Analysis of the Cyanogen Compounds

Of the various methods proposed for the detection of hydrocyanic acid, the following are recommended  

The Prussian Blue Reaction. On addition to an alkaline solution of hydrocyanic acid of a few ml. of ferrous sulphate solution and of ferric chloride solution, then shaking and warming, a blue precipitate of ferric ferrocyanide appears on acidification with hydrochloric acid. In presence of only a trace of hydrocyanic acid, only a greenish-blue colouration appears, due to the formation of a colloidal suspension of the ferric ferrocyanide. On standing (sometimes for as long as 12 hours) the suspension settles to blue floes, leaving the supernatant solution colourless. 

This reaction is specific for hydrocyanic acid and is quite sensitive, detecting 5 mgm. per cu. m. of air according to Kolthoff.  

The Prussian blue reaction may be rendered more convenient in use by employing a reaction paper. For this purpose Ganassini s paper is strongly recommended. It is prepared by immersing a strip of filter paper, just before use, in a mixture of 10 ml. 10% ferrous sulphate solution (containing a trace of ferric salt) and 20 ml. of an alkaline solution of Rochelle salt (30 gm. Rochelle salt, 10 gm. potassium hydroxide and 100 ml. water). This paper should be exposed first to the atmosphere containing hydrocyanic acid and then to hydrochloric vapour, when it becomes greenish-blue. 

The Ferric Thiocyanate Reaction. This test is carried out on a solution of the material to be examined if the sample is gaseous, a solution may be obtained by bubbling it through an alkaline 

---