatment, these electrodes showed a potential which was constant within 100-150 millivolts. The potential of electrodes containing silver chloride reduced about 0.1 % decreased about 500 microvolts after boiling and then became stabilized within about 2 hours at a level approximately 200 microvolts below the initial potential.

The potential of the electrodes contaning silver chloride reduced about 0.5% or more (curves f-l) increased "spontaneously" about 100 microvolts after boiling, whereafter it remained constant. With these electrodes the transition to constant potential was complete already within 10 minutes after the cooling.

Claims (11)

1. An electrode or electrode component for measuring electromotive force and comprising a conductor having a surface of metallic silver and a solid phase of a silver halide and alkali metal halide powder, said solid phase being positioned in relation to the silver surface soyas to permit communication between the silver surface and ions of said halides when the powder is wetted with water, the amount of solid phase of silver halide and alkali metal halide being sufficient to secure that the silver surface of the conductor communicates with a saturated solution of both salts at the maximum operation temperature of the electrode, at least 0.1 mole per cent of the silver halide being reduced to colloidal free silver homogeneously distributed in the silver halide phase.

2. An electrode or electrode component according to claim 1, wherein the silver halide is silver chloride, and the alkali metal halide is potassium chloride.

3. An electrode or electrode component according to claim 1 or 2, wherein the weight ratio of alkali metal halide to silver halide is from 10:1 to 1 0.

4. An electrode according to any of claims 1-3, wherein the powder of the halides when wetted comprises 10-20 per cent by weight of water, 15-45 per cent by weight of alkali metal halide and 40-60 per cent by weight of silver halide.

5. An electrode according to claim 4, wherein the wetted halide powder comprises about 1 5 per cent by weight of water, about 35 per cent by weight of potassium chloride, and about 50 per cent by weight. of silver chloride.

6. An electrode or electrode component according to any of claims 1-5, wherein at least 0.5 mole per cent of the silver halide, preferably from 0.5 to 5 mole per cent of the silver halide, is reduced to free silver.

7. A process for the preparation of a silver halide mass for the use in an electrode, half cell or electrode component according to any of claims 1-6, wherein silver halide in powder form is mixed homogeneously with an amount of reducing agent sufficient for reducing at least 0.1 mole per cent of the silver halide.

8. A process according to claim 7 for the preparation of a silver chloride mass, wherein silver chloride in powder form is soaked with a hydrogen peroxide solution, whereafter the pH of the liquid phase of the resulting slurry is increased to at least 10 by adding a base, preferably an alkali metal hydroxide solution, in a sufficient amount to secure that a pH of at least 10 is maintained also after the reduction.

9. A process according to claim 8, wherein the aount of hydrogen peroxide is sufficient for reducing at least 0.5 mole per cent of silver chloride, preferably from 0.5 to 5 mole per cent of the silver chloride.

1 0. A silver chloride/potassium chloride powder mixture, wherein at least 0,1 mole per dent of the silver chloride is reduced to collidal free silver which is homogeneously distributed in the powder mixture.

11. A powder mixture according to claim 10, wherein at least 0.5 mole per cent of the silver chloride, preferably from 0.5 to 5 mole per cent of the silver chloride, is reduced to colloidal free silver which is homogeneously distributed in the powder mixture.

1 2. A powder mixture according to claim 10 or 11, wherein the weight ratio between silver chloride and potassium chloride is in the range of from 1 0:1 to 1 10 and, especially, about 3:2.