Electrochemistry Volume 8

A Review of Recent Literature

By D. Pletcher

The Royal Society of Chemistry

Copyright © 1983 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-067-1

Contents

Chapter 1 The Electrochemistry of Porous Electrodes: Flooded, Static (Natural) Electrodes By N.A. Hampson and A. J. S. McNeil, 1,
Chapter 2 Electrode Processes in Molten Salts By J. Robinson, 54,
Chapter 3 The Electrochemistry of Transition-metal Complexes By C.J. Pickett, 81,
Chapter 4 The Electrochemistry of Oxygen By D.J. Schiffrin, 126,
Chapter 5 Organic Electrochemistry – Synthetic Aspects By J. Grimshaw and D. Pletcher, 171,

CHAPTER 1

The Electrochemistry of Porous Electrodes: Flooded, Static (Natural) Electrodes

BY N A. HAMPSON AND A.J.S. McNEIL

1 Introduction

The theory of electrochemistry that is presented in the standard textbooks has been obtained by considerations of ideal electrodes, and generally confirmed by experiments with mercury and amalgam electrodes, which present the nearest approach to the ideal situation. The ideal solid electrode is smooth, of accurately known surface area and crystal orientation, structurally perfect, and strain-free. The practical electrochemistry that is encountered in industry is concerned with electrodes that are rough and which present a large number of differently oriented crystal faces to the electrolyte solution. Often, these surfaces are fissured, and they may even contain phase demarcations. The need to present the maximum surface area to the reacting electrode/electrolyte interface has inevitably resulted in the development of quite porous electrodes, such as are commonly found in the electrical storage-battery industry. The lead-acid cell and the Leclanché cell, the two best known to commerce, both contain porous electrodes. Indeed, the authors do not recall a single example of a storage cell consisting of two non-porous solid electrodes.

Porous electrodes can be subdivided into five distinct classes:

(i) a prefatory class of rough electrodes, in which the surface area is somewhat increased over the projected, geometric area of the electrode; all ‘plane’ electrodes are rough to some extent;

(ii) porous or granular electrodes, produced by a specialized process of electrodeposition;

(iii) hydrophobic gas electrodes, whose operation depends critically upon the establishment of a three-component (solid-liquid-gas) interphase;

(iv) flow-through electrodes, with forced input of reactants; and

(v) ‘natural’, flooded, static porous electrodes.

Rangarajan has presented a brief review of the theory and operations of porous electrodes, with a classification of the operating models. The authors are not aware of any review of gas electrodes [class (iii)] that has been made since the survey of fuel cells by Bockris and Srinivasan in 1969. Newman and Tiedemann have on two occasions reviewed the subject of flow-through electrodes [class (iv)]. Work on both of these classes of porous electrodes over the past decade will be reviewed in the next volume in this series.

Although the basic theories have remained unchanged since the review in 1966 by de Levie, a substantial amount of theoretical development and confirmatory experimental work has been carried out in connection with the major porous electrodes of electrotechnology. It is timely, then, to review the progress made in understanding the fundamental electrochemistry of flooded porous electrodes [class (v)]. Although the majority of the papers discussed in this Report are from the main scientific sources, any of the most important contributions that appear uniquely in the published proceedings of symposia have also been included.

It is worthwhile, at this point, to indicate briefly the major avenues of approach that have been followed in studies of porous electrodes. The more important electrochemical relationships are noted, since these are not found in standard works on electrode kinetics.

There are, fundamentally, two approaches which can be taken in order to deal with the porous electrode. First of all, porous electrodes can be considered as extensions of planar electrodes of known electrode-kinetic behaviour. This is the discrete-pore-model approach; historically, it provided the first ex planation of the behaviour of porous electrodes. It was developed to a high degree, notably by Frumkin, Winsel, and de Levies (the review by de Levies contains a thorough account of the early work). Differences from the relationships for the plane electrode arise because, in the ideal porous electrode (i.e. with circular pores), the current, instead of arriving normally to the plane of the electrode, arrives parallel to it. This consideration engenders the concept of the penetration depth, the interlinking of ohmic, concentration, and activation polarizations, and the ‘halving’ of the time–dependent (transient) responses.

The other approach (which, if anything, has been more successful than the pore model) is the macrohomogeneous model. This was first effectively used by Newman and Tobias; the porous electrode was considered to be an ‘average’ of the solid electrode and the electrolyte. Thus, the effective conductance of the porous electrode was the weighted volume average of the respective conductances; diffusion coefficients were similarly averaged, and so on. In this case, however, the electrochemistry cannot be taken from that of the plane electrode; the potential-current relationship must be obtained from the porous electrode by measurement. This is the least satisfying aspect of the macrohomogeneous approach. A possible solution might be to use the pore model to establish the electrochemistry; in general, however, it is clearly better to use experimental methods. The macrohomogeneous model will clearly be the more useful for the electrochemical engineer; however, the single-pore approach is still proving useful in developing our understanding of porous electrodes.

