148
G. HAUGAARD
T H E MECHANISM OF T H E
GLASS ELECTRODE
G. HAUGAARDL Chemical Department, The Carlsberg Laboratory, Copenhagen, Denmark Received March dS, 1040
The glass electrode has been known for many years, but only during the last ten years has it been used to a great extent and become an indispensable instrument in many laboratories. This is due for the most part to a series of investigations performed by MacInnes and coworkers,-especially the study by MacInnes and Dole (8) concerning the importance of the composition of the glass. As a result of these investigations] the glass most suitable for the purpose was produced, having the following composition: 22 per cent “0, 6 per cent CaO, and 72 per cent SiO*.* During the past years several othor investigators have done research work upon the theory of the glass electrode and upon the electrical equipment especially designed for measurements with the glass electrode, as well as upon the applicability of the glass electrode for measuring pH under different conditions. The author intends, in a later and more extensive paper, to discuss this literature; here there will be mentioned only papers that are essential to the present work. The purpose of these investigations is to elucidate experimentally the mechanism of the glass electrode. In the opinion of the author, most of the previous experiments in this direction have failed because the investigators have tried to deduce their theories from the thermodynamical treatment of systems in equilibrium. But we know that thermodynamics alone can not tell us the mechanism of a process. In this investigation] other methods have been used. Potentials of glass electrode systems not in equilibrium have been measured and compared with the absorption of the hydrogen ion into the surface of the glass electrode. The absorption of hydrogen ion by Corning glass powder has been investigated, together with the absorption of water by the glass powder. Conductivity experiments with the glass have also been performed to elucidate how the electricity is transferred through the glass. These experiments will be described first. THE CONDUCTIVITY THROUGH THE GLASS
Cavendisha showed that glass conducts electricity a t high temperatures, and Faraday (3) demonstrated that the conductivity of glass is of an 1 Present address: Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts. Commercially available as Corning glass 015. 8 Cited from B. R. Lorenz, Die Elektrolyse geschmolzener S a k e , Vol. 2, Halle (1905).
’
149
MECHANISM OF T H E GLASS ELECTRODE
electrolytic nature. Warburg (9) found, in 1884, that the sodium ion alone carries nearly all of the current through the glass, and the same result has been attained by later investigators. Most of the conductivity measurements, however, have been carried out a t high temperature and under conditions different from those under which the glass electrode functions. MacInnes and Dole (7) have measured the resistance of differed kinds of glass a t room temperature and have found that glass with the highest content of sodium has the lowest resistance; this is in conformity with the results of Warburg and other investigators. The conductivity measurements given here were made under nearly the same conditions as those under which the glass electrode operates when used for pH determination^.^ The experimental arrangement is given ill
B
A
FIQ.1
A
B
FIQ.2
figure 1. The glass electrode was of the bulb type and was filled with a known amount of 0.02 n hydrochloric acid (the inner liquid). The electrode dipped into a beaker containing a known amount of 0.02 n hydrochloric acid (the outer liquid). The silver voltameter B contained a known amount of 0.02 n silver nitrate. The negative electrode in B was a silver wire, while the positive electrode was a platinum wire. Both the electrodes in A were plathum wires. Five such glass electrodes, each of them in series with a silver voltameter, were combined in parallel. The voltage was 220 volts and the temperature 25°C. The decrease in acidity of both the inner and outer solutions was determined by titration after 561 hr. of electrolysis. Aliquot parts of both solutions were evaporated and weighed. The results of this experiment are tabulated in table 1. Both 4 These are extended experiments similar t o that previously published by the author (Compt. rend. trav. lab. Carlsberg, S6r. chim. 22, 199 (1938)).
150
G. HAUGAARD
the inner and the outer solutions have decreased in acidity during the electrolysis, and this decrease corresponded exactly with the number of milligram-equivalents of silver deposited. From the weight of the evaporated inner and outer solutions, calculated as milligram-equivalents of sodium chloride, it appears that the sodium ion is quantitatively responsible for the transfer of electricity through the glass; otherwise the members of the second, third, fourth, and sixth columns of table 1 could not agree.s When the sodium ions disappear from the inner surface of the glass elec-
TABLE 1 XiLLIQRAH-EQQIVALEN~ LIBERATED FROM TEE QLAW ELECTBODE
ELECTRODE
SILVER VOLTAMETER MILLIQRAYEQUNALENTS
Outez liquid
Inner liquid
Determined by
Determined by
MUYBER
Titration
0.065 0.089
0.080
0.091 0.078
1
Evaporation
Titration
1
0.066 0.096 0.087
0.062
1
HOURS
Evaponrtion
0.005 O.Oo0
0.064
i
’1
AE
0.065 0.108
0.072 0.089
0.096
0.063
’ FIQ. 3
OF
DEPOSITED
0.072
0.013
103 203
300 400
503
ow
703
am
goo
i
ioco
HOURS
FIQ.4
trode during the electrolysis, hydrogen ions a t the same time will go in, and after electrolysis for some time the membrane will have a composition as sketched in figure 2. I n the layer A, hydrogen ions h a t e replaced most of the sodium ions; but the layer B still has nearly the original composition. It was of interest to determine whether the hydrogen ions or the sodium ions have the larger conductance in the glass. A silver voltameter was therefore combined in series with the five glass electrodes, and the number 5 Qualitative test indicates that the amount of calcium released into the inner and outer liquids is negligible.
