Electrochemistry Volume 3

A Review of the Literature Published During 1971

By G. J. Hills

The Royal Society of Chemistry

Copyright © 1973 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-027-5

Contents

Chapter 1 Reversible Electrode Systems and Related Topics by A. K. Covington,
Chapter 2 The Conductance of Electrolyte Solutions by M. J. Wooften,
Chapter 3 The Solid Metal Electrode in Aqueous Solution by N. A. Hampson,
Chapter 4 Ionic Double Layers and Adsorption by D. J. Schiffrin,
Chapter 5 Organic Electrochemistry – Synthetic Aspects by P. M. Robertson,

CHAPTER 1

Reversible Electrode Systems and Related Topics

BY A. K. COVINGTON

1 Introduction

The literature surveyed for this Report covered the period mid-1970 to mid-1972. The format of the previous Report has been followed to facilitate reference back to topics discussed there, and to avoid the need for excessive repetition of references. Water should be understood to be the solvent used in the investigations described, unless another solvent system is specifically mentioned.

Highlights in the work to be described below include an increasing awareness of the importance of surface films (gel layers) on glass electrodes and their influence on time-dependent potentials, and the development of ‘neutral carrier’ complexes of alkali-metal and alkaline-earth ions on which a new range of ion-selective electrodes is based.

Buck has extensively revised and extended the chapter ‘Potentiometry’ in the new Weissberger series. He has also contributed an extensive review (1000 references) to Analytical Chemistry, Review of Fundamentals. Rock has advocated the use of glass or amalgam electrodes in double cells without liquid junction when an electro-active species must be prevented, because of the possibility of chemical reaction, from coming into contact with a reference electrode of the second kind, e.g. to avoid a reaction such as:

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This is essentially the ‘bridging technique’ in Covington’s survey of methods of using reference electrodes. A good recent example is the use of the lanthanum fluoride ion-selective electrode as a reference electrode in nitrate-ion determination with a liquid ion-exchanger ion-selective electrode. The use of two ion-selective electrodes in a cell may present measuring problems since both may have high resistances. Brand and Rechnitz have described an integrated-circuit differential amplifier, which enables these problems to be overcome.

2 Conventional Electrode Systems (Electrodes of First and Second Kinds, Redox Couples)

Feltham and Spiro have contributed a valuable review on the platinized platinum electrode, that most widely used of all types of electrode. As the authors point out, it is remarkable how little is known about the deposition of platinum from lead-containing or lead-free solutions despite the use of the process for 75 years. The addition of lead acetate to the chloroplatinic acid must surely be the most famous electrochemical recipe, and some important features of its role emerge from this careful review and the authors’ own lo Lead can be leached from platinized platinum electrodes prepared in the presence of lead acetate, but only from the first two or three atomic layers where it is present in the form PbO, both acid and oxygen being necessary for the dissolution to occur. The authors consider that the most widely recommended recipe contains too much lead acetate and recommend the following procedure: 3.5% chloroplatinic acid plus 0.005% lead acetate, at a current density of 30 mA cm-2 for up to five minutes for hydrogen-e.m.f. or conductance electrodes. Good stirring is considered essential and no gas should be evolved at the platinum cathode. Chlorine evolved at the anode should be prevented from reaching the cathode by use of an H-type plating cell or similar device.

The Milan electrochemistry group report that hydrogen electrodes function reversibly in up to 99% acetonitrile-water mixtures if the electrodes are ‘smooth’ platinum in the form of a ribbon wound round a glass frit through which hydrogen is bubbled. It is a pity that details are not given about the method of pretreatment of the electrodes. The same workers’ capillary inhibition electrode, which is platinized, and a-palladium electrodes function only up to about 25 % acetonitrile, but quinhydrone electrodes can be used in acetonitrile-water mixtures. The possibilities of using the palladium hydride electrode as a reference electrode at high temperatures (up to 195 °C), where the presence of hydrogen gas may be objectionable, have been explored, along with certain other features of its behaviour.

