Electroplating cathodes for electrochemical synthesis

Packages of undivided electrolytic cells for electrochemical synthesis having a plurality of metal-plated cathodes with corresponding anodes are produced by assembling and permanently fixing the electrodes in substantially parallel-planar relationship as a package and thereafter electroplating the cathodes in package form by applying an electric potential between the anodes and cathodes in an electroplating solution containing the plating metal in complex form, the anodes functioning as non-sacrificial anodes during electroplating. Such electroplating may be accomplished in the electrochemical synthesis cells and may employ the same power application means and controls as the means and controls employed in the electrochemical synthesis.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The invention relates to the production of electrolytic cells for 
electrochemical synthesis, and particularly those cells of the undivided 
variety in which the electrodes are fixed and in substantially 
parallel-planar relationship in the form of an electrode package and in 
which the cathodes are metal-plated, the invention being an improvement in 
the method of production of such cells. 
B. The Background of the Invention 
Electrolytic cells for electrochemical synthesis are well known. Generally 
speaking electrochemical synthesis may involve cathodic processes or 
anodic processes and/or secondary electrolytic processes. Particularly but 
not exclusively with respect to cathodic processes, and more particularly 
with respect to the electrochemical reduction of organic compounds, it is 
common to use metal-plated cathodes. Although any material with a 
sufficiently high electrical conductivity can be employed as a cathode, a 
most important index which characterizes the electrochemical activity of a 
given cathode material is the so-called "hydrogen overvoltage". The 
"hydrogen overvoltage" on a metal may be described by the constants a and 
b of the Tafel equation: 
EQU .eta.=a+b log i 
where .eta. is the hydrogen overvoltage and i is the density of 
polarization current. It is well known that high hydrogen overvoltage 
metals include, for example, lead, thallium, zinc, mercury and cadmium; 
and that such high overvoltage metals retard the discharge of hydrogen 
ions on their surfaces, and for this reason organic compounds are not 
likely to be reduced by atomic hydrogen on such cathodic materials. 
Therefore, they can be reduced mainly by the electrochemical process 
proper, i.e., by direct electron transfer onto the molecule of the organic 
compound. For this reason, cathode surfaces of lead, thalium, zinc, 
mercury or cadmium are preferred for a great many electrochemical 
synthesis reactions involving reduction of compounds. An example of such 
an electrochemical reduction of organic compounds in an undivided cell is 
the reduction of acrylonitrile to adiponitrile, as taught in British Pat. 
No. 1,089,707 (to Tomilov). Wherever it is preferred in electrochemical 
synthesis to use cathodic surface metals selected for the particular 
process desired, for high or for low hydrogen overvoltage or for any other 
individual attribute or property, wherein at the same time, the metal is 
for one reason or another, not suitable for the construction of the entire 
electrode (usually because of the expense or the lack of strength of the 
material) it is a common practice to electroplate the desired metal on a 
more suitable metal such as steel which is strong, readily available and 
inexpensive. A suitable cathode for example, for the 
electrohydrodimerization of acrylonitrile to adiponitrile is 
cadmium-plated steel. 
Electroplating of various metals by any of several methods including the 
cyanide or alkaline method, the acid sulfate method, the pyrophosphate 
method, the fluoborate method and the phytic (hexaphosphoric) acid method, 
is well known. All are described, for example, in U.S. Pat. No. 2,973,308. 
Commercially, electrolytic cells for electrochemical synthesis are 
constructed of electrodes in permanently fixed and substantial parallel 
planar relationship. Metal-plated cathodes are typically electroplated 
from cadmium sacrificial anodes, water-rinsed, drained and sprayed with 
oil, crated for shipment and thereafter shipped to the user location where 
the crates are opened, unpacked, drilled for fastening and then assembled 
into electrode packages. The electrode packages, containing the plated 
cathodes are then made a part of the electrolytic cell. In the course of 
such production, it is not uncommon for the metal-plated cathode, to be 
marred, scratched, or coated with impurities deleterious to the 
electrochemical synthesis. For this reason, any method whereby handling of 
the metal-plated cathodes can be avoided without disproportionate cost 
increases or inconveniences would be a substantial advance in the art and 
such a method is an object of this invention. 
