The solution is subjected to the same treatment as in the search for phosphoric acid, with the exception that, instead of adding acetate of lead to the fluid obtained by taking up the residue left from the alcohol with water, it is divided into two portions which are examined separately. A solution of a lime salt is added to one portion: if oxalic acid be present, a precipitate, which is insoluble in acetic acid or in chloride of ammonium, and effervesces when slightly calcined and treated with hydrochloric acid, is formed. Nitrate of silver is added to the remaining portion of the solution: the formation of a precipitate, which detonates when dried and heated in a glass tube closed at one end, is further evidence of the presence of the acid.
ACETIC ACID.
The solution obtained by treating the alimentary canal with water is distilled, as in testing for nitric and hydrochloric acids, and the following properties verified in the distillate: 1st. It has an acid reaction, and possesses the odor of vinegar; 2nd, unless previously neutralized with a base, it fails to redden the per-salts of iron; 3rd, if the distillate is added to a solution of the per-salts mentioned and sulphuretted hydrogen conducted through the fluid, a black precipitate is formed; 4th, upon boiling the still acid fluid with a small quantity of starch, the property of the latter to become colored in presence of free iodine is not changed; 5th, if heated with an excess of litharge, a basic salt which restores the blue color to reddened litmus paper is produced.
HYDROCYANIC ACID.
The detection of hydrocyanic acid requires special precautions. The substances to be examined are mixed with water, if solids are present, and introduced into a retort provided with a delivery-tube which dips in a solution of nitrate of silver. The retort is then heated over a water-bath. If the evolved vapors produce a precipitate in the silver solution, the heating is continued until a fresh portion of the latter is no longer affected. The operation is now interrupted, hydrochloric acid added to the retort, and heat again applied. Should a second precipitation of cyanide of silver occur, the presence of a cyanide in the suspected materials is indicated; whereas the formation of a precipitate by the simple action of heat would point to the presence of free hydrocyanic acid or cyanide of ammonium.[H] In case the latter compound is present, ammonia will be contained in the distillate.
In order to identify the cyanogen, a portion of the precipitate is collected upon a small filter, washed, dried, and then allowed to fall into a rather long tube, closed at one end, in the bottom of which some iodine has previously been placed. A column of carbonate of soda is then introduced above the precipitate for the purpose of retaining the excess of iodine probably taken. Upon heating the lower end of the tube, white fumes of iodide of cyanogen, which condense in needles upon the cold portion of the tube, are produced. These are easily recognized by aid of a magnifying glass. They are colorless and are readily volatilized by heat. Some ammonia is next added to a solution of protosulphate of iron, the precipitate formed thoroughly washed, and exposed to the air until it acquires a greenish hue. The iodide of cyanogen is then withdrawn from the tube and mixed with potassa-lye and the precipitate mentioned above. The mixture is evaporated to dryness, the residue obtained treated with water and the filtered solution then acidulated with hydrochloric acid. If a solution of a persalt of iron is now added to the fluid, a blue precipitate is formed. The addition of salts of copper produces a reddish precipitation.
The remainder of the precipitate formed in the nitrate of silver solution is heated with sulphur and then boiled with an aqueous solution of chloride of sodium: if cyanogen is contained in the precipitate, a solution of sulphocyanate of soda will be formed, and upon adding sesquichloride of iron an intense red coloration produced.
It is evident that the presence of another acid in the solution examined for hydrocyanic acid would render the detection of cyanides impossible, but in all cases hydrocyanic acid can be separated without arriving at a decision in regard to its original state of combination. Nitric, hydrochloric, and several other acids would not be distilled at the temperature of the water-bath; an examination for these by the methods already described can therefore be instituted simultaneously with the search for hydrocyanic acid.
ALKALIES AND ALKALINE EARTHS.
The separation of these bodies in the caustic state is a matter of difficulty owing to the great tendency they possess to become converted into carbonates; the carbonates of lime, baryta and strontia, moreover, being non-poisonous in their effects, will not be employed with criminal intent, and the carbonates of soda and potassa are extensively used as pharmaceutical preparations. Notwithstanding the small chances of success, the isolation of the compounds under consideration in the caustic state is to be attempted.