The surface resistance of glass electrodes in alkaline solutions

Electroanalytical Chemistry and Interracial Electrochemistry

103

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

THE SURFACE RESISTANCE* OF GLASS ELECTRODES IN ALKALINE SOLUTIONS

ANDERS WIKBY

Department of Analytical Chemistry, University of Ume&,901 87 Ume&(Sweden) (Received 31st January 1972)

The potentiometric behaviour of hydrogen ion selective glass electrodes in alkaline solutions, where partial or full electrode response for alkali ions is generally found, has been extensively studied. The present work contributes very little in this respect since the main purpose has been to extend earlier work I -4 dealing with the surface conductivity of Ingold glass electrodes into the alkaline range of the electrode response. In neutral solutions a surface resistance developed which was related to the formation of a gel layer on the electrode surface a. It was found that the resistance was located at the boundary between the gel layer and the glass and that at this boundary the lithium ion concentration increased sharply from a low average value in the layer to a value characteristic of the glass. The reciprocal surface resistance was found to be proportional to the lithium ion flux from the glass into the solution. The gel layer thickness increased with time of hydration to an extent depending on the lithium ion flux across the boundary. This behaviour, however, is valid only when the outer boundary, gel layer-solution, dissolves at a rate which is small compared with that of the movement of the inner boundary. In neutral solutions at room temperature this was the case with the electrode Ingold LoT 3. In alkaline solutions, however, the situation is expected to be drastically changed, the dissolution rate of the outer glass surface being greatly increased by alkaline attack. It has been pointed out 5'6 and found 7'8 that the attack increases rapidly above pH 9. In addition the inner boundary, gel layer-glass bulk, is expected to move more slowly at high pH values owing to the reduced concentration of hydrogen ions. Douglas and E1-Shamy 8 found that the rate of dissolution of sodium ions and potassium ions from their glasses decreased only slightly when the pH was varied from 2 to 9, but above the latter value the rate suddenly decreased with increasing pH. The break at pH 9 was attributed solely to the decrease in hydrogen ion concentration. Bach and Baucke 9 recently found that the gel-layer thicknesses of their lithium glasses were dependent on pH. The electrodes were hydrated for 6 days at pH values 1 and 9 and the gel-layer thicknesses were found to be 30 nm and 15 nm, respectively. Univalent alkali metal ions in the solution to some extent replace hydrogen ions in the gel layer when the pH of the solution becomes sufficiently high. This effect, * This surface resistance has been measured n o r m a l to the electrode surface. T h e resistance has been found to be located some distance within the physical surface and the n a m e surface should therefore not be interpreted too literally.

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as well as those discussed above, might be expected to influence the surface conductivity and the gel-layer thickness of glass electrodes. The present work involves studies of changes in these parameters in alkaline solutions of different pH containing different amounts of alkali metal ions. EXPERIMENTAL

The pH values of solutions containing sodium hydroxide and tetraethylammonium hydroxide were determined by e.m.f, measurements in a cell Pt, H2(g)lsoln. X IJKCl(satd-)lHgzCl2 (s), Hg(l)

(1)

following the NBS recommended procedure 6. Sodium tetraborate decahydrate (0.01 M) and saturated calcium hydroxide solutions were used as standards ; the pH values were taken as 9.19 and 12.51, respectively at 23.3°C. The e.m.f, of cell (1) when the glass electrode, Ingold LoT, replaced the hydrogen electrode was measured in the same solutions. The glass electrode had an inner solution of a phosphate buffer (pH 7) containing 0.1 M KC1 and an Ag IAgC1 inner reference electrode. The e.m.f, measurements were made at 23.3_+0.5°C with a Radiometer pH meter 4d. An Hildebrand-type hydrogen electrode with two platinum foils was used. The electrodes became stable within about 15 min and the e.m.f, values of the two foils were always the same within 0.2 mV. The glass electrodes were stored in distilled water when not in use. The glass electrode potential was read after 20-25 rain. If there was an alkaline error a slight drift in potential remained after this period. Electrodes were prepared for resistance and leaching measurements by etching for 2 rain in 5~o hydrofluoric acid solution and then hydrating in redistilled water for selected times. Dissolution experiments were performed in polyethylene bottles containing 4 ml of the appropriate solution at 25.0 + 0.3°C. Only the membrane parts were immersed in the solution. The electrodes were transferred to new portions of the solution after different times. The samples were analysed for silicon spectrophotometrically at 815 nm using the blue silicon-molybdate complex. Lithium was determined by flame emission using a Unicam S P 90 atomic absorption spectrophotometer at 670.8 nm. The surface resistance was measured after different times of alkaline attack at 25.0 + 0.1 °C. Details of the resistance measurements have been published earlier 1-3. RESULTS