Reports of drifts of e.m.f. with time of cells involving the quinhydrone electrode have been frequent. A linear e.m.f. drift with time in acetate buffers has been traced to nucleophilic attack by the acetate ion on the p-benzoquinone component of quinhydrone. The reaction was followed by observing changes in the U.V. spectrum of the p-benzoquinone grouping at 246 nm, and correlating these with the e.m.f. changes (0.2 mV h-1). The e.m.f. drift does not preclude the use of the quinhydrone electrode in acetate buffers for, since it is linear and small, its contribution can be eliminated by extrapolation back to zero time. The temperature dependence of the salt error of the quinhydrone electrode in 1 mol kg-1 lithium chloride solution has been determined by Struck and Schneider who, to obtain the results, needed to find the mean activity coefficient of hydrochloric acid in this salt solution. From 1H n.m.r. studies, Hepfinger, Tomkins, and Turner conclude that there are no significant interactions which will cause the activity coefficients of the appropriate substituted quinone and hydroquinone forms to change, and hence preclude the use of the chloranil electrode in acetonitrile.

Izatt and co-workers report a formal potential of -979 [+ or -] 0.005 mV for the Pd|Pd2+ couple in 3.94 mol kg-1 perchloric acid medium. The cell contained an internal platinum wire connection to avoid precipitation of potassium perchlorate at the liquid junction with a saturated calomel electrode, though an intermediate sodium chloride bridge was also incorporated. Lightsuggests a solution of acidic ferrous and ferric ammonium sulphates as a standard (poised’, analogous to ‘buffered’) redox solution for checking-out systems for measuring redox potentials. The search for suitable reference electrodes in non-aqueous solvents continues. The I3-|I- system has been suggested for propene carbonate and the Cu+ |Cu+ for pyridine. The standard potential of the latter couple in acetonitrile has been determinedand refined using an Owen-cell extrapolation method for eliminating the liquid-junction potential.

In the last Report the resurgence of interest in amalgam electrodes was welcomed, and a useful review is now available. Attention is drawn to the quaternary ammonium amalgams as providing a ‘slight chance’ of being useful as electrodes reversible to the popular quaternary ammonium ions. Mussini and Pagella report standard potentials for the calcium amalgam electrode at 25–70 °C. Zinc amalgam- and lead amalgam-lead fluoride electrodes have been used to determine the association constant of ZnF+ from cell measurements. The Cd(Hg)| CdSO4 | Hg2SO4 | Hg cell has been studied in dioxan-water mixtures (up to 60 wt %dioxan) and the reference electrode system Cd(Hg)| CdCl2, NaCl has been suggested for use in dimethylformamide.

Baucke, apparently unaware of a contribution discussed in the last Report, presents a lengthy discussion of the effect of excess solubility of electrode material from electrodes of the second kind on their potentials, He reaches the same conclusion, namely that the effect is principally one of enhanced ionic concentration rather than creation of diffusion potentials. In presumably his last contribution on the subject, since he has now retired from the Chair of Electrochemistry at Birkbeck College, Ives (with Prasad) has described a further ‘improved’ calomel electrode. The ‘banjo’ cell has a large ratio of mercury surface to solution volume (5 ml). Equilibrium time is now reduced to 9 h, with 30 min to reach new equilibrium after a 5 K temperature rise. The new design was tested with one molality of aqueous hydrochloric acid and then used for determination of pK1 and pK2 values over a temperature range for malonic and some substituted malonic acids. We offer our felicitations for a long and happy retirement after 40 years of meritorious contributions to Electrochemistry.

Leuschke and Schwabe report a redetermination by an Owen-cell method of the standard potential of the mercury-mercurous bromide electrode. The value at 25 °C (139.21 [+ or -] 50.04 mV) is in good agreement with work from Ives’ laboratory. In the last section of this paper4 the authors’ conclusion, that the variation of diffusion potential cannot be accounted for by activity coefficient considerations, is erroneous, being based on an incorrect equation (6).

The saturated (KCl) calomel electrode continues to be a popular choice as reference electrode but cannot be used in some non-aqueous solvents because of solubility difficulties. The system Hg | Hg2Cl2 | (C2H5) 4NCl has been suggested for propene carbonate with or without the addition of potassium chloride (sat.). The ‘standard potential’ (E0′ + E1) of the electrode with potassium chloride saturated in methanolic solution has been determined by Russian workers’ over the range 15–40 °C. The standard potential of the mercury-mercury(I) picrate electrode has been reported at 25 °C, and a study by Baldwin suggests that the Ag | Ag3PO4 electrode could be a feasible system for the study of phosphate equilibria. The silver-silver per-chlorate electrode has the advantage of quick response, long-term stability, and reasonably small bias potentials (0.5 mv) as a reference electrode system for propene carbonate, and moreover appears to tolerate the addition of small amounts of water.