The particular configuration of many electrode packages (including packages 
having bipolar interior electrodes such as are commonly employed 
commercially) in electrochemical synthesis, is such that only a portion of 
the metalic element known as the cathode is exposed to the electrolyte and 
actually participates in the electrochemical synthesis. For this reason, 
it is unnecessary, in the preparation of metal-plated cathodes, to metal 
plate the entire cathode element as is customarily done in prior art 
processes. Accordingly, a method whereby the metal-plating of cathodes can 
be restricted to those portions of the cathode which are actually exposed 
to the electrolyte would represent a substantial conservational 
achievement and a separate significant advance in the art, and such a 
method is another object of this invention. 
In some electrochemical synthesis processes, it is desirable, from time to 
time to re-electroplate cathodes, and if such re-electroplating could be 
accomplished without disassembly of the electrode package and/or without 
removal of the package from the cell, a considerable amount of time and 
effort could be saved, such accomplishment being a third object of the 
invention. 
SUMMARY OF THE INVENTION 
According to this invention, electrode packages, for use in undivided 
electrolytic cells for electrochemical synthesis, which have, in 
permanently fixed and substantial parallel planar relationship, a 
plurality of electrodes comprising anoes and cathodes, a major portion of 
which cathodes are metal-plated, are prepared by assembling and 
permanently fixing the electrodes in the substantially parallel planar 
relationship as a package and thereafter electroplating the cathodes while 
in package form by applying an electric potential between the anodes and 
the cathodes, in an electroplating solution containing the plating metal 
in complex form, the anodes functioning as non-sacrificial anodes during 
electroplating. Preferably, the cell employed and means for applying a 
potential between the anodes and the cathodes in the electroplating step 
are the same cell and application means employed in the electrochemical 
synthesis.

DETAILED DESCRIPTION OF THE INVENTION 
The assembly of the electrode package is accomplished by any method known 
in the art not inconsistent by way of structure or material with the 
electrochemical synthesis to which the electrode package will be employed 
in the particular application. In many applications, it has been found 
expedient to place cathodes and anodes in extremely close parallel-planar 
relationship, and to plate but one side as the cathode in the manner 
depicted at FIG. 3. Critically spaced components of a unit such as that 
depicted in FIG. 3 may be held in spaced apart relationship, to the extent 
required in a particular configuration with plastic (polypropylene) 
spacers. 
This invention contemplates the package electroplating of high hydrogen 
overvoltage metal and other metals on base metals. Generally, the plating 
of such metals on steel has been effected by methods known as the cyanide 
or alkaline method, the acid sulfate method, the pyrophosphate method and 
the fluoborate method as well as the phytic acid method, all of which are 
described in U.S. Pat. No. 2,973,308 (herewith incorporated by reference). 
A major requirement in the selection of a particular electroplating 
process to be practiced in accordance with the instant invention is that 
the plating solution contain the plating metal in complex form and that a 
potential be applied between the object to be plated (the cathode) and a 
non-sacrificial anode (preferably carbon steel). Any suitable 
electroplating process employing these principles is acceptable for the 
practice of this invention. 
As is well known in the art, the employment of a reducing agent such as 
hydrazine minimizes the anodic oxidation of the metal complex agent, as 
taught in U.S. Pat. No. 3,770,596, (hereby incorporated by reference), and 
is preferred. 
Due to the close relationship of the electrodes in the package described, 
it is most important that the electroplating be of a smooth and uniform 
consistency. Accordingly, a leveling agent of the polyether surfactant 
type is preferably employed. Polyether surfactants operable in the 
practice of this invention may include aromatic polyethers and aliphatic 
polyethers. Preferably the surfactant is a polyalkoxylated alkyl phenol. 
