Electrolytic process for producing potassium hydroxide

Current efficiency in an electrolytic membrane cell for the production of concentrated potassium hydroxide is considerably increased by employing in the electrolytic membrane cell a membrane selected from a group consisting of an amine modified perfluorosulfonic acid membrane such as a primary amine, diamine, or polyamine modified perfluorosulfonic acid membrane, and a laminated perfluorosulfonic acid acid membrane, and a laminated perfluorosulfonic acid membrane comprised of at least two unmodified perfluorosulfonic acid membranes of different thickness and different equivalent weight.

This invention relates to a process for the electrolytic production of 
chlorine and potassium hydroxide. Potassium hydroxide is used in the 
manufacture of soft soap, alkaline batteries, and in the production of 
textiles and the fabrication of rubber. 
Commercially, potassium hydroxide is produced in electrolytic cells 
employing asbestos diaphragms as a product liquor containing 10-15 percent 
KOH and about 10 percent KCl. The liquor is concentrated by evaporation 
while crystallizing out KCl to provide a concentrated solution containing 
about 45 percent KOH and containing about 1 percent KCl. 
U.S. Pat. No. 3,733,634, issued to A. J. Stacey and R. L. Dotson on Nov. 
20, 1973, describes a process for electrolyzing aqueous sodium chloride 
having a concentration in the range of 120-150 grams per liter in the 
anolyte to produce sodium hydroxide where the concentration is held in the 
range of 31-43 percent. When, however, the concentration of sodium 
chloride in the anolyte is in excess of 250 grams per liter, the caustic 
concentration became unstable and there is a continuous increase in 
caustic concentration. This increasing concentration, however, is 
accomplished by decreased current efficiency. The cell employs 
hydraulically impervious cation-permselective membranes such as the 
unmodified perfluorosulfonic acid membranes. 
U.S. Pat. No. 4,062,743, issued to Byung K. Ahn and Ronald L. Dotson on 
Dec. 31, 1977, discloses a process for improving the reactant efficiency 
in an electrolytic membrane cell for the production of potassium hydroxide 
from aqueous solutions of potassium chloride by maintaining the anolyte 
concentration of potassium chloride at 250 to 350 grams per liter and the 
catholyte concentration of potassium hydroxide from about 410 to about 480 
grams per liter. The electrolytic cell employs an unmodified permselective 
membrane comprised of a copolymer of a perfluoroolefin and a 
fluorosulfonate. However, a catholyte current efficiency of 87 percent 
maximum was achieved at a concentration of potassium hydroxide of about 
450 grams potassium hydroxide per liter. 
There is a remaining need for an electrolytic membrane process for 
producing high purity potassium hydroxide at high KOH concentrations with 
significantly improved current efficiencies using concentrated potassium 
chloride brine. 
OBJECTS 
It is a primary object of this invention to provide an improved 
electrolytic process having a high current efficiency for preparing 
potassium hydroxide. 
It is another object of the present invention to provide a process for 
producing chlorine, hydrogen and potassium hydroxide with reduced energy 
costs. 
A further object of the present invention is to provide a process for 
producing potassium hydroxide of a high purity. 
These and other objects of the invention will become apparent from the 
following description and the appended claims. 
BRIEF DESCRIPTION OF THE INVENTION 
The aforementioned and other objects are achieved in a process for the 
preparation of potassium hydroxide, chlorine, and hydrogen in an 
electrolytic cell by the electrolysis of potassium chloride brine, the 
cell having an anolyte chamber containing an anode and a catholyte chamber 
containing a cathode and wherein the anolyte chamber is separated from the 
catholyte chamber by a cationic permselective membrane, the improvement 
which comprises employing as said membrane a membrane selected from a 
group consisting of an amine substituted perfluorosulfonic acid membrane 
and single film laminate membrane comprised of at least two 
perfluorosulfonic acid membranes laminates, wherein the potassium 
hydroxide concentration in the catholyte chamber is in the range from 
about 300 to about 500 grams potassium hydroxide per liter and wherein the 
membrane is laminated to a fabric of polytetrafluoroethylene and rayon. 
