ELECTROLYTIC ESTIMATION OF ZINC.
I 269
3 volts. In two hours there was deposited (I) 0.1272gram, and (2) 0.1277gram of gold. To the auric chloride were added 50 cc. of water, then the sodium sulphide and the solution warmed carefully until it became clear, and finally diluted to 150 cc. and electrolyzed. The deposited gold was perfectly adherent, and of a shining yellow color, appearing like polished metal. The separation of gold from arsenic, molybdenum and tungsten may be effected by observing the conditions given for the determination of gold from its sodium sulphide solution. Separations of gold from mercury and tin were not effected.
2.4 to
[CONTRIBUTION FROM T H E JOHN
HARRISON LABORATORY OF CHEMISTRY,
No. 86.1
T H E USE OF A ROTATING ANODE IN T H E ELECTROLYTIC ESTIMATION OF ZINC.' BY I,ESLIEHOWARD INGAAM. Received July 26, I904
THEpurpose of the present investigation was to ascertain how rapidly the metal zinc could be electrolytically precipitated with the aid of a rotating anode. The electrolytes emgloyed were aolutions of zinc in the presence of sodium h y c l r *d e, s d i u m acetate. s o d i u i formate and potassiu& y anide. T h e rate of precipitation of zinc from each of the electrolytes was also studied, after which experiments were instituted to discover how quickly the content of metal in zinc blende could be estimated. It is believed that these experiments indicate which electrolyte is preferable in each case, and demonstrate conclusively that the electrolytic procedure for the estimation af zinc is preferable to the ordinary gravimetric or volumetric methods.
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The Appnuafzis.-The depositions were made upon platinum dishes of the usual form. The anode was a platinum wire I mm in diameter, bent into a spiral 2 inches in diameter. The plane 1 From the author's thesis presented to the University of Pennsylvania for the Ph.D. degree.
LESLIE HOWARD I N G H A M .
1270
of the spiral was nearly flat, but slightly depressed in the center in dish-form to conform approximately to the figure which the surface of the liquid assumes in the dish while agitated. The anode was rotated by a small electric moltor made by the Elektrochemische IVerkstatte in Darmstadt, which received its current from the 1x0-volt lightning circuit by means of the usual drop-cord and plug. The motor rotated at about rjoo turns per minute and was provided with a three-step cone-pulley. -q corresponding three-step pulley on the vertical shaft, which carried the anode, permitted the latter to be rotated at three different speeds approximating s o , and I750 revolutio'ns per minute. The belt was usually run on the s i n m pulley of the armature and the largest of the shaft, thus giving the anode the slowest speed. This speed is designated as I, the faster speeds being I1 ant1 111. The latter was never used in the precipitation of zinc. The amount of liquid in the dish was usually about 125 cc. As the agitation of the electrolyte caused the level of the liquid to rise at the walls of the dish, the amount of cathode surface exposed varied constantly. The figures given for current strength will, in every case, be those read directly from the ammeter, but since the cathode surface was kept as near 100 sq. cm. as was possible under the circumstances, these amperages may be considered as equivalent to those implied in the expression N.Dlo,,= ampere.
&
(4) SODIC3I .ICEPATE CLECTROLYTE.
The conditions for the precipitation of zinc from an acetate electrolyte, as given by Exner,I were conformed to in the following experiments : T h e platinum dishes were first coated electrolytically with s i l d th is dmosit wa&d w ith water and alcohol, imited and weighed. The zinc solution was then pipetted into the dish, the desired reagents added and the solution heated nearly to . . H g over a free was not applied during electrolysis, ~ 1 ~ r t r i r a L a u x gexy .. Elided was sufficient to keep the near-il i t c h n , r -l wint. The lack of agreement between the theoretical and required amount of zinc was not thought remarkable, since the specimen of zinc sulphate used was poarly crystallized and appeared to have been formed from a hot supersaturated solution.
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1
This Journal, ~ 5$96 ,
ELECTROLYTIC ESTIMATION O F ZINC.