Typical polyalkoxylated alkyl phenols include polyethoxylated alkyl 
phenols having the formula: 
##STR1## 
wherein R represents an alkyl group of from 4 to 18 carbon atoms, R' is an 
aliphatic radical containing 8 to 20 carbon atoms, m is an integer of at 
least 4 and no more than 100, and X is selected from the group consisting 
of hydrogen, SO.sub.3 M, and PO.sub.4 M.sub.2 where M is selected from the 
group consisting of sodium, potassium, ammonium, magnesium, lead, tin, 
calcium, rubidium, cesium, or any other bath-compatible cation. Operable 
polyether surfactants include nitrogen-containing aliphatic polyethers 
characterized by the following general formula: 
##STR2## 
wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 represent a straight or 
branched chain alkyl group exhibiting 8 to 18 carbon atoms, n is an 
integer of at least 4 and no more than 100, and X is selected from the 
group consisting of hydrogen, SO.sub.3 M, PO.sub.4 M.sub.2 where M is 
selected from the group consisting of sodium, potassium, ammonium, 
magnesium, lead, tin, calcium, rubidium, cesium, or any other 
bath-compatible cation. Polyether surfactants are employed singly in 
amounts of about 1 g./l. to 10 g./l., and in combination from 10 g./l. to 
20 g./l. Typical specific compounds are the following with their 
concentration ranges varying singly from 1 g./l. to 10 g./l. and in 
combination from 10 g./l. to 20 g./l.: 
##STR3## 
Where the cathode is to be cadimum plated, cadmium complexing agents as, 
for example, cyanide (CN-) and ethylenediaminetetraacetate (EDTA) have 
been found satisfactory. Where EDTA has been employed, a greater variety 
of levelers, including hexadecyl trimethylammoniumhydroxyde (C16 TMAOH) as 
well as a polyether surfactant has been found suitable as a leveling 
agent. As stated above, the only source of the metal to be plated upon the 
cathode is in the plating solution. The plating solution may be of 
decreasing concentration of the cadmium complex or the concentration may 
be fixed by reconstituting the solution on a cycle employing methods well 
known in the electroplating art. 
The substrate of the metal-plated cathode (and anode surface) preferably 
consists essentially of carbon steel as opposed as to iron, alloy steel or 
stainless steel. Carbon steel, as defined herein (and by the American Iron 
and Steel Institute [AIS]) is as follows: "carbon steel is classed as such 
when no minimum content is specified or guaranteed for aluminum, chromium, 
columbium, molybdenum, nickel, titanium, tungsten, vandadium or zirconium; 
when the minimum for copper does not exceed 0.40 percent; or when the 
maximum content specified or guaranteed for any of the following elements 
does not exceed the percentages noted: maganese 1.65; silicon 0.60, copper 
0.60." Carbon steels of various compositions are listed in the 1000, 1100 
and 1200 series of AISI and SAE standard steel composition numbers, many 
of which may be found on page 62 of Volume 1, Metals Handbook, 8th Edition 
(1961) published by the American Society for Metals, Metals Park, Ohio. 
Carbon steels are readily distinguishable from steels conventionally known 
as alloy steels and listed in the 1300 and higher series of the 
aforementioned standard steel composition numbers, from the special alloy 
steels that are conventionally known as stainless steels and normally 
contain substantial (usually more than 0.5%) other metals such as nickel 
and/or chromium, and from commercially-pure iron which, by definition, 
contains not more than 0.01% carbon. In general, the carbon steels that 
are preferably used as anode materials in the process of this invention 
contain between about 0.02% carbon (more typically at least about 0.05% 
carbon) and about 2% carbon. Normally, carbon steels such as those of the 
AISI and SAE 1000 series of standard steel composition numbers are 
preferred and those containing between about 0.1 % and about 1.5% carbon 
are typically most desirable. Only a small amount of dissolution of a 
carbon steel anode takes place while electroplating and a soluble iron 
content slowly builds up in the plating solution. 
One of the desirable characteristics of a cathode to be employed in 
electrochemical synthesis is smoothness. Smoothness may be measured in 
terms of Centerline Average (CLA) which, as used herein is determined in 
accordance with the definition of Centerline Average set forth in American 
Standard ASA B46.1-1962 (Surface Texture) published by The American 
Society of Mechanical Engineers, 345 East 47th Street, New York, N.Y. In 
most cases, the Centerline Average of the cathodic surface employed for 
electrochemical synthesis is desirably less than about 70 microinches 
(1.78 microns), preferably less than about 50 microinches (1.27 microns) 
and, for superior results in many cases, less than about 30 microinches 
(0.76 microns). 
After electroplating, if appropriate, the power supply should be 
discontinued and the used plating solution circulated through the cell for 
an additional period of time before being drained from the cell. As 
disclosed in a prospectively copending U.S. patent application, the 
rinsing of cathodes, particularly cadmium plated cathodes with the 
electroplating solution has been found to inhibit fouling of the cathode 
in certain electrochemical synthesis processes such as the 
electrohydrodimerization of acrylonitrile to adiponitrile. 
