Ion-Selective Electrode Reviews

318 Pages·1985·7.261 MB·English

by  J.D.R THOMAS (Eds.)

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ION-SELECTIVE
ELECTRODE REVIEWS
Volume 7
Editor-in-Chief
J. D. R. THOMAS
UWIST, Cardiff, Wales
PERGAMON PRESS
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Copyright © 1985 Pergamon Press Ltd.
All Rights Reserved. No part of this publication may be
reproduced, stored in a retrieval system or transmitted in any
form or by any means: electronic, electrostatic, magnetic tape,
mechanical, photocopying, recording or otherwise, without
permission in writing from the publishers.
This edition 1986
British Library Cataloguing in Publication Data
Ion-selective electrode reviews.—Vol. 7
1. Electrodes, Ion-selective—Periodicals
541.3724Ό5 QD571
ISBN 0-08-034150-0
First published as Ion-Selective Electrode Reviews, Volume 7,
Nos 1 and 2, 1985, and supplied to subscribers as part of their
subscription. Also available to non-subscribers.
Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter
Ion-Selective Electrode Rev. 1985, Vol. 7, pp. 1-2 0191-5371/85 $0.00 + .50
Printed in Great Britain. All rights reserved. Copyright © 1985 Pergamon Press Ltd.
EDITORIAL
To reach the seventh volume represents a significant mile­
stone for ION-SELECTIVE ELECTRODE REVIEWS. It coincides with a
vigorous growth of interest in sensors in general which can be
regarded as conseguent on the general success of electrochemical
sensors, especially those that fall within the scope of ION-SELECTIVE
ELECTRODE REVIEWS.
The range of applications of electrochemical sensors has
been well covered in previous volumes. The present issue complements
this by the extensive review by Dr. K.Vytras on "Potentiometric
Titrations Based on Ion-Pair Formation". The versatility of ion-
selective electrodes will be discussed by Dr. T.R.Yu in his article
on their applications in soil science which is to appear in the next
issue of this volume.
Biosensors have stimulated the imagination and ingenuity of
many research workers. Here the use of enzymes immobilized in
membranes fitted to potentiometric and amperometric electrodes has
greatly extended the scope and use of enzymes catalyzed reactions for
determining substrates. Of course, such reactions can be adversely
affected by inhibitors, but this can be harnessed to good use as
illustrated by Dr. C.Tranh-Minh in his article on "Immobilized Enzyme
Probes for Determining Inhibitors".
Finally, the greatest of the editorial joys of collecting
together the articles of this volume is associated with the first
article. Here, Academician Nikolskii and Dr. Materova address them­
selves to aspects of the substitution of the liguid internal contact
of ion-selective electrodes by solid contacts. But, of course, the
name of Academician Nikolskii has graced the pages of ION-SELECTIVE
ELECTRODE REVIEWS from the beginning, for the celebrated Nikolskii
Eguation is a hallmark of the guality of ion-selective electrodes.
In this article, he and Dr. Materova debate the term "selectivity
coefficient" further and propose "influence coefficient" for our
attention. More significant is their consideration of the stability
conditions of ion-selectj_ve electrode potential with due emphasis on
factors concerning the difficult task of ensuring reversibility and
eguilibrium stability in the transition from electronic to ionic
1
2 Editorial
conductivity in solid contact electrodes. They emphasise that
unless there is an equilibrium and stable process of electron
conductivity change into ionic conductivity and vice versa, the
electrode cannot work well.
"A little onward lend thy guiding hand
To these dark steps, a little further on.
(Samson Agonistes, I: Milton)
^>
J&r~
March 1985 J.D.R.Thomas
Ion-Selective Electrode Rev. 1985, Vol. 7, pp. 3-39 0191-5371/85 $0.00 + .50
Printed in Great Britain. All rights reserved. Copyright © 1985 Pergamon Press Ltd.
