In compound mixtures most and sometimes all of these tests are useless. If the mixture is deeply coloured, none will act characteristically. If carbonate of potass be present in such proportion as is often met with in the shops, the tests cannot be trusted to.
Process for Compound Mixtures.—The following method of analysis is applicable to all mixtures, organic and inorganic. Add water, if necessary, and filter; and if the fluid which passes through is tolerably free from colour, test a little of it with solution of starch and chlorine. If the colour is too deep to admit of this trial, or the test on trial does not act, unite the fluid and solid parts and transmit sulphuretted hydrogen to convert any free iodine into hydriodic acid. Drive off the excess of gas, supersaturate with a considerable excess of potass, filter, and evaporate to dryness. Char the residue at a low red heat in a covered crucible; pulverize the charcoaly mass, and exhaust with water. This solution will probably act characteristically with starch and chlorine; but on the whole it is better in the first instance to remove some of the salts by evaporating to dryness, and exhausting the residuum with alcohol. The alcoholic solution contains the hydriodate of potass, with some other salts; and on being evaporated to dryness, a residuum is left, on which, when dissolved in water, the starch and chlorine will act characteristically. No other test is necessary; and frequently no other test will act, on account of coexisting salts.
I have found that a grain of iodide of potassium may thus be easily detected in six ounces of urine, which must be considered a very complicated fluid. In the solution ultimately procured nitrous acid struck a pale brown tint, and on the addition of solution of starch a dark-blue precipitate was formed; which, after being sufficiently diluted, disappeared under ebullition, leaving a colourless fluid. On cooling, no change took place; but on the subsequent addition of a drop of sulphuric acid, the blue colour and precipitation were immediately restored. No other reagent acted characteristically, although there was a sufficient quantity of solution to try the starch test ten times at least.
Dr. O'Shaughnessey has proposed a more complex method by precipitation with chloride of platinum.[1] Professor Orfila says it is sufficient to boil and filter the suspected matter, and to heat first the liquid and then the solid part with solution of chloride, when violet vapours of iodine are disengaged, which may be condensed and subjected to various tests.[2] I have not compared this method with the one I have been in the practice of using; but, notwithstanding the strong assurances of its proposer, its superiority in point of delicacy seems dubious, although no one can deny its simplicity.[3]
Action and Symptoms in Man.—From the experiments of Devergie on animals, iodide of potassium seems to be in large doses an irritant, though not a powerful one. Two drachms in an ounce of water killed a dog in three days with violent vomiting, and signs of