The hydrogen-silver halide cell remains a valuable route to thermo-dynamic data. We conclude this section by mentioning recent determinations of standard potentials and related information: Ag I AgBr in methanol-water, Ag | AgCl in ethanol (up to 80%)-water, Ag | AgCl in propan-1-ol, propan-2-ol (95%)-water, butan-1-ol, t-butyl alcohol (up to 8%)-water, glycerol, and glycerol (up to 70%)-water; Ag | AgCl and Ag | AgBr in the isodielectric mixture methanol-propene glycol (the following three papers give autoprotolysis constants, amalgam standard potentials, and proton-transfer solvent effects); Ag | AgCl in DMF (5 and 10%)-water, in DMSO (5, 10, 20, and 40%)-water; Ag | AgCl, Ag | AgBr, and Ag | AgI in acetonitrile (up to 20%)-water; and some supplementary data for ethanol-water, acetone-water, and dioxan-water.

3 Glass Electrodes

We continue to distinguish glass electrodes from the new range of ion-selective electrodes, if only for historical reasons. Advances in solid-state electronics have rendered the measurement of small potentials from high-impedance sources no problem, and all-solid-state, digital-read-out pH meters are now available. Groups at Oxford and Reading have demonstrated that an integrated-circuit operational amplifier with digital voltmeter read-out, or a vibrating-condenser electrometer backed-off with a vernier potentiometer, can equally well be used for potentiometric titrations with 0.01 mV discrimination, using commercially available high-resistance glass electrodes. McBryde has pointed out that interpretation of pH as -logaH+ and a suitable estimate of the activity coefficient in order to get the hydrogen ion concentration, is not always the best method, particularly when a supporting medium of high ionic strength is used, as favoured by so many ‘complex-ion chemists’. Calibrations are given for three popular supporting electrolytes at several concentrations, thus enabling the pH meter readings to be converted into hydrogen-ion concentrations. The principles are exemplified by the determination of the concentration quotient of sulphosalicyclic acid.

Deviation from hydrogen-electrode function and time-drifts are often noted in solutions uncongenial to the electrode glass. A full account has appeared of work9 mentioned in the previous Report, where potential drifts in solutions containing various oxyanions have been attributed to the take-up of the anions into the glass, and such behaviour has been detected radiochemically. Other French workers have failed to be able to interpret errors of Corning 015 composition glass electrodes in HCl+NaCl solutions, using Eisenman’s equation for mixed H+/Na+ response. This is not surprising since Naresponse would only be expected at higher pH. Karlberg and Johanssonhave confirmed that electrodes which show high sodium errors in water show high errors in isopropyl alcohol solutions, and conversely ‘0–14’ electrodes show low errors. Japanese workers challenge the usual statement that alkaline errors are steadier and more reproducible than acidic errors and have followed potentials over many days by direct comparison with hydrogen-gas electrodes. It has been found in the Reporter’s laboratory and elsewhere that the extent of the alkaline error can decrease after prolonged soaking in water, and after repeated contact with alkali or alternation between acidic and alkaline solution.

The use of glass electrodes in solvents other than water is frequent, but the demonstration of strict hydrogen-ion response is not always easy. Two papers’ describe the use of glass electrodes in dimethylformamide. In liquid ammonia at — 38 °C, deviations from response to NH4 are attributed to alkali-metal-ion function, the order of selectivity being very different from that in water. Studies of the lithium-ion response of Beckman cation-responsive electrodes in propene carbonate and the effect of interfering ions are reported. Shults and co-workers have continued their work on the effect of methanol on cation-responsive aluminosilicate glass electrodes.