SOLID CONTACT IN MEMBRANE
ION-SELECTIVE ELECTRODES
B. P. Nikolskii and E. A. Materova
Chemical Department, Leningrad State University,
199164 Leningrad, U.S.S.R.
CONTENTS
1. INTRODUCTION
2. THEORETICAL ^ART
2.1 Ion-exchange theory of ion-selective electrodes
2.2 Buffer properties of membrane systems
2.3 The region of transition from one ISE function to
another
2.4 Influence (selectivity) coefficients
2.5 Stability conditions of ion-selective electrode
potential
3. LITERATURE REVIEW OF ISEs WITH SOLID INTERNAL CONTACT
3.1 Electrodes with solid crystal membranes
3.1.1 Membranes with mixed electronic and ionic
conductivity of membranes
3.1.2 LaF3 membranes
3.2 Glass electrodes with solid internal contact
3.3 Poly(vinyl chloride)(PVC)matrix electrodes with solid
contact
3.3.1 Solid contact ISEs with internal electrodes of
the second type
3
B. P. Nikolskii and E. A. Materova
3.3.2 Solid contact ISEs with redox systems in the
organic phase
3.3.3 ISEswith a combined glass-PVC membrane
3.3.4 Coated-wire electrodes (CWEs)
3.3.5 Selectrodes
.4. CONCLUSION
5. SYMBOLS AND NOTATION
6. REFERENCES
KEYWORDS: Buffering properties of membrane systems; influence co­
efficients; ion-selective electrodes with solid internal
contact; potential stability of ion-selective electrodes;
selectivity and influence coefficients.
1. INTRODUCTION
The last two decades have seen increasing interest in
methods of analysis and production control based on the application
of ion-selective electrodes (ISEs). This branch of chemistry may be
given the title of "ionometry". The great practical significance of
ionometry provides the initiative for various ISE improvements.
This article deals with one such improvement, namely, sub­
stitution of the liquid internal contact by solid contact (SC)
between the metal conductor and the ion-selective membrane. Several
advantages of ISEs with the correctly arranged solid contact if com­
pared with the usual ones (with liquid filling) show the importance
of this improvement:
1. Ion-selective electrodes with solid contact (ISE SC) can
function in any space position, namely, vertical, horizon­
tal and "upside down", and can endure rotation, vibration,
shaking and weightlessness.
2. Many electrodes of this type can be used at temperatures
above 100°C, which is essential for sterilization and at
temperatures below 0°C, which is important for winter
transportation.
3. Time stability and potential reproducibility of SC-electro-
des are often higher than those of electrodes with liquid
filling.
4. Many SC constructions do not demand the use of noble metals.
5. In some special cases ISE SC can have other advantages which
are mentioned later.
Solid Contact ISEs
To realize these advantages, the true electro-chemical
equilibrium in the system is necessary. However, not all the ISE SC
construction designs described in the literature satisfy these
requirements. Therefore, this problem will be considered in detail.
2. THEORETICAL PART
2.1 Ion-exchange theory of ion-selective electrodes
The first ISE was a glass electrode for the determination
of hydrogen ion concentration. After the classical work of Haber
and Klemensievicz in 1909 [1], a number of attempts to find a suit­
able composition of glass providing the highest selectivity of
electrodes to H+ ions was made [2-5]. The greatest success in this
field was achieved by Perley [6] with further developments in other
works [7-9]. A review of works in this field up to 1967 was made by
Izard [10]. Later on glasses of complex compositions were synthe­
sized. Among them there were glasses for electrodes selective to
other ions (sodium, lithium, potassium, ammonium, silver, thallium)
[11,12], as well as electrodes with electron function for measuring
the oxidation potentials of solutions [13].
In 1935-37 an ion-exchange theory of glass electrode was
developed [14,15]. This was elaborated upon by many authors (Simon,
Nikolskii, Shultz, Eisenman [12, 16-19,40]. Nowadays, this theory i
generally accepted.
Since the principles of ISE SC action are based mainly on
this theory, it is useful to summarize its main ponts. An outline
consideration is sufficient. Usually an ISE device with internal
liquid contact can be represented by the following scheme:
Examined Membrane Reference halfcell
solution +
A Internal solution | Ag Cu
A+ (f <P (f[ KC1, AC1, AgCl ^
e D
If the membrane is selective to A ions then the internal
solution should contain the salt of this ion. This provides the
reversibility of the processes which take place when current passes
through the cell. The theory suggests [20, p.11] that between the
surface membrane layer and the solution in contact with it there
exists an ion-exchange equilibrium. Let us consider the simplest
case:
A(M) + B (S) *— A (S) + B (M)
where (S) and (M) denote the solution and membrane respectively; B
is an interfering ion. A and B may be also anions. It is supposed
that in the membrane medium all ions are in a "free state" (complete
dissociation). The equilibrium of this ion-exchange process is
determined by the equation:
6 B. P. Nikolskii and E. A. Materova
where a and b - A + and B+ activities in solution;
ä and b - their relative concentration in the membrane*;
K - and equilibrium constant (exchange constant).
by changing the proportion a/b in the solution, it is possible to
shift ä/B ratio in the membrane phase in any direction. The ion-
exchange equilibrium constant K characterises the relative select­
ivity of absorption of A+ and B+ ions by the membrane from the solu­
tion.
