By CHARLES A. KOHN, B.Sc., Ph.D., Assistant Lecturer in Chemistry,
University College, Liverpool.
The first application of electrolysis to chemical analysis was made by
Gaultier de Claubry, in 1850, who employed the electric current for
the detection of metals when in solution. Other early workers followed
in this direction, and in 1861 Bloxam published two papers (J. Chem.
Soc., 13, 12 and 338) on "The application of electrolysis to the
detection of poisonous metals in mixtures containing organic matters."
In these papers a description is given of means for detecting small
quantities of arsenic and of antimony by subjecting their acidulated
solutions to electrolysis. The arsenic was evolved as hydride and
recognized by the usual reactions, while the antimony was mainly
deposited as metal upon the cathode. The electrolytic method for the
detection of arsenic, in which all fear of contamination from impure
zinc is overcome, has since been elaborated by Wolff, who has
succeeded in detecting as little as 0.00001 grm. arsenious oxide by
this means (this Journal, 1887, 147).
In a somewhat different manner the voltaic current is made use of in
ordinary qualitative analysis for the detection of tin, antimony,
silver, lead, arsenic, etc., by employing a more electro-positive
metal to precipitate a less electro-positive one from its solution.
The quantitative electrolytic methods of analysis, some of which I had
the honor of bringing before the notice of the Society some time back
(this Journal, 1889, 256), have placed a number of methods of
determination and separation of metals in the hands of chemists, which
can be employed with advantage in qualitative analysis, especially in
case of medical and medico-legal inquiry. These methods are not
supposed to supersede in any way the ordinary methods of qualitative
analysis, but to serve as a final and crucial means of identification,
and thus to render it possible to detect very small quantities of the
substances in question with very great certainty. As such they fulfill
the required conditions admirably, being readily carried out,
comparatively free from contamination with impure reagents, and
capable of being rendered quantitative whenever desired.
In conjunction with Mr. E.V. Ellis, B.Sc., I have examined the
applicability of the electrolytic methods for the detection of the
chief mineral poisons (with the exception of arsenic, an electrolytic
process for the detection of which has already been devised, as
described), viz., antimony, mercury, lead, and copper.
_Antimony_.--The method employed in the case of antimony is that
adopted in its quantitative estimation by means of electrolysis, a
method which insures a complete separation from those metals with
which it is precipitated in the ordinary course of analysis--arsenic
and tin. This fact is of considerable importance in reference to the
special objects for which these methods have been worked out.
The precipitated sulphide is dissolved in potassium sulphide, and the
resultant solution, after warming with a little hydrogen peroxide to
discolorize any poly-sulphides that may be present, electrolyzed with
a current of 1.5-2 c.c. of electrolytic gas per minute (10.436 c.c. at
0° and 760 mm. = 1 ampere), when the antimony is deposited as metal
upon the negative electrode. One part of antimony (as metal) in
1,500,000 parts of solution may be thus detected, a reaction thirty
times more delicate than the deposition by means of zinc and
potassium. The stain on the cathode, which latter is best used in the
form of a piece of platinum foil about 1 sq. cm. in diameter, is
distinct even with a solution containing 1/28 mgrm. of antimony; and
by carefully evaporating a little ammonium sulphide on the foil, or
by dissolving the stain in hot hydrochloric acid and then passing a
few bubbles of sulphureted hydrogen gas into the solution, the orange
colored sulphide is obtained as a satisfactory confirmatory test. The
detection of 0.0001 grm. of metal can be fully relied on under all
conditions, and one hour is sufficient to completely precipitate such
small quantities.
_Mercury_.--Mercury is best separated from its nitric acid solution on
a small closely wound spiral of platinum wire. The solution to be
tested is acidified with nitric acid and electrolyzed with a current
of 4-5 c.c. (c.c. refer to c.c. of electrolytic gas per minute). The
deposition is effected in half an hour. The deposited metal is removed
from the spiral by heating the latter gently in a test tube, when the
mercury forms in characteristic globules on the upper portion of the
tube. As a confirmatory and very characteristic test, a crystal of
iodine is dropped into the tube, and the whole allowed to stand for a
short time, when the presence of mercury is indicated by the formation
of the red iodide. 0.0001 grm. of mercury in 150 c.c. of solution can
be clearly detected.
Wolff has applied this test under similar conditions, using a special
form of apparatus and a silver-coated iron anode (this Journal, 1888,
454).
_Lead_.--Lead is precipitated either as PbO_{2} at the anode from a
nitric acid solution or as metal at the cathode from an ammonium
oxalate solution. In both cases a current of 2-3 c.c. suffices to
effect the deposition in one hour.