1271
The following results were obtained :
$
a
4
a
rl)
0.2503 0.2503 0.2503
0.2503 0.2503 0.5"s 0.5005 0.5005 0.5005 0.5005
0.5005 0.5005
0.3504 0.3504 0.3504 0.3504
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
4 4 4 4 6 6 4 6 6 6 6 6 6 6 6 6
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
I2
I I1 I1 I1 I1 I I I
I2
I
0.5210
I7
I I1 I1 I I I I
0.522'1 0.5 204 0.52nA
I2
I4 14 I4 I7 I2
I3
I7 17 I1 I1 I1 I1
0.2615 0.2614 0.2617 0.2619 0.2626 0.52l"
0.5217
0.5191
0.360-
0.3665 0.3660 0.3658
I 2
3 4 5 6
7 8 9 IO
I1 12
I3 14 I5
16
After the deposit was weighed the zinc and silver were removed together by heating gently with nitric acid (sp. gr. 1 . 2 ) . The gray deposit dissolved very readily, leaving the platinum dish clean and bright. This was then resilvered and weighed as described above. "his mode of cleaning the silver and zinc from the dish was subsequently found to be unnecessary, however, as a dilute solution of sulphuric acid will remove the zinc and leave the silver practically unattacked. Although this was not discovered until later, it will be convenient to describe here the manipulation which was finally adopted. After the dish with its deposit was weighed it was filled to the brim with sulphuric acid previously diluted with about fifty times its volume of water ; the dish was then set aside until visible action ceased. The solution was then poured into a beaker and the dish rinsed with water and heated to a faint redness over a free flame while held in nickel forceps. I t was then cooled under the faucet and filled a second time with the dilute acid, rinsed after a few moments, heated as before and given a third treatment with the same acid. After the weak acid had been poured away the dish was given a final rinsing in clean water, wiped dry externally and ignited, cooled in a desiccator and weighed. l3y this treatment the dish lost only about
LESLIE HOWARD INGHARI,
1272
0.5 mg. &silver earllftme ' it was used and one coat of silver sufficed for over a hundred determinations of zinc. T h e entire time spent in cleaning the dish need not exceed six minutes. Some annoyance was esperienced at the outset in the form of high results and a brown anodic stain, which was found to be due to lead acetate in the sodium acetate, and entirely disappeared upon the remora1 of the lead. All the deposits from a sodium acetate electrolyte consisted of exceedingly minute flat crystals, gray in color and very adherent. (E)
SOnTUhl H Y D R O S I D E ELECTROLYTE,
The conditions given by Exner for the deposition of zinc from a solution of sodium zincate in the presence of a large excess of alkali mere verified in the following experiments : For each experiment 25 cc. of solution (== 0.2490 gram of zinc) were taken, together with the stated amount of sodium hydroxide in stick form. -4fter all was dissolved the liquid was washed into a platinum dish which had been previously coated with silver and weighed, diluted to about IZj cc., then electrolyzed with the current and pressure given in the tabular exhibit below. At the expiration of the stated time the liquid in the dish was siphoned off and replaced by cold water and the circuit interrupted. The following are among the results obtained : Zinc taken.
Gram.
%'it
.
0.2490 0.2490 0.2490 0.2490 0.2490 0.2490 0.2490 0.2490 0.2490 0.2490 0.2490
NaOH.
Am-
Grams. peres.
8
8 8 8 8 8 8 8 8 8 8
55 5 5
Mwtes.
5 5 5
I5
7
5
'6
5
6 6 6 6 6 6
5 5 5 5 5
me.
Volts.
15
20
'5 15 I5 1.5
1 . 5
15 1.5 1.5
Zinc found. Gram.
KsSO4.
No.
0.248r - 4 I 0.2476 '19 2 0.2492 t & 3 0.2489 4 0.2487 - 9 5 0.2492 + %6 0.2492 + A 7 0.2489 - 1 8 0.2488 -1 9 0.2494 t q IO 0.2492 +a11
Gram. 0.0 0.0 0.0
0 .o 0.0
0.0 0.0
equivalent "