On the membrane-solution boundary, potential difference
arises according to the equilibria?
A+(S) * A+(M) and B+(S) * B+(M)
The condition of these equilibira can be presented (Ρ,Τ-const) in
the form:
dG = >*dn - M dn + z Ff dn = 0
A A
and dG = A^dn - A dn + z F^dn = 0
JD toJD aD
Here G - Gibbs free energy of the membrane-solution system,
jW· and /x. - chemical potentials of ions in a membrane and
solution, respectively,
dn - number of A+ and B+ moles passing from solution into
the membrane,
z, and z - ion charges (equal to 1 in our case),
R
F - Faraday constant, and
tf - potential difference between membrane and solution.
Hence
f = ^Α -Μκ - MB M B
As μ = Μ^ + RT In a, M = /^ + RT In b
κ B
μ = Ä° + RT In Έ, M = <^° + RT In b
κ B
a -go , -, b
^= ^J + S l o gf ^ + S 0l o g| (2)
S
where S = 2.303 ¥RT' ?w1?-0Ύ ·>AA <ΛΩΚΟ 'ΎA>ß ,^0OB =^,A„O + S ^κ
If we assume that the sum of a + "b is constant, it is not difficult
to derive from (1) and (2) the basic equation for membrane potential
relative to the solution ^14]
Cf> = φ° + S log (a + Kb) (3)
If the diffusion potential arising within the membrane due to the
difference between A+ and B+ mobilities is taken into account, then
for the membrane potential
If = Cf° + s log (a + K b) (4)
A>ß
* The ratio of the activity coefficients of ions A and B in the
membrane is supposed to be independent of exchange degree.
Solid Contact ISEs 7
where Κ „ = K —, and TL and ΰ^ are the mobilities of the ions in
AΛ · B — A ID
UA
the membrane.
2.2 Buffer properties of membrane systems
As already mentioned, an outline of the theory is sufficient
to fulfil the aims of this article. Hence, equation (1), (2) and (3)
will mainly be used, i.e., account will not be taken of the
differences in ion mobilities. This theory was first developed for
glass electrodes, but as a first approximation, it can be used for
the majority of other ISE types.
It is useful to consider three cases:
1. a» K b. According to equation (1), "a» b, i.e., the
membrane exchange capacity is occupied by A ions mainly.
Membrane potential in this case will be determined
according to equation (3) only by A ions.
2. a<< K b. Then "a«b, i.e., the membrane is occupied mainly by
B ions and its potential will be a function of the activity
of B ions only.
3. a« K b, ~âç&h, i.e., the membrane contains approximately
equal quantities of both ions. Its potential is equally
changed by alteration of the activity of both the ions in
the solution.
By considering the first case it can be seen that doubling
of the interfering B+ ion activity halves the -5 ratio (a = constant).
According to equation (1), the ^ ratio should also be halved since
a very small quantity of B ions enter the membrane to displace an
equivalent number of A ions. This process will practically not change
the concentration of A ions in the membrane, since a » B. Hence the
value of b can increase almost twice without breaking this inequality*
The membrane potential will_practically not change since in
equation (2)^ ^ , fe b/b ratio remains almost unaltered*!*
= + s
Thus, it is possible to say that the reason for the insens-
itivity of membrane potential to B ion activity in solution is the
high degree of membrane exchange capacity favouring A + ions.
x It is assumed that the total content of A and B ions in the
solution is much larger than in the membrane.
xx A more precise calculation according to equation (2) gives the
following result. Suppose the B ion content_in the membrane con­
stitutes 1% of its exchange capacity, i.e., b = 0.01 (a + b).
After doubling the concentration of this ion in the solution, its
content in the membrane increases not twice but by -=— * *. = 1.98
times. The corresponding shift of membrane potential will be:
*<P = 59 log^
98