Referring now to the drawing, FIG. 1 shows an arrangement of substantially 
parallel planar fixed electrodes which is suitable for electrochemical 
synthesis and for electroplating in accordance with this invention. The 
electrodes are anodes (1) and cathodes (2) which are held in fixed 
parallel planar relationship by non-conductive backing (4). An 
electroplating solution (3) passes between anodes (1) and cathodes (2). 
Referring now in detail to FIG. 2, electroplating solution (21), containing 
the cadmium complex, leveling agent and anode depolarizer is pumped by 
means of circulating pump (22) through heat exchanger (23) and flow meter 
(24) to cell (25) where electroplating takes place. Passing through cell 
(25), the solution is pumped to off-gas separator (26) where most of the 
off-gas is separated from the liquid which drains into stirred vessel 
(27). The gas itself is passed to condenser (28) for removal of 
condensable material. Between heat exchanger (23) and flow meter (24) is a 
filtration stream comprising pressure gauge (29) filter (210) and flow 
meter (211). Stirring motor (212) and thermometer (213) are included in 
stirred vessel (27). 
Referring now in detail to FIG. 3, the essential portions of the simplified 
cell are cathode (31) and anode (32), which are separated by plastic 
spacer (45). A circulation chamber is defined by cathode (31), anode (32) 
and the inside perimeter of plastic spacer (45). The electroplating 
solution is fed through aperture (36) and slot (39) of polyethylene feed 
block (37) through slot (41) of neoprene gasket (34) to the aforementioned 
circulation chamber, and from the circulation chamber through slot (40) of 
neoprene bottom gasket (34), slot (38) of polyethylene feed block (37), 
and out through aperture (35) of polyethylene feed block (37). The entire 
assembly, including micarta upper and lower plates (42) and (43) and 
conductor plate (44) is assembled in fixed parallel-planar relationship. 
Plastic spacer (45) on anode (32) assures uniform spacing of the element 
from cathode (31). Spacer (45), in this particular embodiment is 0.178 cm 
thick. 
EXAMPLES 
Example 1 
A 1500 ml plating solution containing 32.0 g Cd.sup.++ /0.4 g polyethylene 
glycol (PEG) equal parts number average molecular weight (MW) 
1000/1450/per liter with a CN.sup.- /Cd.sup.++ mole ratio of 8 at pH 12.5 
was circulated at one foot/second and 30.degree. C. through the cell 
depicted in FIG. 3 at a current density (CD) of 0.0084 amp/cm.sup.2 for 
300 minutes. The 20.4 g Cd deposited on the 230 cm.sup.2 cathode 
represents a 100% cathode current efficiency and had an average 3.5.+-.0.1 
mil plate thickness with centerline averages (CLA's) of 18-19 microinches 
and the 0.22 moles CN.sup.- /F lost shows the amount of CN.sup.- that is 
lost in the absence of hydrazine. 
Example 2 
A 1500 ml plating solution containing 68.5 g Cd.sup.++ /0.31 moles H.sub.2 
NNH.sub.2 /0.4 g PEGs (equal parts MW 1000 and 1450)/per liter with a 
CN.sup.- /CD.sup.++ mole ratio=4 at ph=12.30 was circulated at one 
foot/second and 30.degree. C. through the cell at a CD=0.008 amp/cm.sup.2 
for 300 minutes. The 34.7 g Cd deposited on the 416 cm.sup.2 cathode area 
represents a 100% cathode current efficiency and had a 3.72.+-.0.18 mil 
average plate thickness with CLA's of 40-58. The plating solution 
increased by 4 ppm Fe and the current efficiency for anoidcally oxidizing 
H.sub.2 NNH.sub.2 was 97%. 
Example 3 
A 1500 ml plating solution containing 67.8 g Cd.sup.++ /0.4 g PEG (equal 
parts MW 1000 and 1450)/0.30 moles H.sub.2 NNH.sub.2 /per liter with a 
CN.sup.- /Cd.sup.++ mole ratio=4 at pH=12.63 was circulated at one 
foot/second and 30.degree. C. through the cell at a CD (Current 
Density)=0.01 amp/cm.sup.2 for 240 minutes. The 36.6 g Cd deposited on the 
416 cm.sup.2 cathode area represents a 100% cathode current efficiency and 
had a 3.89.+-.0.11 mil average plate thickness with CLA's of 60-91. The 
current efficiency for anodically oxidizing H.sub.2 NNH.sub.2 was 95.6% 
and the plating solution increased by 3 ppm Fe. This cathode was run over 
192 hours in an electrolysis cell and its cathode gave normal ADN yields 
and showed no signs of fouling. 