DETAILED DESCRIPTION OF THE INVENTION 
The electrolytic cell employed in this invention may be a commercially 
available or a custom-built electrolytic cell of a size and electrical 
capacity capable of economically producing the desired potassium hydroxide 
product. 
A particularly advantageous electrolytic cell which may be employed in the 
practice of this process has separate anolyte and catholyte chambers, 
using as a separator a selected permselective cation exchange membrane. 
Located on one side of the membrane partition, the anolyte chamber has an 
outlet for by-product chlorine gas generated, and an inlet and an outlet 
for charging, removing, or circulating potassium chloride solution. On the 
opposite side of the membrane partition, the catholyte chamber has an 
inlet for water, an outlet for removing potassium hydroxide product and an 
outlet for removing by-product hydrogen liberated at the cathode by the 
electrolysis of water. 
A gas disengaging space is generally located in each of the anolyte and 
catholyte chambers within the electrolytic cell. 
The membrane cell can be operated on a batch or flow-through system. In the 
latter system, anolyte and catholyte are continuously circulated to and 
from external solution storage vessels. 
Hydrogen gas is removed as formed from the catholyte chamber and collected 
for use as a fuel or otherwise disposed of. Any excess chlorine gas is 
likewise removed as formed from the anolyte chamber and collected. 
Typical electrochemical cells which may be employed in the preparation of 
aqueous solutions of potassium hydroxide are disclosed in U.S. Pat. No. 
4,062,743, supra, and hereby incorporated by reference in its entirety. 
Several groups of materials are suitable for use as membranes in the 
process of this invention. 
A first group of membranes includes amine substituted polymers such as 
diamine and polyamine substituted polymers of the type described in U.S. 
Pat. No. 4,030,988, issued on June 21, 1977 to Walther Gustav Grot and 
primary amine substituted polymers described in U.S. Pat. No. 4,085,071, 
issued on Apr. 18, 1978 to Paul Raphael Resnick et al. Both of the above 
patents are incorporated herein in their entirety by reference. 
With reference to the diamine and polyamine substituted polymers of U.S. 
Pat. No. 4,030,988, supra, the basic precursor sulfonyl fluoride polymer 
of U.S. Pat. No. 4,036,714, issued on July 19, 1977 to Robert Spitzer, and 
incorporated herein in its entirety by reference, is first prepared and 
then reacted with a suitable diamine, such as ethylene diamine, or 
polyamine to a selected depth wherein the pendant sulfonyl fluoride groups 
react to form N-monosubstituted sulfonamido groups or salts thereof. The 
thickness of amine substituted polymers of the first group is in the range 
from about 4 to about 10 and preferably in the range from about 5 to about 
9 mils. 
The selected depth is typically in the range from about 1.0 to about 7.0 
and preferably from about 1.2 to about 1.5 mils. 
In preparing the basic precursor sulfonyl fluoride as described in the '714 
patent above, the preferred copolymers utilized in the film are 
fluoropolymers or polyfluorocarbons although others can be utilized as 
long as there is a fluorine atom attached to the carbon atom which is 
attached to the sulfonyl group of the polymer. A preferred copolymer is a 
copolymer of tetrafluoroethylene and perfluoro 
(3,6-dixoa-4-methyl-7-octenesulfonyl fluoride) which comprises 10 to 60 
and preferably 25 to 50 percent by weight of the latter. Surface sulfonyl 
groups are then converted to form diamine and octyamino groups or salts 
thereof through the reaction of the diamine, such as ethylene diamine. 
With only surface conversion of the sulfonyl halide groups, further 
conversion of the remaining sulfonyl halide groups to the ionic form is 
most desirable. The prior art techniques of conversion of the--SO.sub.2 X 
groups with X as chlorine or fluorine may be undertaken such as by 
hydrolysis. The techniques set forth in Connolly and Gresham, U.S. Pat. 
No. 3,282,875 and/or Grot, U.S. Ser. No. 178,782 and now U.S. Pat. No. 