E.m.f. values of the glass electrode are plotted in Fig. 1 vs. pH as measured by the hydrogen electrode. The upper straight line was obtained in solutions containing no alkali metal ions. The slope of the line between the standard solutions was nearly Nernstian being 59.3 mV per pH unit as compared with a slope of 58.8 mV per pH unit obtained with the hydrogen electrode. The three curves lying under the straight line display the behaviour of the glass electrode in solutions containing NaC1 and NaOH with a sodium ion concentration of 0.11, 1.0-1.1 and 5.0-5.1 M respectively. The electrode responded to sodium ions to various degrees as expected. Figure 2 shows results similar to those in Fig. 1 except that the interfering ion was potassium at concentrations of 0.10, 1.0 and 4.0 M. J. Electroanal. Chem., 39 (1972)

SURFACE RESISTANCE OF GLASS ELECTRODES IN ALKALI

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Fig. 1. E.m.f. values obtained in the cell Hg (1), Hg2C12 (s) I KCI (satd.) II soln. X I glass [ 0.1 M KCI buffered to pH 7 I AgCI, Ag plotted vs. pH determined with hydrogen electrode. Solutions X : (+, lower) 0.01 M Na2B407 . 10 H20, (+, upper) satd. Ca(OH)2, ((3, lower) 0.01 M (C2Hs)4NOH and 0.01 M (C2Hs)aNCI, (Q, upper) 0.1 M(C2Hs)4NOH and 0.01 M(C2Hs)4NC1. Other solns, composed of NaOH and NaCI. Sodium ion concn.: (A) 0.11, ([]) 1.0-1.1, (0-)5.0-5.1 M. Fig. 2. E.m.f. values obtained in the cell reported in Fig. 1 vs. pH. Solns. (+), (+), ((3), (O), as Fig. 1. Other solns, composed of (C2Hs)4NOH and KC1. Potassium ion concn. : (&) 0.1, (ff]) 1.0, (Q) 4.0 m .

In Fig. 3 the increase in surface resistance of Ingold LoT electrodes has been plotted v s . storage time in the four solutions in which the electrode behaved ideally. The pH values of the solutions were 9.2, 11.9, 12.5 and 12.9. The lowest curve in the Figure corresponds to results obtained in previous studies at pH 7.0. The value at zero time represents the surface resistance of the electrode at the beginning of the experiment. The electrodes were hydrated for about 44 h before the experiment was started. Figure 4 shows the value of the surface resistance of Ingold LoT electrodes

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Fig. 3. Increase in surface resistance of Ingold LoT electrodes (hydrated 2 days) vs. storage time in solns, of various pH. (Lowest curve) 2 mM NH4C1, 2 mM NH4HzPO ~ adjusted to pH 7 with NH 3 ; ( 0 ) 0.01 M Na2B40 7 • i0 H2 O, pH 9.2; (A) 0.01 M (CzHs)4NOH and 0.01 M (C2Hs)4NC1, pH 11.9 ; ([5]) satd. Ca (OH)2, pH 12.5; ((3) 0.1 M(C2Hs)aNOH and 0.01 M(C2H5}4NC1, pH 12.9. Fig. 4. Variation in surface resistance vs. storage time in different solns. Electrodes hydrated for different times in redist: water prior to storing in different solns. : (G) hydrated 50 h, storing soln. : 0.1 M (C2Hs)4NOH + 0.01 M (C2H 5)4NC1 ; (~') hydrated 0 h, storing soln. : 0.1 M NaOH + 0.01 M NaC1; (!?)hydrated 4 h, storing soln. : as (V); (A) hydrated 44 h, storing soln. : as (•); (G) hydrated 43 h, storing soln. : 0.1 M NaOH + 1 M NaCI; (0) hydrated 43 h, storing soln. : 0.1 M NaOH + 5 M NaC1. d. Electroanal. Chem.,

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plotted vs. storage time in 0.1 M tetraethylammonium hydroxide or in 0.1 M sodium hydroxide containing 0.01, 1.0 and 5.0 M sodium chloride. The electrode showed increasing alkaline error in these solutions as can be seen from Fig. 1. The Figure gives results obtained with etched electrodes as well as electrodes which were hydrated in redistilled water for about 4 and 46 h respectively. In Fig. 5 the increase in surface resistance of an Ingold LoT electrode hydrated for about 44 h in redistilled water is plotted vs. storage time in solutions which contained 0.1 M tetraethylammonium hydroxide and 0, 0.1, 1.0 and 4.0 M potassium chloride. J'. R/M

