Table Of ContentCopyrigkited
By
William Gates Moulton
1952
UNIVERSITY OF ILLINOIS
THE GRADUATE COLLEGE
January 7, 1952
I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY
WILLIAM GATES MOULTON
SUPERVISION BY-
MOBILITIES OF IONS IN COLLODION MEMBRANES
ENTITLED
BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF PHILOSOPHY
4. w^
/ In Charge of Thesis
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Head of Denartmcnt
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M440
MOBILITIES OP IONS IN COLLODION MEMBRANES
BY
WILLIAM GATES MOULTON
B.S., Western Illinois State Teachers College, 1946
M.S., University of Illinois, 1948
THESIS
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEOREE OF DOCTOR OF PHILOSOPHY IN PHYSICS
IN THE GRADUATE COLLEGE OF THE
UNIVERSITY OF ILLINOIS, 1952
URBANA. ILLINOIS
ACKNOWLEDGMENTS
I wish to express my gratitude to
Mr, R. A. Kronihout for many helpful discussions
and suggestions, to my wife for calculating
some of the data and checking all of the com
putations, and to Mr. A. E. Vawter for his help
with the electron miscroscope studies. I wish
also to thank Professor A. 0. Hanson for the
loan of the vibrating reed electrometer.
I especially wish to thank Professor
J. H. Bartlett for directing this research.
INDEX
Page
I. INTRODUCTION 1
II. TECHNIQUES 7
A. Transference Numbers 7
B. Resistance Measurements 12
C. Preparation of the Membranes . .. 19
III. RESULTS 22
A. - Transference Numbers, Resistance
and Ionic Conductance of the
Membranes 22
B. Experiments at High Current
Densities 39
IV. SUMMARY 62
V. APPENDIX 1 65
VI. APPENDIX 2 70
VII, REFERENCES 73
VIII. VITA 75
1
I. INTRODUCTION
One of the most universal processes which occurs in
living systems is the transport of ions and molecules through
membranes. Transport occurs across the membrane of single
cells, and across membranes which contain many cells, such as
stomach mesentary. Some of the transport Is simply free
diffusion; in some the transport can be explained only if the
membrane is "selective", i. e., one kind of ion or molecule
can traverse the membrane more easily than other kinds; in
some, the so-called "active transport", the cell supplies
metabolic energy to produce the transport.
Measurements of the osmotic pressure produced when
a collodion membrane is used to separate an electrolytic
solution from water show that the collodion membrane is
(6)(16)(17)
selective. Since the collodion membrane is a much simpler
system to study than a living membrane, it has been exten
sively investigated. It is used as a model for some of the
transport processes in living systems; for example in the
explanation of the transfer observed in such systems as kidney
(12)
tubules and intestine. Perhaps the most extensive studies of
the collodion membrane have been carried out by K. Sollner and
(20)
his co-workers.
The transference numbers of the ions in the membrane
have been used by most workers to characterize the semi-per-
2
meable, or "selective" properties of the membrane. The technique
for measuring transference numbers which has been used most
extensively is to separate electrolytic solutions of different
concentrations with a collodion membrane and measure the po
tential between the two sides of the membrane with a salt
bridge and calomel electrode arrangement. The transference
number of the anion is then calculated from the Henderson
equation:
V «(l-2t") SLln °1
T~ TJg
where: V • the potential observed across the membrane
t = the transference number of the anion
R - the gas constant
T ~ the absolute temperature
F = the Faraday
C]_ and Cg are the ionic concentrations on
opposite sides of the membrane.
With an appropriate choice of units the above equation reduces to:
J" ' " • » I. . — I I. • 111 I • I. • IM» , • . I • n • .
