Sodium picrate paper

The liberation of hydrocyanic acid due to complete hydrolysis of cyano-genic glycosides can be determined by a simple colour test using sodium picrate paper (yellow), which turns red (sodium isopurpurate) in contact with HCN. 

For qualitative (presence-absence) or semiquantitative determinations of cyanide, the easiest and most reliable method is a simple test with either Feigl-Anger or sodium picrate paper. The reactions on which these tests are based as well as other methods for detection of cyanogenic glycosides are described by Hegnauer (1986). 

Prepare as many picrate papers as you need. Soak the papers in a puddle of sodium picrate in the Petri dish. Lift the papers so that they are arranged in a circle round the edge of the dish. 

Guignard s Sodium Picrate PaperSodium picrate paper on exposure to gaseous hydrocyanic acid assumes a blood-red colour by the formation of sodium isopurpurate. ... 

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