A Note on the Detection of Cyanides

Abstract

Since hydrocyanic acid is both a very weak and an easily volatile acid, it would seem that one ought to be able to separate the cyanides from admixture with these other substances by making the solution containing them slightly acid and distilling off the hydrocyanic acid. Any of the well-known tests could then be applied to the distillate. There is this difficulty, however, that ferro, and ferricyanides and sulfocyanates will all yield hydrocyanic acid when an acid solution containing them is distilled at the boiling temperature. Hilger and Tamba state that cyanides may be detected in the presence of ferroand ferricyanides by acidulating with tartaric acid and passing a current of carbon dioxid through the solution, taking care not to warm the solution above sixty degrees. On account of the restricted conditions this method is not adapted to use by students. The cyanides are salts of very weak acid, while the ferroand ferricyanides and sulfocyanates are salts of strong acids; as the result of this, the cyanides are, upon going into solution in water, largely hydrolytically split up with the formation of much free hydrocyanic acid, while the hydroferrocyanic acid, etc., are not formed. As a result of this, hydrocyanic acid will distil from a solution of potassium cyanide although the latter is highly alkaline. The hydrolysis of all salts of weak acids is greater if there is present at the same time in the solution the cations of some weak base; hence the hydrocyanic acid will distil much more readily from the potassium cyanide solution if there be added to the latter some ammonium chlorid.