The transference number of the 1 kind of ion in a solution
is defined by the relation tj.- CIZJUJ/^ CJSJUI
where: Cjf* the concentration of the t^kind
v
of ion
th
zj= the valence of the i " kind of ion
Ui= the mobility of the ittt]£ind of Ion
If there are only two kinds of ions present in the solution, both
having the same valence but opposite charge, the transference
number of the negative ion may be written f * U" /UVtP«I~/lVl~-
I / I. where I™ and I are the electrical currents carried by the
negative ion and positive ion respectively, and I is the total
current.(7)
3
It was found by this method that the anion transference
number is much smaller in the membrane than in free solution,
suggesting that the anion is inhibited. It was observed that
the more impure the collodion is, the smaller the anion trans
ference number is. However, it was found that the purest
collodion compounds can be made selective if the membrane is
oxidized, either by immersing the membrane in NaOBr or NaOH.
Supposedly, the oxidation of the membrane produces dissociable
groups on the walls of the membrane, probably carboxyl groups.
The following explanation of transport in membranes
is generally accepted at present: 'The membrane consists of
a mosaic of interstices which are interconnected. Negative
dissociable groups are attached to the walls of the membrane,
either due to impurities in the collodion or due to the car
boxyl groups produced by oxidation. These dissociable groups
are surrounded by a diffuse layer of ions of the opposite
charge. When an anion attempts to pass through the membrane,
its passage Is inhibited by interaction with the negative
groups. The cations pass through the membrane by exchanging
with the cations in the diffuse layer around the fixed groups.
Wyman studied the free diffusion of phosphoric acid
(26)
through a collodion membrane utilizing P32 as a tracer. He
found the diffusion constant of the H2PO4" ion to be less than
the diffusion constant of the HJP0 molecule in the membrane.
4
He was able to interpret most of his data in terms of the
above model.
The measurement of transference numbers by potentlo-
4
metric techniques involves the following difficulties:
1. Since different concentrations must be used on the
two sides of the membrane, if the transference numbers are a
function of concentration and if it is desired to study the
concentration dependence, only average values over a concen
tration region may be measured.
2. One sees from the form of the Henderson equation
that since the anion transference number is small, a small
error in the measurement of the potential will introduce large
errors in the transference number.
3. The Henderson equation is an approximate equation,
and may be in errori10'Hermans has shown that for some cases
the error in the equation may be quite large.^1'
In view of the inherent limitations in the measurement
of transference numbers by potentlometrie techniques, it was
felt that a direct measurement of the anion transference number
was needed in order to obtain quantitative Information on its
behaviour as a function of concentration, temperature, and
oxidation of the membrane. The fraction of electric current
carried by the anion can be measured directly if the anion is
tagged with a radioactive tracer. Measurements of trans
ference numbers by this method will be discussed.
In most of the previous work it has been assumed that
the anomalous concentration potential observed across the
membrane is due to the inhibition of the anion in the membrane,
5
while the oatlon mobility remains about the same in the
membrane as in free solution. Measurements of the anion
and cation mobilities as a function of concentration and
temperature will be discussed.
The electrolyte used in these measurements was phos
phoric acid. This electrolyte was chosen for three reasons:
1. Phosphoric acid is a weak acid for which the dls-
(18)
sociation constant has been measured. Thus, one can calculate
the ionic concentration at any molar concentration from the law
of mass action, and concentrations, rather than activities,
can be used for all calculations over the range of concentration
-4 -1
studied, 10 molar to 10 molar.
2. The phosphate ion is an ion commonly present in
living systems.
3. Pgg is a convenient isotope to use for tracer
studies of this kind since it emits only high energy beta par
ticles (1.7 mev). It is sufficiently short-lived (14.7 days)
to make it safe to work with in solution, but sufficiently
long-lived so that It is not necessary to correct for the
decay during the course of an experiment lasting only a few
hours.
It was found that as long as sufficiently low current
densities were used for making the measurements, the trans
ference numbers and the membrane resistance measured were in
dependent of the current. However, if current densities which
were too high were used to make the measurements, both the
transference numbers and the resistance were found to depend
