An introduction to electrochemistry

565 Pages·1942·26.664 MB·English

by  Glasstone, Samuel

AN INTRODUCTION
TO
ELECTROCHEMISTRY
BY
SAMUEL GLASSTONE, D.Sc., PH.D.
Contultanl, Untied Stales Atomic Energy Commirsion
TENTH PRINTING
(AN EAST-WEST EDITION)
AFFILIATED EAST-WEST PRESS PVT. LTD.
NEW DELHI.
Copyright 1942 by
LITTON EDUCATIONAL PUBLISHING, INC.
No
reproduction in any form ofthis book, in whole
or in part (except for brief quotation in critical
articles or reviews), may be made without written
authorizationfrom thepublishers.
First Published May 1942
AFFILIATED EAST-WEST PRESS PVT. LTD.
East-West Student Edition - 1965
Second East-West Reprint - 1968
Third East-West Reprint 1971
Fourth East-West Reprint - 1974
Price in India Rs. 12 00
Sales Territory : India, Ban^la Desh, Burma, Ceylon,
Malaysia, Pakistan and Singapore.
Reprinted in India with the special permission ofthe
original Publishers, Litton Educational Publishing,
Inc. New York, U.S.A. and the copyright holders.
This book has been published with the assistance
of the joint Indian-American Textbook Programme
Published by K.S. Padmanabhan for AFFILIATED EAST-
WESTPRESS PVT. LTD., 9Nizamuddm East,New Delhi 13,
India, and printed by Mohan Makhijani at Rckha Printers,
New Delhi.
To V

PREFACE
The object of this book is to provide an introduction to electro-
chemistry in its present state of development. An attempt has been
made to explain the fundamentals of the subject as it stands today, de-
voting little or no space to the consideration of theories and arguments
that have been discarded or greatly modified. In this way it is hoped
that the reader will acquire the modern point of view in electrochemistry
without being burdened by much that is obsolete. In the opinion of the
writer, there have been four developments in the past two decades that
have had an important influence on electrochemistry. They are the ac-
tivityconcept, theinterionicattractiontheory, theproton-transfertheory
of acids and bases, and the consideration of electrode reactions as rate
processes. These ideas have been incorporated into the structure of the
book, with consequent simplification and clarification in the treatment of
many aspects of electrochemistry.
This book differs from the author's earlier work, "The Electrochem-
istry of Solutions/' in being less comprehensive and in giving less detail.
While the latter is primarily a work of reference, the present book is
more suited to the needs of students of physical chemistry, and to those
of chemists, physicists and physiologists whose work brings them in con-
tact with a variety of electrochemical problems. As the title implies,
the book should also serve as an introductory text for those who in-
tend to specialize in either the theoretical or practical applications of
electrochemistry.
In spite of some lack of detail, the main aspects of the subject have
been covered, it is hoped impartially and adequately. There has been
some tendency in recent electrochemical texts to pay scant attention to
the phenomena at active electrodes, such as ovcrvoltage, passivity, cor-
rosion, deposition of metals, and so on. These topics, vihich are of
importance in applied electrochemistry, are treated here at Mich length
as seems reasonable. In addition, in view of tho growing interest in
electrophoresis, and its general acceptance as a branch of electrochem-
istry, a chapter on clectrokinetic phenomena has boon included.
No claim is madeto anythingapproaching completeness in the matter
of references to the scientific literature. Such reformers as arc given arc
generally to the more recent publications, to review articles, and to
papers that may, for one reason or another, have some special interest.
Referencesarealsofrequentlyincludedtoindicatethesourcesfromwhich
data have been obtained for many of the diagrams and tables. Since
no effort was made to be exhaustive in this connection, it was felt that
an author index would be misleading. This has consequently been
VI PREFACE
omitted, but where certain theories, laws or equations are usually asso-
ciated with the names of specific individuals, such names have been in-
cluded in the general index.
In conclusion, attention may be drawn to the problems which are to
be found at the end of each chapter. These have been chosen with the
object of illustrating particular points; very few are of the kind which
involve mere substitution in a formula, and repetition of problems of the
same type has been avoided as far as possible. Many of the problems
are based on data taken directly from the literature, and their solution
should provide both valuable exercise and instruction. The reference to
the publication from which the material was taken has been given in the
hopethat when workingtheproblemthestudentmaybecomesufficiently
interested to read the original paper and thus learnforhimselfsomething
of the methods and procedures of electrochemical research.
SAMUEL GLASSTONE
NORMAN, OKLAHOMA
March 1942
CONTENTS
CHAPTER PAGE
PREFACE v
I. INTRODUCTION 1
II. ELECTROLYTIC CONDUCTANCE 29
III. THE THEORY OF ELECTROLYTIC CONDUCTANCE 79
VI. THE MIGRATION OF IONS 107
t
i.V. FREE ENERGY AND ACTIVITY 131
REVERSIBLE CELLS 183
ELECTRODE POTENTIALS 226
VIII. OXIDATION-REDUCTION SYSTEMS 267
IX. ACIDS AND BASES 306
X. THE DETERMINATION OF HYDROGEN IONS 348
XI. NEUTRALIZATION AND HYDROLYSIS 370
XII. AMPHOTERIC ELECTROLYTES 418
XIII. POLARIZATION AND OVERVOLTAGE 435
XIV. THE DEPOSITION AND CORROSION OF METALS 482
< XV. ELECTROLYTIC OXIDATION AND REDUCTION 504
XVI. ELECTROKINETIC PHENOMENA 521
INDEX 547