MECHANISM OF T E E GLASS ELECTRODE
151
of coulombs passed was determined a t intervals by titration. These results are given graphically in figure 3. The current can be obtained from the slope of the curve in figure 3, as i = d coulombsldt, where t is measured in seconds. As the potential was constant a t 220 volts, the resistance ( R )of the systems could be deduced. As all other resistances of the system are negligible compared with that of the glass electrode, the measured resistance is the resistance of the five glass electrodes combined in parallel The results of these simple calculations are given graphically in figure 4. It is apparent that the resistance of the glass electrode increases markedly during the electrolysis, which shows that the hydrogen ions have a lower conductance in the glass than the sodium ions that they have displaced. THE HYDROGEN-ION ABSORPTION OF A FRESH GLASS SURFACE COMPARED WITH THE CORRESPONDING POTENTIAL CHANGES
It is a well-known fact that new glass electrodes are not suitable for exact measurements until they have been soaked in water for some time. The continuous variation in potential observed with a new electrode may be connected with the property of glass of exchanging sodium ions for hydrogen ions. I n a previous paper (4) the author has shown a distinct correlation between the amount of exchange of sodium ions for hydrogen ions in powdered Corning glass suspended in 0.1 n hydrochloric acid and the drift of the potential of a glass electrode, when one side of the glass membrane was fresh (Le., had not been soaked in water) and the other side had been soaked in 0.1 n hydrochloric acid for a week or longer. This experiment has been repeated with the important change that the alterations in potential now are compared with the absorption of hydrogen ions by the fresh glass electrode surface itself and not, as previously, with the absorption of hydrogen ions by glass powder. The experimental arrangement is shown in figure 5. The glass electrode of Haber’s bulb type is placed with half of the bulb dipped into a cup attached with wax to a table which can rotate in order to stir the solution. I n the cup is a known amount of a buffer solution of which the composition is as follows: 30 cc. of Sorensen’s ( M / 1 0 ) citrate solution diluted 20 cc. of 0.001 n hydrochloric acid, and quinhydrone one hundred times is added. Into the solution dips the siphon of a calomel electrode and a platinum wire for measuring pH with the quinhydrone electrode. Into the inner solution of the bulb there dips, as usual, a silver chloride electrode. When a and c are connected with the potentiometer, the pH of the solution in the cup can be measured, and when a and b are connected with the potentiometer the potential of the glass electrode can be measured. If the fresh glass surface dipping into the solution in the cup exchanges sodium ions for hydrogen ions, the composition and pH of the solution in the cup will alter during the experiment in a way corresponding to the
+
152
G . HAUGAARD
increasing amount of sodium ions and decreasing amount of hydrogen ions. T o test this idea, the pH values of a series of buffer solutions with compositions the same as that of the solution in the cup a t different times during the experiment were measured, and from these the amount of exchanged sodium ions was determined. The pH of the solution in the cup varied during the experiment from 4.59 to 4.81. The potential of the glass electrode system had to be corrected, owing to this alteration. This correction varied from 0 millivolt to 12.6 millivolts. The corrected potential values of the glass electrode system plotted against the time in minutes are given in figure 6, on the curve through the solid circles. The hydrogen absorption of the glass surface computed from the pH change is given as milligram-equivalents x lo5 per cm.*
FIQ. 5
of surface, the curve through the open circles. The two curves illustrate convincingly the correlation between the amount of hydrogen taken up by the surface and the variation of the potential. This is a typical result from a series of experiments. In figure 7 , for the given time 1, the potential of the system, rt,is plotted against the logarithm of br, the amount of hydrogen ion absorbed per unit area of surface. The points fall upon a straight line, which can be represented by the empirical equation rt =
59 log br
+ constant
The value of approximately 59, found for the slope of the line, is close to the value (59.1) at 25°C. of the factor R T / ( F log e ) from the Nernst
153
MECHANISM OF THE GLASS ELECTRODE
equation. This agreement seems to indicate that the electrochemical potential of the absorbed hydrogen ions at the beginning of the soaking of 2x1
I1 2
15
m
3
! i 10
5
05
I
I
I
I
0
10
20
30
200
0 0 40
Xinutea
FIQ. 6
:2 0
240
-
200 -0 1
+o 1
0 109
+03
b
FIG. 7
the glass is proportional to the logarithm of the amount of hydrogen ions taken up per unit area of the surface.