Here, again, 0.0001 grm. of metal in 150 c.c. of solution can be
easily detected. With both solutions this amount gives a distinct
discoloration to the platinum spiral, on which the deposition is best
effected. As a confirmatory test the deposited metal is dissolved in
nitric acid and tested with sulphureted hydrogen, or the spiral may be
placed in a test tube and warmed with a crystal of iodine, when the
yellow iodide is formed. This latter reaction is very distinct,
especially in the case of the peroxide.
Of the above two methods, that in which an ammonium oxalate solution
is used is the more delicate, although it cannot be employed
quantitatively, owing to the oxidation of the metal that takes place.
An addition of 1 grm. of ammonium oxalate to the suspected solution is
sufficient.
_Copper_.--0.00005 grm. of copper can be very readily detected by
electrolyzing an acid solution in the usual way. A spiral of platinum
wire is employed as the cathode, and the presence of the metal
confirmed for by dissolving it in a little nitric acid, diluting with
water and adding potassium ferrocyanide.
To detect these metals in cases of poisoning, the organic matter with
which they are associated must first be destroyed in the usual way by
means of hydrochloric acid and potassium chlorate, and the
precipitates obtained in the ordinary course of analysis, then
subjected, at suitable stages, to electrolysis. As the solutions thus
obtained will be still contaminated by some organic matter, it is
necessary to pass the current for a longer time than indicated above.
On the other hand, _urine_ can be tested directly for these poisons.
The presence of mercury or of copper may be detected by acidifying the
urine with 2-3 c.c. of nitric acid (conc.), and electrolyzing as
described. 0.0001 grm. of metal in 30 c.c. of urine can be detected
thus, or 1 part in 300,000 of urine.
Lead does not separate well as peroxide from urine, but if ammonium
oxalate be added, and the lead deposited as metal, the reaction is
quite as delicate as in aqueous solution, and 0.0001 grm. of lead can
be thus detected.
With antimony it is advisable to precipitate it first as sulphide, but
it can be detected directly, though not so satisfactorily, by
acidifying the urine with 2-3 c.c. of sulphuric acid (dil.), and
electrolyzing with a current of 1-5 to 2 c.c. In this case also it is
precipitated as metal upon the cathode (cp. Chittenden, Proceedings
Connecticut Acad. Science, Vol. 8).
In the presence of urine it is advisable to continue the passage of
the current for about twice the time required in the case of aqueous
solutions.
That an approximately quantitative result can be obtained under the
above conditions was shown in several cases in which deposition of
0.001 grm. of metal was confirmed with considerable accuracy, the
spiral or foil being weighed before and after the experiment.
A comparison of the delicacy of these tests with the ordinary
qualitative tests for antimony, mercury, lead, and copper by means of
sulphureted hydrogen, showed that the two were equally delicate in the
case of antimony and of copper, but that in that of mercury and of
lead the electrolytic test was at least eight times the more delicate.
These comparisons were made in aqueous solutions. In testing urine the
value of the electrolytic method is still more evident, for here the
color of the liquid interferes materially with the reliability of the
ordinary qualitative tests when only very small quantities of the
metals referred to are present.
Beyond the detection of mineral poisons, qualitative electrolysis can
only offer attraction to analysts in special cases, and the data on
the subject are to be found in the many electrolytic methods already
published. Beyond testing for gold and silver in this manner, I have
not therefore examined the applicability of these methods further.
The detection of small quantities of gold and silver is of
considerable importance, and advantage can be taken of the ease with
which they are separated from potassium cyanide solution by the
electric current for this purpose.
_Silver_.--Silver is obtained as chloride in the course of analysis.
To confirm for the metal electrolytically, this precipitate is
dissolved in potassium cyanide and the resulting solution electrolyzed
with a current of 1-1.5 c.c. A spiral of platinum wire is employed as
the anode, from which the silver may be dissolved by means of nitric
acid, and tested for by hydrochloric acid or by sulphureted hydrogen.
0.0001 grm. of silver in 150 c.c. of solution can be detected thus,
and one hour is sufficient for the deposition.
_Gold_.--Gold is deposited under similar conditions to silver from
cyanide solutions. The deposit, which is rather dark colored, can be
dissolved in aqua regia and confirmed for by the Cassius' purple test.
Here again 0.0001 grm. of metal in 150 c.c. of solution can be
detected without any difficulty.
As gold and silver are both extracted from quartziferous ores by
treatment with potassium cyanide solution according to the
MacArthur-Forrest process of gold extraction (this Journal, 1890,
267), this electrolytic method should prove very useful. By
electrolyzing the resulting solution a mixture of gold and silver will
be deposited upon the cathode, which can then be parted by nitric acid
and tested for as described.