IV. Sulphur Test (Liebig).—The liquid to be examined is placed in a somewhat shallow glass dish or beaker, covered almost airtight with a watch-glass, moistened on the under surface with a drop or two of yellow ammonium sulphide. [The ordinary sulphide is commonly yellow enough for the purpose, or, if not, can be made so by warming with a little flowers of sulphur.] After warming gently for a short time (the periods recommended by different authorities vary from half a minute to ten minutes), great care being taken that the steam does not condense and cause the solution on the watch-glass to drop back into the liquid, the cover is removed, dried on a water bath to drive off any excess of ammonium sulphide, treated with a drop or two of water, and a drop of not too acid ferric chloride free from nitric acid and diluted till nearly free from colour. If HCN be present, it will have formed sulphocyanide with the ammonium sulphide, and will therefore generate a blood-red colour with the ferric chloride. If a colour be produced, continue the addition of ferric chloride till no further deepening occurs. The reaction is made quantitative by comparing the tint with that produced by a known quantity of sulphocyanide and ferric solution (Herapath). But there are difficulties in making it exact.
This is the most delicate test for HCN, detecting 1/7930th of a grain of HCN in a very dilute liquid, whereas Prussian blue does not discover less than 1/780th of a grain (Taylor, Ann. Ch. Pharm. lxv., 263). Salts of acetic, formic, and meconic acids give red colours with ferric chloride, but (1) meconic acid is not volatile; (2) the red from acetic and formic acids is at once removed by a slight excess of dilute hydrochloric acid, sulphocyanide is not; (3) sulphocyanide-red is destroyed by solution of mercuric chloride, the others are not.
The above tests are sufficient, but the following additional ones have been at different times proposed.
V. Guaiacum Test.—Paper dipped in fresh tincture of guaiacum, containing about 3 per cent. of the resin, then dried, then moistened with dilute cupric sulphate solution (2 per cent.), becomes blue in HCN vapour. But the same effect is produced without HCN by almost all oxydants, such as chlorine, bromine, or iodine, ferric chloride, nitric and nitrous acids, chromic acid, peroxide of hydrogen, ozone (Mohr’s Toxicologie), also by ammonia, hypochlorous acid, soluble chromates, &c. (Blyth).
VI. Uranium Test.—A grain or two of pure ferrous salt (ammonio-ferrous sulphate will do), and the same quantity of uranium nitrate, are dissolved in half an ounce of water. Two or three drops of this are placed on a white plate, and a drop of the suspected liquid added. A purple precipitate, or a greyish purple colour in weak solutions, indicates HCN. Cobalt nitrate may be used instead of the uranium salt, and is nearly as delicate. (Carey Lea, American J. of Science [3] ix., 121.)
VII. A hot solution of potassium cyanide mixed with picric acid gives a deep blood-red —“picrocyanic” acid). Free HCN does not give this reaction, and therefore must first be neutralized by an alkali. Said to be more delicate than the iron tests. (C. D. Braun, Zeitschr. f. anal. Ch. iii., 464.)
VIII. Slightly alkalize the distillate with potash, add a few drops of cupric sulphate, and afterwards just enough hydrochloric acid to dissolve the excess of cupric hydrate: white cuprous cyanide will remain undissolved. “This test will detect 1/20000 of HCN in solution.” (Lassaigne, Ann. de Chimie, xxvii., 200.) But a similar effect is produced by hydriodic acid, and potassium iodide might have been administered.
IX. Mix the HCN with excess of alkali, add cobalt chloride and tartaric acid: on exposure to air a deep brown-red colour will be produced. (C. D. Braun, loc. cit.).
X. If to a solution of HCN, potash be added in excess, and then a little very finely pulverised, or precipitated, mercuric oxide, the latter will dissolve. Mercuric oxide is soluble in alkaline fluids only in presence of HCN. (Fresenius, Qual. Anal.).