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Fig. 5. Increasein surfaceresistanceof hydrated (2 days) LoT electrodesvs. storage time in 0.1 M (C2H5)4" NOH soln. containing: (O) 0.01 M(C2H~)4NCI,(z~) 0.1 M KCI, ([]) 1.0 M KC1, (O) 4.0 M KC1. Fig. 6, Accumulatedamounts of silicondissolvedfrom LoT electrodesvs. storagetime. Electrodeshydrated for differenttimes and then stored in differentsolns., details as in Fig. 4, Accumulated amounts of silicon obtained simultaneously with the experiments reported in Fig. 4 are plotted vs. storage time in the respective sodium salt solutions in Fig. 6. The dissolution of lithium ions from one electrode was investigated at a pH of about 12 with a sodium ion concentration of about 0.02 M. The electrode had previously been hydrated for 44 h in distilled water. The surface resistance measured after storage for 50 h in the solution was 6 times higher than the resistance found for the same electrode type when stored at pH 7 under otherwise identical conditions. The lithium ion flux was found to be about 10 times lower in the alkaline solution than at pH 7 when the corresponding comparison was made. DISCUSSION Influence o f p H

When the pH of a solution was increased but the electrode still responded only to hydrogen ions, it was found that the surface resistance increased more rapidly with storage time at the higher pH values, see Fig. 3. This behaviour is in agreement with earlier explanations of the origin of the surface resistance given in refs. 2 4 and summarized in the introduction to this paper. If the supply of hydrogen ions in the solution becomes insufficient the rate at which the boundary gel layer-glass bulk moves inwards towards the dry glass will be retarded, as can be seen from the exchange reaction J. Electroanal. Chem., 39 (1972)

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H + (soln.) + Li + (glass bulk)-,H + (glass bulk) + Li + (soln.) This effect is comparable with that found when the electrode was transferred from water to isopropanol solution 4. The absence of sufficient amounts of hydrogen ions and water molecules in the latter solvent caused the surface resistance to increase sharply. The present result, however, gives the additional information that hydrogen ions even in the presence of water molecules may be essential for the hydration processes to occur. The importance of the presence of water molecules, is not, however, excluded.

Influence of alkali metal ions In the range of alkaline error of the hydrated glass electrode there was an additional increase in the surface resistance with storage time. This can be seen in Figs. 4 and 5. Comparison of the results obtained in the two Figures indicates that the effect from sodium ions was much greater than that from potassium ions. The increase in resistance was also more rapid at higher alkali metal ion concentrations. With increasing alkaline error, hydrogen ions in the gel layer are successively replaced by alkali metal ions from the solution. For sodium ions, full alkali response is probably obtained in the solution consisting of 5 M sodium chloride in 0.1 M sodium hydroxide, as the e.m.f, of the cell was almost unaffected by a change in pH (see Fig. 1). If a significant proportion of hydrogen ions has been replaced by sodium ions in the gel layer, the inner phase boundary may move inwards towards the glass bulk by exchange processes involving sodium, lithium and hydrogen ions. It is assumed that no direct exchange occurs between lithium ions in the glass and alkali metal or hydrogen ions in the gel layer due to differences in sizes of the ions. The differences in the rate of movement of the boundary may then be explained on the basis of the unique ability of the hydronium ion to react with the silica network as proposed earlier 3 and as suggested by Csfikvfiri et al. 1°. Similar structure-breaking processes would not occur with the alkali metal ions assumed to be devoid of hydration shells. By comparing the results in Figs. 1 and 2 it may be seen that the Ingold LoT electrode showed similar selectivity for sodium and potassium ions up to 1.0 M concentrations. Above this concentration, however, the electrode responded preferentially to sodium ions so that full potassium ion response was never reached. One reason for this may be differences in sodium and potassium ion activities at higher concentrations of the sodium and potassium chloride solutions. Another reason may be that exchange involving potassium ions is less favourable owing to their larger size. Such behaviour is not uncommon among zeolites which show a sharp sieve action as described by Helfferich 11. Since the density of the inner part of the gel layer is probably higher than the outer part, it will be assumed that potassium ions do not penetrate into the barrier region where the surface resistance is located. Consequently, the latter will be influenced only slightly by potassium ions as shown in Fig. 5. Changes in gel-layer thickness The above considerations hold only if the glass electrode has a gel layer separating the dry glass and the alkaline solution because of the very high dissolution rate of the unhydrated electrode glass. If an etched electrode, initially exhibiting zero surface resistance, is placed in an alkaline solution of sufficiently high pH, the disJ. Electroanal. Chem., 39 (1972)