3,784,399 may be employed. Illustratively, the unconverted sulfonyl groups 
of the polymer may be converted to the form --(--SO.sub.2 NH).sub.m Q 
wherein Q is H, NH.sub.4, cation of an alkali metal and/or cation of an 
alkaline earth metal and m is the valence of Q. Additionally, the 
unconverted sulfonyl groups may be formed to --(SO.sub.3).sub.n Me wherein 
Me is a cation and n is the valence of the cation. Preferred definitions 
of Q include NH.sub.4 and/or cation of an alkaline earth metal 
particularly sodium or potassium. Preferred definitions of Me include 
potassium, sodium and hydrogen. 
As employed in the present context, a di- or polyamine is defined as an 
amine which contains at least two amino groups with one primary amino 
group and the second amino group either primary or secondary. Additional 
amino groups may be present so long as the above-defined amino groups are 
present. 
Specific amines falling within the above definition are included within the 
disclosure in U.S. Pat. No. 3,647,086, issued to Mizutani et al on Mar. 7, 
1972, which disclosure of amines is incorporated by reference herein. 
Typical membranes of the first group prepared from ethylene diamine which 
may be employed in the process of this invention include (a) a homogeneous 
film about 7 mils thick of about 1200 equivalent weight perfluorosulfonic 
acid resin which has been chemically modified by ethylene diamine 
converting a depth of about 1.5 mils to the perfluorosulfonamide, (b) a 
homogeneous film about 7 mils thick of 1150 equivalent weight 
perfluorosulfonic acid resin which has been chemically modified by 
ethylene diamine converting a depth of about 1.5 mils to the 
perfluorosulfonamide, and (c) a homogeneous film about 7 mils thick of 
1150 equivalent weight perfluorosulfonic acid resin which has been 
chemically modified by ethylene diamine converting a depth of about 1.2 
mils to the perfluorosulfonamide. 
For the above mentioned amine-substituted membranes, a laminated inert 
cloth supporting fabric may be employed. 
The thickness of the laminated inert cloth supporting fabric is in the 
range from about 3 to about 7 and preferably from about 4 to about 5 mils. 
The inert cloth supporting fabric is typically comprised of 
polytetrafluoroethylene, rayon, or mixtures thereof. 
A preferred example of a diamine substituted polymer is a perfluorosulfonic 
acid polymer comprised of a homogeneous film about 7 mils thick, of about 
1150 equivalent weight perfluorosulfonic acid resin which has been 
chemically modified on one side by ethylene diamine converting a depth of 
about 1.5 mils of the polymer to perfluorosulfonamide. The unmodified side 
is laminated to a fabric of polytetrafluoroethylene resin. The fabric is 
characterized by having a basic weave pattern, a thread count of about 
6.times.6 polytetrafluoroethylene, 24.times.24 rayon per centimeter, a 
denier of about 200 polytetrafluoroethylene and 50 rayon, a fabric 
thickness of about 4.6 mils and an open area (Optical) of about 70 percent 
by volume after rayon removed. 
The ethylene diamine treated side of the membrane is oriented toward the 
cathode in the electrolytic cell. 
Also included in the first group are polymers similar to the above '988 
patent which are prepared as described in U.S. Pat. No. 4,085,071, supra, 
wherein surface sulfonyl groups of the backbone sulfonated fluorine 
polymers are reacted to a selected depth with a primary amine such as with 
heat treatment of the converted polymer to form N-monosubstituted 
sulfonamido groups or salts on the sulfonyl fluoride sites of the 
copolymer through the reaction of the primary amine. 
With only surface conversion of the sulfonyl halide groups, further 
conversion of the remaining sulfonyl halide groups to the ionic form is 
most desirable. The prior art techniques of conversion of the --SO.sub.2 X 
groups with X as previously defined may be undertaken such as by 
hydrolysis. The techniques set forth in Connolly and Gresham, U.S. Pat. 
No. 3,282,875 and/or Grot, U.S. Ser. No. 178,782 and now U.S. Pat. No. 
