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.