154
G. HAUGAARD DO THE HYDROGEN IONS I N THE GLASS CARRY WATER?
Owing to the known attraction of the proton for water, it would be supposed that the hydrogen ions in the glass are solvated. T o test this supposition, the following experiment has been performed. Samples of glass powder, 2 g. each, were shaken with a known amount of 0.02 n hydrochloric acid. After different times, samples of the glass powder suspension were filtered through a glass filter, an aliquot part of the filtrate was titrated, the glass powder was washed with water, and the excess of water was removed by centrifugation. The very last trace of water can, of course, not be removed from the surface in this way; the glass filters containing glass powder were therefore dried to constant weight over 27 per cent sulfuric acid. The tension of water over 27 per cent sulfuric acid is 19 mm. a t 25"C., while the tension is 23 mm. over water. Firmly bound water can not be removed under these circumstances, but presumably only the TABLE 2
~
TIME
WEIQET OF QLASS E $ ~ ~ ~ ~ ~ & - DRIED o F OVER 27 6 PER HYDROGEN I O N CENT H B O ~mnz18 ABSORBED
I
hour8
03' 153
654
OF
DRIED OVER
Pa,
1
DATA
OF
As YILLIGRAYEQUIVALENTS
'1
d/b
1
mg. ~
1 ~
0.317 0.334 1.226
2.80 2.98 3 96 7.63 9.18
~
1
1 ,
0.155 0.165 0.220 0.423 0.509
~
1
1
0.49 0.49 0.46 0.50 0.42
water which covers the surface of the glass powder is removed. After constant weight was reached, the glass filters were placed in a dessicator over phosphorus pentoxide and again dried to constant weight. This last step will remove the more firmly bound water, and the loss in weight will correspond to the amount of bound water. The results of a series of such measurements are tabulated in table 2. The first column of the table contains the time in hours that the glass powder has been exposed to the 0.02 n hydrochloric acid. Thesecond column gives the milligram-equivalents of hydrogen taken up by the glass powder. The third column gives the weight differences between the powder dried over 27 per cent sulfuric acid and later over phosphorus pentoxide; and the fourth column, these values calculated as milligramequivalents of water. The fifth column shows the amount of water taken up or bound by 1 mg.-equiv. of absorbed hydrogen ion. The absorbed hydrogen ion seems, from this investigation, to bind about 0.5 mole of
155
MECHANISM O F T H E GLASS E L E C T R O D E
water. The experimental value found for this ratio depends, however, upon the arbitrarily chosen conditions of the drying of the surface, and the true value might be higher than that actually found. If one equivalent of hydrogen ion transferred from the solution to the surface carries X equivatents of water, we must add a term for the corresponding work to the Nernst formula. Under this circumstance the equation is:
where A ; and A&, are the activities, respectively, of hydrogen ion and water in the solution on one side of the glass membrane, while Aff and are the same quantities with respect to the surface. K involves here not only the potentials of the ;eference electrodes (calomel and silver chloride-silver electrodes) but also the potential which refers to both the hydrogen and water activities on the other side of the membrane. TABLE
3
Absorption of alcohol and water by glass powder PER CENT ALCOEOL I N DEBICCATOR
WEIQBT INCREASE O F QLASS
ALCOHOL CONTENT O F
mr.