3,784,399 may be employed. Illustratively, the unconverted sulfonyl groups 
of the polymer may be converted to the form --(--SO.sub.2 NH).sub.m Q 
wherein Q is H, NH.sub.4, cation of an alkali metal and/or cation of an 
alkaline earth metal and m is the valence of Q. Additionally, the 
unconverted sulfonyl groups may be formed to --(SO.sub.3).sub.n Me wherein 
Me is a cation and n is the valence of the cation. Preferred definitions 
of Q include NH.sub.4 and/or cation of an alkaline earth metal 
particularly sodium or potassium. Preferred definitions of Me include 
potassium, sodium and hydrogen. 
With respect to the diamine or polyamine substituted polymers of the '988 
patent and the primary amine polymers of the '071 patent described above, 
the modifications are generally performed on only one side of the 
membrane. The thickness of the diamine and polyamine substituted polymers 
is in the range from about 4 to about 10 and preferably in the range from 
about 5 to about 9 mils. The depth of the modification is in the range 
from about 1.0 to about 7.0 and preferably from about 1.2 to about 1.5 
mils. 
The amine treated side of the membrane is also oriented toward the cathode. 
A second group of materials suitable as membranes in the process of this 
invention includes perfluorosulfonic acid membrane laminates which are 
comprised of at least two unmodified homogeneous perfluorosulfonic acid 
films. Before lamination, both films are unmodified and are individually 
prepared in accordance with the basic '714 patent previously described. 
The first film has a thickness in the range from about 0.5 to about 2 mils, 
of about 1500 equivalent weight perfluorosulfonic acid resin, and the 
second film has a thickness in the range from about 4 to about 6 mils, of 
about 1100 equivalent weight perfluorosulfonic acid resin. 
After lamination together to form a single film, the resulting membrane is 
positioned in the electrolytic cell with the thinner, higher equivalent 
weight side of the resulting film oriented toward the catholyte chamber. 
Typical laminated membranes of the second group which may be employed in 
the process of this invention include (a) a homogeneous film about 1 mil 
thick of about 1500 equivalent weight perfluorosulfonic acid resin and a 
homogeneous film about 5 mils thick of about 1100 equivalent weight 
perfluorosulfonic acid resin; (b) a homogeneous film about 1.5 mils thick 
of about 1500 equivalent weight perfluorosulfonic acid resin and a 
homogeneous film about 5 mils thick of about 1100 equivalent weight 
perfluorosulfonic acid resin; (c) a homogeneous film about 2 mils thick of 
about 1500 equivalent weight perfluorosulfonic acid resin and a 
homogeneous film about 4 mils thick of 1100 equivalent weight 
perfluorosulfonic acid resin; and (d) a homogeneous film about 1.5 mils 
thick of about 1500 equivalent weight perfluorosulfonic acid resin and a 
homogeneous film about 4 mils thick of about 110 equivalent weight 
perfluorosulfonic acid resin. 
For selected laminated membranes, a laminated inert cloth supporting fabric 
may be employed. The thickness of the laminated inert cloth supporting 
fabric is in the range from about 3 to about 7 and preferably from about 4 
to about 5 mils. The inert supporting fabric is typically comprised of 
polytetrafluoroethylene, rayon, or mixtures thereof. 
At least one electrode is positioned within the anolyte chamber and one 
electrode within the catholyte chamber. For maximum exposure of the 
electrolytic surface, the face of the electrode should be parallel to the 
plane of the membrane. 
Examples of materials which may be employed as an anode include 
commercially available platinized titanium, platinized tantalum, or 
platinized platinum electrodes which contain, at least on the surface of 
the electrodes, a deposit of platinum or titanium, platinum on tantalum or 
platinum on platinum. Also effective are anodes composed of graphite, or 
anodes comprised of a metal oxide coated substrate such as ruthenium 
dioxide or titanium and others as described in U.S. Pat. No. 3,632,498, 
issued to H. B. Beer on Jan. 4, 1972 which is incorporated herein in its 
entirety by reference. When such electrodes are employed as anodes, anodic 
chlorine overvoltage is minimized. Any electrode construction capable of 
effecting electrolytic production of potassium hydroxide from a brine 
containing potassium chloride may be employed in the process of this 
invention. 