Example 4 
A 1500 ml plating solution containing 67.4 g Cd.sup.++ /0.4 g PEG (equal 
parts MW 1000 and 1450)/0.30 moles H.sub.2 NNH.sub.2 /per liter with a 
CN.sup.- /Cd.sup.++ mole ratio=4 at pH=11.80 was circulated at one 
foot/second and 50.degree. C. through the cell at a CD=0.01 amp/cm.sup.2 
for 240 minutes. The 36.7 g Cd deposited on the 416 cm.sup.2 cathode 
represents a 101% cathode current efficiency and had a 3.92.+-.0.12 mil 
plating thickness with CLA's of 26-42. The current efficiency for 
anodically oxidizing H.sub.2 NNH.sub.2 was 101% and the plating solution 
increased by 3 ppm Fe. 
Example 5 
A 1500 ml plating solution containing 64.1 g Cd.sup.++ /0.4 g PEG (equal 
parts MW 1000 and 1450)/0.31 moles H.sub.2 NNH.sub.2 /per liter with a 
CN.sup.- /Cd.sup.++ mole ratio=8 at pH=11.79 was circulated at one 
foot/second and 30.degree. C. through the cell at CD=0.01 amp/cm.sup.2 for 
242 minutes. The 36.8 g Cd deposited on the 416 cm.sup.2 cathode area 
represents a 101% cathode current efficiency and had a 3.81.+-.0.19 mil 
average plate thickness with CLA's of 9-18. The plating solution increased 
by 4 ppm Fe and the current efficiency for anodically oxidizing H.sub.2 
NNH.sub.2 was 97.5%. An average of 0.007 moles CN.sup.- /Faraday was lost 
in this series of plating fourteen cathodes which shows that less CN.sup.- 
is lost in the presence of hydrazine. No HCN was detected in the offgas. 
EXAMPLE 6 
A 1500 ml plating solution containing 45.7 g Cd.sup.++ /0.4 g PEG (equal 
parts MW 1000 and 1450)/3600 ppm Fe.sup.+++ (added as K.sub.3 
Fe(CN).sub.6)/per liter with CN.sup.- /Cd.sup.++ mole ratio=8 at pH=12.67 
was circulated at one foot/second and 30.degree. C. through the cell at a 
CD=0.008 amp/cm.sup.2 for 300 minutes. The 34.5 g Cd deposited on the 416 
cm.sup.2 cathode area represents a 94% cathode current efficiency and had 
3.75.+-.0.15 mil plating thickness with CLA's of 6-9. A sample of the 
cadmium plating (3.75 mils thick) was dissolved in nitric acid and less 
than 40 ppm Fe was found in the plate. The cathode was run over 268 hours 
in an electrolysis cell producing good ADN yields and showed no signs of 
fouling which shows that a satisfactory cadmium cathode can be plated in 
the in the presence of high Fe.sup.+++ concentrations. 
Example 7 
A 1500 ml plating solution containing 47.5 g Cd.sup.++ /0.4 g PEG (equal 
parts MW 1000 and 1450)/per liter with ethylenediaminetetraacetate 
(EDTA)/Cd.sup.++ mole ratio=1.5 at pH 8.1 was circulated at one 
foot/second and 30.degree. C. through the cell at a CD=0.008 amp/cm.sup.2 
for 300 minutes. The 12.5 g Cd on the 230 cm.sup.2 cathode area represents 
a 65% cathode current efficiency and had 2.64.+-.0.06 mil plating 
thickness with a CLA of 14. The iron content of the plating solution 
increased 22 mmp and 0.148 moles EDTA/faraday was lost during electrolysis 
which shows the amount of EDTA lost in the absence of hydrazine. 