2 The Development of Theories

The Single-pore Model. — We summarize the state of the theory of porous electrodes at the time of the review by de Levie in 1966.

In spite of the clearly evident need to consider porosity of the electrode in relation to electrode-kinetic investigations of many types, it is not customary to do so. This arises from two causes. One is that, to a first approximation, porous and rough electrodes behave as smooth electrodes of enhanced surface area. The other is that porosity is difficult to incorporate into electrode kinetics, because of lack of definition of the porous electrode. Simplification of the porous electrode to give, for example, a parallel array of pores of uniform diameter is an obvious first extension from the planar electrode. In considering this model, the pore is essentially one–dimensional and the resistance of the electrolyte is uniformly distributed along its length. The simplest approach is to assume that the pore is of uniform cross–section and completely filled with solution. Frumkin 6 considered the curvature of the equipotential surfaces within the pore; however, by replacing the equipotential surfaces by the mean values which lies in planes perpendicular to the axis of the pore, the problem is avoided, as the model becomes essentially one-dimensional.

de Levie has shown that the transmission-line representation leads to the expression:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

for the potential drop (E0 – Ex) over the distance x from the mouth of the pore (x = 0) within a pore of length l. The important constant ρ has the form:

ρ = (Rω/RD)½ (2)

where Rω is the ohmic resistance of the solution within the pore for unit pore length (as distinct from that of the bulk solution) and RD is the charge-transfer resistance for unit length. The quantity ρ-1 has the dimensions of length and is called the ‘penetration depth’. The current at the mouth of the pore(total current) is given by:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The pore behaves as a resistance, according to:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

If we assume a cylindrical pore, the value of Rω in the pore can be calculated as:

Rω)-1 = πa2κ (5)

for a radius a cm and solution conductance κ [OMETGA]-1 cm-1, and with RD terms of the exchange-current density i0 using:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

The penetration depth becomes:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

Hence, the penetration depth decreases with decreasing κ and pore radius and with increasing i0. A number of workers have attempted to get exact solutions for the current-potential characteristics of a circular pore, assuming linearity of the characteristic, but, without a detailed knowledge of the structure of a pore, there seems little point in this, particularly as the more complex situations yield solutions which agree with the simple ones to within 5%, provided that the pores are of significant depth.

The simple use of the specific charge-transfer resistance for the impedance of the electrode surface only holds for small polarizations, generally of the order of a few millivolts. For more significant polarizations, when the planar electrode (under charge-transfer control) is expected to follow Erdey-Gruz kinetics, 14 the Tafel equation can be applied in the region for which the overpotential (η) exceeds some tens of millivolts. For a cylindrical pore, the Tafel slope dE/d(ln i) is double that which would characterize the corresponding planar electrode.

The fact that the form of the kinetic equations for an electrode involves the product of both charge-transfer and mass-transport characteristics clearly emphasizes these two as equally important modes of limitation of the current for a porous electrode. Changes of concentration within the pores of the electrode can obviously be just as important as the Tafel behaviour in limiting the current. de Levie catalogues the early work and shows that a limitation due to diffusion results in a current-potential relationship that is similar to the doubled (planar) Tafel behaviour of the charge-transfer-limited system.

The penetration depth is a function of both i0 and the concentration at the opening of the pore; even at the mouth of the pore, the concentration (cx = 0) is different from that of the bulk (cb), and the current into the porous electrode results from Fick’s first law, as:

ix=0 = (cb – cx=0)πa2zFD (8)

where δ is the thickness of the diffusion layer, provided that the front of the electrode can be treated as flat. In terms of porous parameters:

ix=0 = ρcbD’ tanh ρl/(1 + ρδ tanh ρl) (9)

where D’ is an ‘effective’ coefficient for diffusion, expressed as D’ = πa2zFD in terms of the diffusion coefficient, D. When the potential becomes very large, the penetration depth is small and the diffusion-limited current becomes that of a flat electrode of the same dimensions as the projected area of the porous one. This illustrates why the use of flooded porous electrodes is more effective for slow electrode reactions, where transport of mass is less significant.

The case [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (where Sr and So are reduced and oxidized species, respectively) was considered by Austin and Lerner, who established the expression:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

where [varies] is the cathodic charge-transfer coefficient and cr and co refer to concentrations of reduced and oxidized species, respectively, either in the bulk solution (cb) or at the mouth of the pore (cx = 0).

The penetration depth, ρ-1, accordingly has a maximum value at:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (11)

which is at or near the standard potential of the redox system: Dr and Do are diffusion coefficients of the reduced and oxidized species, respectively.

Generally, impedance measurements are made with small perturbations of amplitude, so that the rate equation may be considered linear. Thus, for a frequency ω, the pore exhibits an impedance:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (12)

where ZP is the impedance per unit pore length.