Examples of materials which may be employed as the cathode are carbon 
steel, stainless steel, nickel, nickel molybdenum alloys, nickel vanadium 
alloys, mixtures thereof and the like. Any cathode material that is 
capable of effecting the electrolytic reduction of water with either high 
or low hydrogen overvoltage may be used as cathode construction material 
in the process of this invention. 
The cathode and anode may each be of either solid, felt, mesh, foraminous, 
packed bed, expanded metal, or other design. Any electrode configuration 
capable of effecting anodic electrolytic production of potassium hydroxide 
from a brine containing potassium chloride may be used as anodes or 
cathodes, respectively, in the process of this invention. 
The distance between an electrode, such as the anode or the cathode, to the 
membrane is known as the gap distance for that electrode. The gap distance 
of the anode to membrane and the cathode to membrane are independently 
variable. Changing these respective distances concurrently or individually 
may affect the operational characteristics of the electrolytic cell and is 
reflected in the calculated current efficiency. For the process of this 
invention for each electrode, the electrode current efficiency is defined 
as the ratio of the number of chemical equivalents of product formed 
divided by the electrical equivalents consumed in forming that product x 
100. This may be expressed mathematically by the following equation (1): 
##EQU1## 
where A=Mass of product produced in grams. 
B=Equivalent weight of product produced in grams per equivalent. 
C=Quantity of electricity consumed in making desired product in ampere 
hours. 
D=Faraday's Constant of 26.81 ampere hours per equivalent. 
In general, preferably anode to membrane and preferably cathode to membrane 
gap distances can be defined for any concentration of potassium chloride 
employed as the anolyte in the membrane electrolytic cell. When using 
potassium chloride solution as the anolyte at a concentration in the range 
from about 200 to about 300 grams potassium chloride per liter, the 
preferable anode to membrane gap distance is in the range from about 0.1 
to about 2.5 centimeters, and the preferable cathode to membrane gap 
distance is in the range from about 0.1 to about 1.7 centimeters. 
The anolyte is comprised of an aqueous solution of potassium chloride. The 
solution charged to the electrolytic cell may be made by dissolving solid 
potassium chloride in water, preferably deionized water, or the solution 
may be obtained by regenerating spent solution of potassium chloride. 
Minor amounts of sodium chloride, sodium bromide, potassium bromide, 
potassium sulfate, sodium sulfate, potassium dithionate, sodium 
dithionate, sodium bisulfate, potassium bisulfate, Na.sub.3 PO.sub.4, 
K.sub.3 PO.sub.4 or mixtures thereof may be present. The concentration of 
potassium chloride ranges from about 200 to about 300 and preferably from 
about 250 to about 285 grams potassium chloride per liter in the anolyte 
feed. 
The aqueous solution of potassium chloride described above is supplied to 
the anolyte chamber of the electrolytic cell at a concentration described 
above and at a flow rate in the range from about 5 to about 20 milliliters 
per minute. 
In starting up an electrolytic cell employing a selected permselective 
membrane from among these previously described, the cell is first 
assembled employing the selected membrane. Potassium chloride solution at 
the desired concentration is charged to the anolyte chamber until it is 
substantially full, leaving sufficient space at the top to collect and 
remove chlorine product. An aqueous solution of alkali metal hydroxide 
such as potassium hydroxide, sodium hydroxide or mixtures thereof of the 
desired concentration is fed into the catholyte chamber until 
substantially full, leaving sufficient space at the top to collect and 
remove hydrogen gas product. 
In the operation of the process of this invention, a direct current is 
supplied to the cell and a voltage of about 3.8 volts is impressed across 
the cell terminals. To initially obtain the desired concentration of 
potassium hydroxide, little or no alkali metal hydroxide such as potassium 
hydroxide solution may be withdrawn from the catholyte chamber until the 
desired concentration is obtained. 