Example 8 
A 1500 ml plating solution containing 67.4 g Cd.sup.++ /no leveling agent 
and no H.sub.2 NNH.sub.2 added/per liter with a EDTA/Cd.sup.++ mole 
ratio=1.67 at pH=12.8 was circulated at one foot/second and 30.degree. C. 
through the cell at a CD=0.008 amp/cm.sup.2 for 300 minutes. The 35.1 g Cd 
on the 416 cm.sup.2 cathode area represents a 101.4% cathode current 
efficiency but the surface was heavily ridged. The iron content of the 
plating solution increased by 1 ppm and 0.14 moles EDTA/Faraday was lost 
during electrolysis which shows the amount of EDTA lost in the absence of 
hydrazine, and shows how rough the surface becomes without a leveling 
agent being present. 
Example 9 
A 1500 ml plating solution containing 60.0 g Cd.sup.++ /2.0 g C.sub.16 
TMAOH/0.30 moles H.sub.2 NNH.sub.2 /per liter with a EDTA Cd.sup.++ mole 
ratio=1.12 at pH=12.50 was circulated at one foot/second and 30.degree. C. 
through the cell at a CD=0.008 amp/cm.sup.2 for 300 minutes. The 34.5 g Cd 
on the 416 cm.sup.2 cathode area represents a 99.7% cathode current 
efficiency and had 4.10.+-.0.20 mil plating thickness with CLA's of 40-43. 
The iron content of the plating solution increased 1 ppm and the 0.03 
moles EDTA/faraday lost during electrolysis shows that smaller amounts of 
EDTA are lost in the presence of hydrazine. 
Example 10 
A 1500 ml plating solution containing 30.3 g Cd.sup.++ /0.2 g C.sub.16 
TMAOH/0.30 moles H.sub.2 NNH.sub.2 /per liter with a EDTA/Cd.sup.++ mole 
ratio=1.5 was circulated at one foot/second and 30.degree. C. through the 
cell at CD=0.008 amp/cm.sup.2 for 261 minutes. The 30.3 g Cd deposited on 
the 416 cm.sup.2 cathode area represents a 100.0% cathode current 
efficiency and had a 3.32 mil plating thickness with CLA's of 86 to 115. 
The iron content of the plating solution increased by 1 ppm; the current 
efficiency for anodically oxidizing H.sub.2 NNH.sub.2 was 100.0% and the 
0.011 moles EDTA/faraday lost during electrolysis shows that smaller 
amounts of EDTA are lost when hydrazine is present. 
Example 11 
A 1500 ml plating solution containing 60.0 g Cd.sup.++ /-1.0 g PEG's (equal 
parts MW 1000 and 1450)/0.45 moles H.sub.2 NNH.sub.2 /per liter with an 
EDTA/Cd.sup.++ mole ratio=2.1 at pH=11.7 was circulated at two feet/second 
and 40.degree. C. through the cell at a CD=0.012 amp/cm.sup.2 for 104 
minutes. The 18.6 g Cd on the 416 cm.sup.2 cathode area represents a 102% 
cathode current efficiency which characterizes the high efficiency to be 
expected when using solutions of pH.gtoreq.11.7. The 1.92.+-.0.08 mil 
plating thickness had CLA's of 12-19 microinches. 
Example 12 
A 1500 ml plating solution containing 40 g Cd.sup.++ /1.0 g PEG's (equal 
parts MW 1000 and 1450)/0.45 moles H.sub.2 NNH.sub.2 /per liter with an 
EDTA/Cd.sup.++ mole ratio=2.1 at pH 11.0 was circulated at one foot/second 
and 40.degree. C. through the cell at a CD=0.016 amp/cm.sup.2 for 78.5 
minutes. The 14.0 g Cd on the 416 cm.sup.2 cathode area represents only a 
76.5% cathode current efficiency which shows how the current efficiency 
can be decreased by operating with a solution of pH less than 11.7. The 
1.75.+-.0.10 mil plating thickness had CLA's of 59-96 microinches which 
reflects the roughing of the surface as the current density is increased. 
In-plate plating of multi-electrode cell packages comprising 
"bi-electrodes" (having a carbon steel side as the anode for one-cell and 
a plated side which is the cathode for the adjacent cell and having no 
independent source of electrical potential as by independent electrical 
current) may be accomplished in the same manner as depicted in the 
drawing, and explained herein. It should be pointed out however that a 
variance will inevitability occur in the plating thickness between the 
plated cathode surfaces of interior bipolar and exterior plates of the 
package. Such variances are believed to be consistent with good 
performance and long life in where subsequent electrochemical synthesis 
the commercial packages contain as many as 200 plates.