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solution rate will exceed that of hydration. No gel layer is built up and the surface resistance remains zero. Such behaviour shown by an etched Ingold LoT electrode can be seen in Fig. 4 (lower curve). From Fig. 6, the upper curve, it can be seen that the rate of dissolution of silicon from the electrode is high. The rate levels off after about 10 h and becomes about 50 times higher than the rate attained in redistilled water 3. The intercept at zero time is probably due to droplets formed by the etching procedure on the surface which are dissolved rapidly when the electrode is transferred to water, as discussed earlier 3. The preceding observations show that the gel-layer thickness of an hydrated electrode will decrease with time when the electrode is placed in an alkaline solution. This is contrary to what was found in neutral solutions 3. In the presence of an alkaline error the boundary gel layer-glass bulk moves considerably more slowly than at pH 7 and simultaneously the boundary gel layer-solution is dissolved much more quickly. The former statement is supported by the rapid increase in surface resistance and the very low lithium ion flux from the glass into the solution. The rate of dissolution of the gel layer was determined by measuring the dissolved silicon as shown in Fig. 6. The dissolution was studied in parallel with resistance measurements in a solution containing 10 mM NaC1 in 0.1 M sodium hydroxide for two electrodes which were hydrated for 4 and 44 h, respectively, in redistilled water. The amount of lithium ions dissolved away from the electrodes after these hydration times was earlier determined to be 0.20 and 0.62/zmol respectively3. The corresponding amounts of silicon remaining in the two gel layers are then 0.24 and 0.75/~mol since the mole fraction lithium to silicon of the glass is 0.82 and a negligible amount of silicon is dissolved during the hydration. If it is assumed that the inner boundary does not move inwards towards the dry glass when the electrode is in the alkaline solution, it is possible to calculate from the dissolution rate of silicon, shown in Fig. 6, the time required for the gel layer to be completely removed. The times are 14 and 44 h respectively for the two electrodes which were hydrated for 4 and 44 h. In Fig. 4 it can be seen that the surface resistances of the respective electrodes began to decrease after 16 and 50 h. It may be concluded that the gel layers were etched away after these times in the alkaline solution, thus eliminating the surface resistance. Subsequent etching in hydrofluoric acid showed that the ratio of lithium to silicon in the remaining surface region was characteristic of the glass bulk, which supports the conclusion made. In addition, the inner boundary had moved very slowly during the storage of the electrodes in the solution as required by the initial rapid increase in surface resistance. In the solution containing 0.1 M NaOH and 5 M NaC1 the reversal in surface resistance did not occur within 70 h. As can be seen from Fig. 6 the dissolution rate of silicon was considerably slower than that obtained in the case discussed above, indicating that the gel layer should not be eliminated in this solution within about 100 h. ACKNOWLEDGEMENTS The author thanks Dr. Ingold, Ztirich, for gifts of electrodes, Prof. G. Johansson for valuable discussions, Mrs. K. Palmgren for help with experiments and Dr. M. Sharp for reading the manuscript. This work was supported by grants from the Swedish Natural Science Research Council. J. Electroanal. Chem.,

39 (1972)

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SUMMARY

Changes in surface resistance of Ingold LoT electrodes due to the attack of alkaline solutions of different pH, pNa and pK values have been followed for about 50 h. There were increases in resistance due to diminishing hydrogen ion concentrations of the solutions as well to increasing concentrations of alkali metal ions which produced the well-known alkaline error of glass electrodes. The dissolution rate of silicon from the electrodes due to the alkaline attack on the silica structure was considerably higher than that found in neutral solutions. These observations have been interpreted on the basis that the inner sharp boundary between the gel layer and the glass bulk moves at a considerably lower rate than that found in neutral solution while the outer boundary gel layer-solutions moves very much faster. The gel layer thus decreases in thickness with the time of storage in the alkaline solutions. This is contrary to what is found at pH 7. The high silicon dissolution rate and a very low lithium dissolution rate as well as a total elimination of the gel layer after definite times of storage in some alkaline solutions, support the general interpretation given above.

REFERENCES 1 A. Wikby and G. Johansson, J. Electroanal. Chem., 23 (1969) 23. 2 A. Wikby, J. Electroanal. Chem., 33 (1971) 145. 3 A. Wikby, J. Electroanal. Chem., 38 (1972) 429. 4 A. Wikby, J. Electroanal. Chem., 38 (1972) 441. 5 H. T. S. Britton, Hydrogen Ions, Vol. I, Van Nostrand, Princeton, New York, 4th ed., 1956, chap. 7. 6 R. G. Bates, Determination ofpH, Wiley, New York, 1964. 7 D. Hubbard, E. H. Hamilton and A. N. Finn, J. Res. Natl. Bur. Stand., 22 (1939) 339. 8 R. W. Douglas and T. M. M. El-Shamy, J. Amer. Ceram. Soc., 50 (1967) 1. 9 H. Bach and F. G. K. Baucke, Electrochim. Acta, 16 (1971) 1311. 10 B. Cs~ikv~iri, Z. Boksay and G. Bouquet, Anal. Chim. Acta, 56 (1971) 279. 11 F. Helfferich, Ion Exchange, MacGraw-Hill, New York, 1962. J. Electroanal. Chem., 39 (1972)