Alternatively, the catholyte chamber is filled with deionized water prior 
to the start of electrolysis. U.S. Pat. No. 4,062,743, supra, discloses 
general methods for starting up electrolytic cells employing alkali metal 
halide brines such as potassium chloride brine. During electrolysis, a 
portion of the spent potassium chloride solution is removed from the 
anolyte chamber of the cell after partial depletion. The spent solution is 
treated and reconstituted with fresh potassium chloride to achieve the 
desired feed potassium chloride concentration, and then is recycled to the 
cell anolyte chamber for electrolysis. 
The rate of which potassium chloride solution is supplied to the anolyte 
chamber during electrolysis is in the range from about 2 to about 20 and 
preferably from about 5 to about 15 milliliters per minute at a current 
density of about 2 kiloamperes per square meter. 
When employing a cell with an amine modified or laminated permselective 
membrane as in the present invention, potassium ions are transported 
across the membrane from the anolyte chamber into the catholyte chamber 
during electrolysis. The concentration of the potassium hydroxide produced 
in the catholyte chamber is essentially determined by the amount of water 
added to this chamber from a source exterior to the cell and from water 
transferred through the permselective membrane. 
The percent depletion of KCl in the KCl brine during electrolysis is the 
percent KCl electrolyzed and in the range from about 5 to about 40 and 
preferably about 10 to about 30%. 
In a preferred embodiment, the catholyte KOH concentration is maintained 
within the desired range by feeding water into the catholyte chamber at a 
rate of about 0.05 to about 0.2 milliliter per minute per kiloampere per 
square meter of cathode surface. The amounts of water added controls the 
concentration of the potassium hydroxide in the catholyte, which, in turn, 
affects the ion transport properties of the membrane. 
Electrolysis of the potassium chloride brine is conducted at current 
densities of from about 1.0 to about 5.0, and preferably from about 1.5 to 
about 2.5 kiloamperes per square meter of anode working surface. 
The operating temperature of the membrane cell is in the range from about 
40.degree. to about 150.degree. C., and preferably of about 70.degree. to 
about 100.degree. C. 
The operating pressure of the cell is essentially atmospheric. However, 
sub- or superatmospheric pressures may be used, if desired. 
The catholyte is removed from the electrolytic cell at a KOH concentration 
in the range from about 300 to about 500 and preferably from about 350 to 
about 480 grams potassium hydroxide per liter. 
After removal from the cell, the potassium hydroxide solution may be used 
as is or may be further concentrated by evaporation. 
The concentration of potassium chloride in the catholyte product is minimal 
and is generally less than about 0.1 weight percent KCl. This minimal 
amount of potassium chloride migrates from the anolyte chamber during 
electrolysis. 
Chlorine produced in the anolyte chamber and hydrogen produced in the 
catholyte chamber are recovered from the cell as formed and are recovered 
by well-known methods.

The following examples are presented to define the invention more fully 
without any intention of being limited thereby. All parts and percentages 
are by weight unless indicated otherwise. 
EXAMPLE 1 
Potassium hydroxide, hydrogen gas and chlorine gas were continuously 
prepared in a divided flow-through polytetrafluoroethylene cell having an 
anolyte chamber containing an anode and a catholyte chamber containing a 
cathode and exterior dimensions which were about 23 centimeters in height, 
about 13 centimeters in width, and about 9 centimeters in depth. An 
ethylene diamine modified permselective cation exchange membrane as 
described below was employed to separate the catholyte chamber and the 
anolyte chamber. 
An anode was positioned vertically in the anolyte chamber. The anode was a 
23/4 inch by 23/4 inch section of metallic mesh comprised of a titanium 
substrate coated with a mixed oxide of ruthenium oxide and titanium oxide. 
The coating was obtained by painting the titanium substrate with butyl 
titanate and ruthenium trichloride and then oven fired to form the oxides. 
The finished anode was of the type described in U.S. Pat. No. 3,632,498, 
supra, was secured on one side to a 5/16 inch diameter circular titanium 
rod centrally inserted through one side of the anolyte chamber. 
A cathode was positioned vertically in the catholyte chamber. The cathode 
was a 23/4 inch by 23/4 inch section of nickel wire mesh. The cathode mesh 
was secured on one side to 5/16 inch diameter circular nickel rod which 
extended into the catholyte chamber through the opposite side wall of the 
catholyte chamber. 
