ELECTROLYTIC PRECII’ITIZTION O F CUPROUS OXIDE ____ BY D. MILLER
One of the problems of long standing which has confronted the electrochemist is the question of making white lead electrolytically which shall be able to compete in quality with the product made b>- the “Dutch Method.” Practically all the electrolytic white leads which have been made are failures apparently for two reasons. They are too transparent and absorb too much oil. This may be attributed to the fact that electrolytic white lead is crystalline while the other is amorphous. The problem then becomes simply a question of determining under what conditions we shall be able t o obtain electrolytically a white lead which shall be dense and amorphous. Before taking up the above problem, it was thought that a study of the characteristics of electrolytic precipitates, obtained under varying conditions, might, in the future, help in its solution. For the sake of convenience in the laboratory, i t was decided to work with cuprous oxide instead of white lead, because it was known that the color varies markedlj- with the conditions of precipitation. ll‘ith this end in view, a hot solutiori of sodium chloride was electrolyzed between copper electrodes and the cuprous oxide thus formed examined under the microscope, the color as well as the size of the particles being noted. The effect of varying the temperature, current density, and concentration of the electrolyte mas studied as well as the effect of adding a little gelatin t o the solution.
Variation of Temperature The precipitates we obtained a t various temperatures between 60’ and 100’ showed marked differences. The color and size of the particles varied directly with the temperature, the darker and larger aggregates being formed a t the higher temperatures. In order to be certain that the pre-
Electrolytic Prccip itation o j Cupvnzcs Oxide
257
cipitates contained no cuprous h!-droxide, samples were taken in which the electrolysis had taken place and boiled for not less than fifteen minutes. This operation produced no change either in the size or the color of the particles. Since cuprous oxide is knom-n to precipitate from solutions a t I O O ~ ,it \vas supposed t h a t cuprous hydroxide would dehydrate a t the same temperature. Since no color change was observed, it was assumed for a while t h a t the precipitates were all cuprous oxide and t h a t the color changes were not connected with any change of composition. Subsequent analyses by Rlr. Gillett showed t h a t this was not the case and t h a t the precipitates have compositions varying continuously from t h a t corresponding t o cuprous hydroxide t o that corresponding to cuprous oxide. The yellower the precipitate, the more nearly the analysis will correspond to the formula for cuprous hydroxide while the intense red precipitates approximate fairly closely to the formula for cuprous oxide. This introduces an annoying complication because the change in the size of the particles very probably depends on the change in composition as well as the changes in the other factors. Fortunatelq-, it was found that it Iras cornparatively easq- to duplicate the colcrs with a very fair degree of accuracy. In order to avoid circumlocutions, the word cuprous oxide will be used in this paper to mean precipitates obtained a t 60' or upwards, regardless of the actual composition of the precipitates. These results of the temperature 1-ariations are tabulated in Tables I and 111. Variation of Current Density An increase in the current density causes the particles to become smaller and lighter in color. This is what would necessarily be expected since sinall particles are always produced when a precipitation is rapid and larger ones when it is slower. Alta high current density, the cuprous oxide is precipitated rapidly and consequently the particles are
D. Miller
258 A
small. ment.
IO%
solution of NaCl was used in this experi-
TABLEI Color changes Current den-1 sity Amp,dni2 I
1
IO00
Dark red
1.125 2.25 3.75
70°
so0
90°
--+
7 .j
.1
r ---f
------+-
Light red
I
Light red
---f
.1
6.0
60°
Light orange
The arrows indicate that the colors grade in the direction in which they point between the shades named above. No appreciable color changes took place when solutions, precipitated at 60' or 70°, were boiled for a short time.
TABLEI1 Sizes of particles in thousandths of a millimeter
_-
-
I
A m p dm2 ~~
1
IOOO
goo
1
-
6.4 3 2 3 0
2 6 2 4
1
I
Soo
I-
4 3 2 2
8 0 8 1
2 0
70°
I
1
60'
I
I
,
'
4 0 3 0 2 6
2 0 2 0
I I I
3.2 3.0 2.4 I .6 1.2
These measurements were made with a microscope in the usual way, a micrometer eye-piece, which had been previously calibrated, being used and the crystals floated in glycerine. They represent the average diameter of the single particles. JVhile they may not mean much as absolute measurements, still they do show that the particles are smaller a t the lower temperatures and higher current densities than a t the higher temperatures and lower current densities respectively. From these two tables, the following conclusions can be drawn. The color varies with the size of the crystals the darker color being present when the particles
Electrolytic PvccipitatioTt
OJ
Cuprous Oxide
2 59
are large. Since the samples were not analyzed when prepared, it is impossible to distinguish accurately between the change in color due to change in chemical composition and that due t o change in size of particle. The change in composition is, however, the more important factor. Variation of Concentration The current densitj- was held constant a t 3.75 amperes per square decimeter while the concentration of the electrolyte was varied. In the samples obtained, we find that neither the color differences nor the variation in size of the particles are as marked as in the preceding case. The figures given in Table ITT are practically worthless as regards any difference in the size of the particles. If it were possible to get greater magnification, one niight find that a t the higher concentrations, the particles were smaller, as the data to a certain extent seem to indicate. The objective used in making the measurements in these tables was I 6”. As regards changes in color, it is easy to detect, that at the higher concentrations the precipitates are lighter in color. This, presumably, is due to a slight increase in the solubility of the cuprous oxide with increasing concentration of sodium chloride
TARLEI11 Color differences Conceti tratioii Percent
5 20
IO00
I
90°
800
70°
Brick-red -+
.1
___-__
Saturated I Light brick-red
-e-
+ -+ +
60°
Orange
.1 Orange-red
Here again the arrows indicate that the colors grade in the directions in which the arrows point between the shades named above.
D . 1Vilter
260
TABLEIV Sizes of particles in thousandths of a millimeter ~
~
~-~~
-
Concentration Percent
-___
Iooo
60'
70°
-
~
2 2 2 2 I
3 4 3 0 3.2
5 IO
I5
__
.o .o .o .o .6
The above measurements represent the average diameter
of the single particles. Addition of Gelatine The addition of gelatine t o the electrolyte caused the cuprous oxide t o be precipitated in an amorphous form. One-tenth of a gram of gelatine was added t o each beaker containing about 300 cubic centimeters of a 10% NaC1 solution. The current density used was 3.7s amperes per square decimeter. The colors of the precipitate varied from a brownish red, obtained a t 100' to a yellow formed at 60'. With larger amounts of gelatine, the precipitate assumes a greenish-yellow tint, even a t roo'. The sizes of the particles were as follows. Sizes of particles in thousandths of a millimeter ~
-
~
90°
1000
-
3 2
-
~
~
2 4
70"
so0 2 0
__
60' --
-
1 6
__
__ I 2
The temperatures as given in this article are only approximate values, and the caption 100' merely means that the solution was heated to boiling. The conclusions to be drawn from this work are as follows : I. The color of the cuprous oxide depends on the composition and size of the particles, the color being darker the larger the particles.
Electrolytic Prccifiitatio?z
01 Cupvous Oxide
26 I
2. The particles are larger, the higher the temperature and the slower the precipitation. The general rules of precipitation, therefore, hold whether the substance is precipitated chemically or electrolytically. This work was suggested by Professor Bancroft and was carried out under his supervision.
Covllell 1~~lz.ucuszt).