CHAPTER
I
INTRODUCTION
Properties of Electric Current. When plates of two dissimilarmetals
are placed in a conducting liquid, such as an aqueous solution of a salt
or an acid, the resulting system becomes a source of electricity; this
source is generally referred to as a voltaic cell or galvanic cell, in honor
of Volta and Galvani, respectively, who made the classical discoveries in
this field. If the plates of the cell are connected by a wire and a mag-
netic needle placed near it, the needle will be deflected from its normal
position it will be noted, at the same time, that the wire becomes warm.
;
If the wire is cut and the two ends inserted in a conducting solution,
chemical action will be observed where the wires come into contact with
the liquid; this action may be in the form of gas evolution, or the libera-
tion of a metal whose salt is present in the solution may be observed.
These phenomena, viz., magnetic, heating and chemical effects, are said
to be caused by the passage, or flow, of a current of electricity through
the wire. Observation of the direction of the deflection of the magnetic
needle and the nature of the chemical action, shows that it is possible
to associate direction with the flow of electric current. The nature of
this direction cannot be defined in absolute terms, and so it is desirable
to adopt a convention and the one generally employed is the following:
ifamanwereswimmingwiththeelectriccurrentand watchingacompass
needle, the north-seeking pole of the needle would turn towards his left
side. When electricity is passed through a solution, oxygen is generally
liberated at the wire at which the positive current enters whereas hydro-
gen or a metal is set free at the wire whereby the current leaves the
solution.
It is unfortunate that this particular convention was chosen, because
when the electron was discovered it was observed that a flow ofelectrons
produced a magnetic effect opposite in direction to that accompanying
the flow of positive current in the same direction. It was necessary,
therefore, to associate a negative charge with the electron, in order to be
in harmony with the accepted convention concerning the direction of a
current of electricity. Since current is carried through metals by means
of electrons only, it means that the flow of electrons is opposite in direc-
tion to that of the conventional current flow. It should be emphasized
that there is nothing fundamental about this difference, for if the direc-
tionofcurrentflowhad been defined in theopposite manner, theelectron
would have been defined as carrying a positive charge and the flow of
electrons and of current would have been in the same direction. Al-
2 INTRODUCTION
though a considerable simplification would result from the change in
convention, it is too late in the development of the subject for any such
change to be made.
E.M.F., Current and Resistance: Ohm's Law. If two voltaic cells
are connected together so that one metal, e.g., zinc, of one cell is con-
nected to the other metal, e.g., copper, of the second cell, in a manner
analogous to that employed by Volta in his electric pile, the magnetic
and chemical effects of the current are seen to be increased, provided
the same external circuit is employed. The two cells have a greater
electrical driving force or pressure than a single one, and this force or
pressure * which is regarded as driving the electric current through the
wireiscalled theelectromotiveforce,orE.M.F. Betweenanytwopoints
inthecircuitcarryingthecurrentthereissaidtobeapotentialdifference,
the total E.M.F. being the algebraic sum of all the potential differences.
By increasing the length of the wire connecting the plates of a given
voltaic cell the effect on the magnetic needle and the chemical action
are seen to be decreased: the greater length of the wire thus opposes the
flow of current. This property of hindering the flow of electricity is
called electrical resistance, the longer wire having a greater electrical
resistance than the shorter one.
It is evident that the current strength in a given circuit, as measured
by its magnetic or chemical effect, is dependent on the E.M.F. of the cell
producing thecurrent and theresistanceofthe circuit. Therelationship
between these quantities is given by Ohm'slaw (1827), which states that
the current strength (/) is directly proportional to the applied E.M.F. (E)
and inversely proportional to the resistance (R) thus
;
is the mathematical expression of Ohm's law. The accuracy of this law
has been confirmed by many experiments with conductors of various
types: it fails, apparently, for certain solutions when alternating currents
of very high frequency are employed, or with very high voltages. The
reasonsfor this failureof Ohm'slaw areof importancein connection with
tho theoryofsolutions (see Chap. III). Itisseen from equation (1) that
the E.M.F. is equal to the product of the current and the resistance:
a consequence of this result is that the potential difference between any
two points in a circuit is given by the product of the resistance between
those points and the currentstrength, the latter being the same through-
out the circuit. This rule finds a number of applications in electro-
chemical measurements, as will be evident in due course.
*Electrical force or pressure does not have the dimensions of mechanical force or
pressure;thetermsareused,however,byanalogywiththeforceorpressurerequiredto
producetheflowof afluidthroughapipe.