The membrane employed was a homogeneous film of cationic exchange membrane 
(about 7 mils thick), about 1150 equivalent weight perfluorosulfonic acid 
resin which had been chemically modified by ethylene diamine, converting a 
depth of about 1.5 mils to perfluorosulfonamide, and laminated with a 
fabric backing of polytetrafluoroethylene resin and rayon. The fabric had 
a basic weave pattern, a thread count of about 6.times.6 
polytetrafluoroethylene, 24.times.24 rayon per centimeter, a denier of 
about 200 polytetrafluoroethylene and 50 rayon, a fabric thickness of 
about 4.6 mils and an open area (Optical) of about 70 percent by volume 
after rayon removed. 
The membrane was soaked for about 16 hours in about a 25 percent by weight 
aqueous sodium hydroxide solution which was maintained at a temperature of 
about 85.degree. C. 
Thereafter, the membrane was removed from the sodium hydroxide solution and 
while still damp with the sodium hydroxide solution was placed in the 
cell. 
The membrane was positioned vertically in the center of the cell and formed 
a catholyte chamber which was about 7.6 centimeters in width, about 1.7 
centimeters in depth, and about 17.8 centimeters in height and an anolyte 
chamber which was about 7.6 centimeters in width, about 1.9 centimeters in 
depth, and about 17.8 centimeters in height. 
Both anode and cathode were positioned parallel to the cell membrane. The 
ethylene diamine modified side of the membrane was oriented toward the 
catholyte chamber. The anode to membrane gap distance was set at about 0.3 
centimeter and the cathode to membrane gap distance was set at about 0.3 
centimeter. The cell was fully assembled. 
The anolyte chamber was filled with a saturated potassium chloride solution 
containing about 280 grams potassium chloride per liter of solution. The 
catholyte chamber was filled with an aqueous solution of sodium hydroxide 
containing about 30 percent sodium hydroxide by weight. 
The direct current electrical leads were connected to the cell and the 
current was turned on at a current density of about 0.1 kiloampere per 
meter square. The cell temperature was increased from about 25.degree. to 
about 55.degree. C. at a rate about linear with time of about 8C..degree. 
per hour by employing an electrical resistance heater in the catholyte 
chamber. The cell was held at the above conditions for about 16 hours 
before cell operation. 
The cell was further warmed to an operating temperature of about 85.degree. 
C. at a rate about linear with time of about 8C..degree. per hour. The 
current was gradually increased about every 15 minutes by a current 
density increment of about 0.2 kiloampere per meter square to a final 
current density of about 2.0 kiloamperes per meter square. 
During electrolysis, the anolyte solution was continuously supplied at the 
rate of about twelve millimeters per minute to the anolyte chamber of the 
electrolytic cell by regulating the flow from a head tank of anolyte 
solution. A receiving tank was connected to the outlet process connection 
on the anolyte chamber to collect depleted potassium chloride brine for 
treatment, regeneration and subsequent reuse as feed potassium chloride to 
the electrolytic cell. In addition, a storage flask was connected to the 
outlet process connection on the catholyte chamber to collect product 
potassium hydroxide. A source of deionized water was connected to a 
process inlet of the catholyte chamber. The vapor outlet of the anolyte 
chamber was connected to a vented scrubber to collect chlorine generated 
in the anolyte chamber of the cell. Hydrogen generated in the catholyte 
chamber of the cell was collected in a process hydrogen header system. 
After electrolysis was started in the cell, and the concentration of KOH in 
the catholyte was in the range from about 300 to about 500 grams KOH per 
liter of solution, deionized water was supplied to the catholyte chamber 
at about 0.35 milliliter per minute. 
The portion of the catholyte containing the sodium hydroxide employed 
during start-up of the cell was collected and segregated from the 
catholyte product potassium hydroxide. 
Spent potassium chloride was continuously removed from the anolyte chamber 
and had a concentration of about 240 grams potassium chloride per liter of 
solution. 