An understanding of the complex behaviour of glass electrodes in such varied media will only follow a better appreciation of the surface reactions of glass in contact with the solution. It is encouraging that more work is being directed to this end, and that advances are being made. It is now widely believed that many glasses are not homogeneous. Hair has suggested that two OH bands in the i.r. spectra of alkali-metal silicate glasses, viz. that at 2.8 µm (found only in pure silica) and another at 3.6 µm, which increases in intensity with Na2O content of the glass, are diagnostic of a pure silica micro-environment and a sodium silicate micro-environment in these glasses. With aluminosilicate glasses two analogous environments are again envisaged, the water distributing itself between the two phases which give rise to the 2.8 µm and 3.6 µm OH bands. A good linear correlation is found between the percentage OH in the 2.8 µm band and logKpotNaK the Eisenman selectivity constant, throughout the whole range of glass compositions for which data are available. NAS glasses become potassium-selective above an NA+/Al3+ ratio of 2.5. Potassium selectivity is attributed to the sodium silicate phase which gives rise to the 3.6 µm band. Sodium silicate glasses are known to be leachable to yield porous glasses, and it is suggested that response to K+ is due to the formation of porous hydrated layers; a suggestion based on similarities to sintered porous glasses, where ionic selectivity appears to be a molecular-sieve effect. The presence of micro-inhomo-geneities in glass makes questionable some attempts to correlate electrode response properties with bulk glass composition. Since the extent of phase separation will depend on the heat treatment of the glass, inconsistencies in behaviour of nominally identical electrodes are to be expected. Whitfield, in a careful study of the effect of the membrane geometry of the glass electrode on the asymmetry potential and its variation with pressure and temperature, concluded that only by flame annealing of the bulbs could reproducible results be obtained. This is a departure from commercial manufacturing practice for blown bulbs. The long-accepted statement that strain-free flat glass membranes have small and stable asymmetry potentials was confirmed. The best form of electrode which minimizes the asymmetry potential and its variation, and which has low resistance coupled with mechanical robustness, is a double-bulb electrode formed by fusing together two normal bulbs with stems at an angle of about 30 °C. This type is advocated for deep-sea measurements, where frequent standardization is impossible and conditions are extreme (5 °C, 1000 bar).

In continuation of work described in detail in the last Report, Wikby has applied his constant-current pulse method to determine changes in the surface resistance of glass electrodes when subjected to various solutions, either acidic, neutral, or non-aqueous. The surface resistance is obtained by resolution of the constant-current polarization curve, the contributions from the surface resistance and capacitance having time constants of the order of seconds as opposed to the contributions from the bulk glass, which are in the millisecond region. The relative merits of the constant-current pulse and a.c. methods are not clear, although in principle the same information should be obtainable from either. Surface films can only be detected by the a.c. method at frequencies less than 1 Hz. In the earlier paper it was shown that certain electrodes had high surface resistance, which decreased when the electrode was soaked in water (hydrated or ‘conditioned’). Additional experiments now show that this high resistance is located at the inside surface of the electrode and can be removed by etching with hydrofluoric acid. Electrodes treated in this way show an increase in surface resistance on conditioning, and the increase continues even after the e.m.f. becomes steady (e.g. after 70 h). In isopropyl alcohol solutions of lithium chloride there was found to be a take-up of Cl- ions that was dependent on the acidity, with a parallel increase in surface resistance; this suggests that the rise in resistance can be attributed to a blocking of the conduction mechanism by HCl molecules. When a partially hydrated electrode is transferred to isopropyl alcohol, the surface layer stops growing, as shown by controlled etching experiments of the type devised by Hungarian workers, and there is an increase in surface resistance. By etching small layers and remeasuring the surface resistance it was shown that the impediment to the conduction process is situated at the interface between gel layer and bulk glass. The Hungarian group has refined the successive etching treatment and chemical analysis so as to give the increased sensitivity necessary to investigate the much smaller gel layers on lithia electrodes. The layer thickness is pH-dependent, and in ethanol only a thin layer (2 x 10-5 cm) is built up, about ten times smaller than that in water. Dobos has used tritium radio-tracer experiments to determine the concentration profiles of water as well as those of the alkali-metal and alkaline-earth ions in the gel layer. The water concentration falls as expected, fairly sharply, and then more slowly until the gel layer-bulk glass interface is reached. Wikby’s experiments are in general agreement with the Hungarian work (the glasses used are different), the necessary information not always being available to derive the thickness of the gel layer from the number of moles of silicon of the network dissolved by the etchant. In some of Wikby’s experiments an inner-filling solution containing 0.25% HF was used to remove continuously the surface resistance contribution from the inside surface of the glass bulb. Presumably this has no effect on the e.m.f. of the system. A new method of analysing the surface layers of leached glass has been evolved by workers at the Jena Glaswerk, Mainz. The surface is sputtered with argon ions of high energy in a vacuum and is successively removed in layers, the average penetration of the ions being 6 nm. The luminescence produced by lithium ions in the glass under the bombardment is detected by a photomultiplier and used to obtain the concentration profile across the layer, its thickness being obtained by comparison with interferometric measurements of the depth of etch pits produced after long bombardment. The method seems promising and has been used to investigate the effect of pH on the lithium concentration profile in the gel layer. It is interesting that the presence of lithium in the conditioning solution resulted in layer concentrations being obtained which were greater by a factor of 2. Further experiments with xenon and krypton bombardment and analysis of the concentration profiles of other ion components are promised.

(Continues…)Excerpted from Electrochemistry Volume 3 by G. J. Hills. Copyright © 1973 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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