5 20 50 65 85
43.09 37.08 6.74 6.28
ms. 1.20 5.90 1.90 2.25
GLASS
PERCENTAQE OF ALCoaoLWATER MIXTURE ABSORBED
2.78 15.9 28.2 35.8 40.1
THE ABSORPTION O F ALCOHOL BY GLASS S U R F A C E S
Glass is also capable of taking up other substances than water,-for instance, alcohol and acetone. Concerning alcohol a simple desiccator experiment will show this. Different samples of fresh glass powder were placed in desiccators over different alcohol-water mixtures. After 410 hr. the increase in weight of the samples and the alcohol content of the powder were determined by the use of a microbalance. The content of alcohol was determined according to the method of Widmark (10) used for the determination of alcohol in blood. The glass powder was added directly to the solution of chromic acid and the iodometric titration of the alcohol was carried out 24 hr. later in order to be sure that the whole amount of alcohol had reacted. The results of this experiment are tabulated in table 3. I t appears from this table that glass has the ability to take up alcohol and water together. Experiments similar to those tabulated in table 3
,
156
G. HAUGAARD
have not been done with alrohol-water mixtures, but there is very little doubt that such experiments should give similar results, Le., the absorbed hydrogen ion will in this case bind water and alcohol. Consequently we must add a third term to the Nernst equation, representing the work of transferring alcohol from the solution to the glass-alcohol-water phase. The following equation must be valid in this case:
These equations are similar t o the equation reported by Dole (1) in 1932. Dole worked with strong salt solutions and alcoholic solutions, and found deviations from the hydrogen electrodes which followed the formula just given. DISCTTSSION OF T H E THEORY OF THE GLASS ELECTRODE
The theory of Horowitz (5) seems to be fairly well in line with the experiments given here. Horowita assumes that the glass electrode works as a hydrogen electrode on account. of the ability of the glass to exchange sodium ions for hydrogen ions.
FIG.8 On the basis of the above experiments, the following picture can be given of what happens when a fresh glass curface comes in contact with an acid, neutral, or very weakly basic solution (Le., within the range where the glass elertrode acts only as a hydrogen electrode): At first the glass will take up water and the sodium salt of silicic acid will dissociate under the influence of the water taken up, hydrogen ions at the same time being absorbed,-in other words, the sodium salt of the weak silicic acid is partially hydrolyzed at the surface by the water, forming in the surface layer a skeleton of silicic acid. Hydrogen ions react readily with this surface, which affords an easy entrance of the hydrogm ions into the glass. I n the middle of the glass there is a layer of intact sodium salt, as sketched in figure 8. With the passage of feeble currents through the membrane dur-
MECHANISM OF T H E GLASS ELECTRODE
157
ing the compensation of the electromotive force of the system, there will be a slight movement of the salt layer toward one side, depending on the direction of the current. This “transport” theoretically does not require the doing of any thermodynamic work. This theory is in many respects an extension of that of Horowita and of a theory given later by MacInnes and Belcher in the following words ( 7 ; page 3328) : “A simpler assumption, and one which accounts for all the phenomena within the p H range in which the glass electrode functions quantit,atively, is that the hydrogen ion, or proton, can pass the electrolyte-glass boundary at a lower pot#ential than any other positive ion and that negative ions do not pass the boundary at all. This does not involve any assumptions as to the conduction in the glass. It is quite probable that other ions than hydrogen move in the body of the glass.’, Later in the same paper (page 3330), the authors emphasize the importance of the water absorption. The absorption of water by glass causes a swelling of the surface which has been demonstrated interferometrically by Hubbard, Hamilton, and Finn ( 6 ) . These authors have also shown that a corrosion of the surfme takes place when the glass is immersed in solutions the p H values of which are higher than 9. SUMMARY
The transference of electricity through glass under conditions approximating those under which the glass electrode works has been studied. The mechanism of the glass electrode has been in part elucidated by experiments with electrodes, the glass membranes of which were in equilibrium with the surrounding solution only on one side. The hydrogen-ion uptake of glass surfaces has been measured in connection wit,h the absorption of water. Experiments with alcohol-water absorption have also been reported. The theory of the glass electrode has been discussed on the basis of the experimental results given. REFERENCES (1) DOLE,M.: J. Am. Chem. SOC.64, 3095 (1932).
(2) (3) (4) (5) (6) (7)
(8) (9) (IO)
DOLE,M.: J. Am. Chem. SOC.63, 4260 (1931). FARADAY, M.: Ostwald’s Klassiker, No. 86. G . : Nature 140, 66 (1937). HAUGAARD, K . : 2.Physik 16, 369 (1923). HOROWITZ, HUBBARD, D., HAMILTON, E. H . , A N D FINN, .4. C.: J. Research Sat]. Bur. Standards 42, 339 (1939). MACINNES, D. A . , AND BELCHER, D . : J. Am. Chem. SOC.63, 3315 (1931). MACINYES, D. A., AND DOLE, M. : J. Am. Chem. SOC.62, 29 (1930). WARBURG:Ann. Physik 21, 622 (1884); Ber. 17, R193 (1884). WIDMARK,ERIKh‘f. P.: Biochem. 2.131, 473 (1922).