The operating temperature of the cell was maintained at about 85.degree. C. 
and the operating pressure of the cell was about atmospheric. Cell voltage 
was about 3.75 volts. 
After above five hours (about 51 ampere hour of electrical energy), 
electrolysis was stopped. During that time, about 337 grams of potassium 
hydroxide solution having a concentration of about 400 grams KOH per liter 
was prepared. The cell current efficiency was calculated using equation 
(1) on the basis of the potassium hydroxide produced and was calculated to 
be about 96.8 percent. 
Table I presents selected operating conditions and calculated catholyte 
current efficiencies for a series of similar tests as Examples 2-6 of 
electrolysis of potassium chloride solutions employed to prepare aqueous 
solutions of KOH of varying concentrations, utilizing the previously 
described electrolytic cell and ethylene diamine modified 
perfluorosulfonic acid membrane of Example 1. 
The percent depletion KCl in KCl brine during electrolysis was about 14.3 
percent. 
TABLE I 
__________________________________________________________________________ 
Electrolysis of KCl at a Current Density of About 2 KA/M.sup.2 at About 
85.degree. C. 
Example 2 3 4 5 6 
__________________________________________________________________________ 
Water (mls per minute) 
0.7 0.5 0.4 0.3 0.13 
Voltage (volts) 3.52 
3.64 
3.90 
3.94 
4.14 
Spent KCl Concentration (grams per liter) 
263 245 263 244 269 
Fresh KCl Concentration (grams per liter) 
280 280 280 280 280 
Product KOH Concentration (grams per liter) 
305 355 435 460 476 
Catholyte Current Efficiency (%) 
97.5 
97.5 
92.4 
90.0 
87.2 
Percent depletion KCl in KCl brine 
6.1 8.9 6.1 9.3 3.9 
during electrolysis 
__________________________________________________________________________ 
COMATIVE EXAMPLE A 
An electrolytic cell was operated under similar conditions as Examples 1-6 
except that the membrane of the cell of Comparative Example A was an 
unmodified cation permselective membrane of about 7 mils thickness having 
an equivalent weight of about 1200 supported by a layer of 
polytetrafluoroethylene cloth as taught in the prior art. 
Current efficiencies were calculated for catholytes of various KOH 
concentrations, as shown in Table II. 
TABLE II 
______________________________________ 
Electrolysis of KCl at a Current Density 
of about 2KA/M.sup.2 
Catholyte Concentration of 
Cathode Current 
KOH (grams per liter) 
Efficiency 
______________________________________ 
325 80 
350 80 
400 80 
425 84 
440 87 
465 86 
475 84 
485 80 
______________________________________ 
A comparison of these results with Examples 1-6 shows that the catholyte 
current efficiency for the ethylene diamine modified perfluorosulfonic 
acid membrane of this invention as shown in Examples 1-4 was about 92 to 
about 98 percent in a KOH concentration range of about 300 to about 435 
grams per liter. As the KOH concentration was increased in the range from 
about 460 to about 476 grams per liter as shown in Examples 5-6, the 
catholyte current efficiency decreased gradually with increasing KOH 
strength to about 87.2 percent. 
In marked contrast, the catholyte current efficiency for the modified 
perfluorosulfonic acid membrane of Comparative Example A was about 80 
percent in the KOH concentration range of about 325 to about 400 grams KOH 
per liter. As the KOH concentration was increased in the range greater 
than about 400 to about 500 grams per liter, the calculated catholyte 
current efficiency increased from 80 percent to a maximum of about 87 
percent at about 420 grams KOH per liter. The calculated catholyte current 
efficiency continued to decrease with increasing KOH concentration. 
Thus it can be seen that over the concentration range of about 325 to about 
400 grams KOH per liter, the catholyte current efficiency of the membranes 
of this invention was at least about 17 percentage points greater than the 
current efficiency of the prior art membrane. Throughout the other 
concentration ranges, the ethylene diamine modified membrane of this 
invention was at least about 4 percentage points and generally as high as 
about 8 percentage points greater than the catholyte current efficiency of 
prior art membranes.