For a semi-infinite pore, the phase angle of Z0 (the impedance of the pore) is half the planar impedance (since Rω is a real quantity), and |Z0| is proportional to ZP½ Thus a ‘squaring’ operation is a simple way of correlating the impedance of a porous electrode with that of the corresponding flat electrode. The phase angle of the impedance of the pore is a function of the depth of the pore, and deep pores clearly contribute more significantly to the impedance of the electrode than do shallow ones.

The part of the impedance of a porous electrode that is due to double-layer charging is given by:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13)

where [square root of (-1)] and C is the capacitance per unit length of pore. When the pore is infinite in length it exhibits a phase shift of 45° between current and potential, and it appears as a simple Warburg impedance. The reciprocal penetration depth:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)

increases with ω; at sufficiently high values of ω, shallow pores will behave as inifnitely long ones. Using the equation for the impedance (Z) of an electrode reaction that is controlled by double-layer charging and the electrode reaction:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)

the charge-transfer resistance is given by :

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (16)

and the familiar semicircle in the diagram in the complex plane for the planar electrode becomes lemniscate.

If the diffusion processes that are linked with the electrode reaction can be described in terms of semi-infinite linear diffusion, as was done by Winsel, then a Warburg dependance with a phase angle of 45° for the surface impedance of the pore becomes a phase angle of 22 1/2° for a semi-infinite pore.

The transient responses of porous electrodes are extended in the time domain, and steady states may take a considerable time to achieve. Equations show that the double-layer charging process is time-dependent, with a time constant of l2RωC. The charging process travels down the pore and reaches different depths l at different times; deviations from simple semi-infinite behaviour must be considered where the charging process reaches the bottom end of the pore. The problem of coupling the double-layer charging process with the transfer of charge at an electrode and the attendant diffusion is formidable. Double-layer charging is a much faster process than diffusion, and it has been suggested that the processes are separable. Ksenzhek solved the Fick equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (17)

for a galvanostatic pulse to show that the potential increased linearly with time, at a rate that is proportional to the concentration (c) of electro-active species, providing an effective concentration impedance Z’, as was observed for the oxidation of hydrogen on porous nickel.

The Macrohomogeneous Model. — de Levies primarily considered the single-pore model, thinking of the porous electrode as a combination of single pores, essentially without cross-links. The macroscopic current is then the sum of the contributions of all the single pores. Prediction of the overall performance of the electrode, however, would require detailed knowledge of the distribution of particle sizes, which would be difficult to determine as well as to relate to practical (e.g. sintered) electrodes. In contrast, the macroscopic or continuum approach describes the whole electrode-electrolyte system as two continua (one of the electrode matrix and the other of the solution, filling in all the void fraction of the electrode). Both phases are assumed to be homogeneous, isotropic, and spatially complementary. The avoidance of detailed structural description (e.g. for tortuosity factor) is reflected in mathematical simplicity, although the mathematical expressions are analogous to those of the single-pore model.

Newman and Tobias were the first to consider explicitly the continuum approach, their initiative being followed by Grens and Tobias and by Micka. de Levies gives a brief summary of this new development as it stood in 1966.

The macrohomogeneous model, developed by Newman and Tobias, disregards the actual structural detail of the pores and regards the porous electrode as the superposition of two continua, i.e. the solution and the matrix. The current density in the electrolyte solution that is due to a flux Ni of species i is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (18)

For the material balance,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (19)

where ε is porosity, ci is the concentration of species i, and jin is the pore- wall flux of species i, averaged over interfacial area a. For electroneutrality,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (20)

and

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (21)

where i1 and i2 are current densities in the matrix and in the electrolyte in the pores, respectively.

The electrochemistry is ex pressed by the general relationship:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (22)

where αa and αc are transfer coefficients in the anodic and cathodic directions, respectively, and ηs is the surface overpotential.

Transport processes for the matrix phase are introduced, using Ohm’s law:

i1 = -σ[nabla]Φ1 (23)

where σ and Φ1 are the conductivity and electric potential of the matrix, respectively. For a dilute solution, within the pores, the solutes move by processes of diffusion, dispersion, migration, and convection:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (24)

where Da is a dispersion coefficient, ui is the mobility of species i, Φ2 is electric potential in solution, and v is the fluid velocity.

These seven equations (18) — (24) are solved together, after various simplifications and adjustments, as required by the special features of the systems considered.

Theoretical Modelling of Porous Electrode Systems. — Theoretical developments cannot be separated into two distinct streams, springing from either the single-pore or the continuum theory. These approaches are complementary rather than exclusive, and investigators have employed both, in theoretical as well as experimental work. By the early 1970s, as Newman and Tiedemann remarked, the basic theoretical groundwork had been developed to the point where it could begin to be applied to almost any electrode system. What appears to be a distinctive feature of the work of the past decade is that investigators have begun to study specific electrode systems, adapting the general theoretical principles to their particular electrochemical character.

(Continues…)Excerpted from Electrochemistry Volume 8 by D. Pletcher. Copyright © 1983 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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:

[FORMULA NOT REPRODUCIBLE IN ASCII]

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.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.