Hydrogen cyanide occurs in the distillates of a number of plants. It plays an important part in the medicinal value of oil of bitter almond and oil of cherry laurel. The presence of hydrogen cyanide can be ascertained qualitatively by means of the Prussian blue test.
Procedure: To 1 cc. of the oil in a test tube add 1 cc. of distilled water, a few drops of a 10% aqueous sodium hydroxide solution and a few drops of a 10% ferrous sulfate solution.125 Shake thoroughly and acidify with dilute hydrochloric acid. The precipitate of ferrous and ferric hydroxides dissolves, and in the presence of hydrogen cyanide the characteristic precipitate of Prussian blue appears.
In order to determine quantitatively the amount of hydrogen cyanide in an oil the titrimetric method of "The United States Pharmacopoeia"126 has proved satisfactory. This method is based upon precipitation of the cyanide by silver nitrate solution.. The end point of the reaction can be determined by the red color of silver chromate. Part of the hydrogen cyanide found in oil of bitter almond and oil of cherry laurel is bound with benzaldehyde in the form of cyanhydrin ; in order to liberate this hydrogen cyanide, a small amount of freshly precipitated magnesium hydroxide is added.
Procedure: Dissolve 0.75 g. of magnesium sulfate in 45 cc. of distilled water. Add 5 cc. of 0.5 X sodium hydroxide solution and 2 drops of a 10% solution of potassium chromate, and titrate the solution with 0.1 N silver nitrate solution to the production of a permanent reddish tint; this requires but a few drops of the silver nitrate solution. Pour this mixture into a 100 cc. Erlenmeyer flask containing 0.5 g. of oil of bitter almond accurately weighed. Mix well and titrate again with 0.1 N silver nitrate solution until a red tint, which does not disappear upon shaking, is produced. Conduct this titration as rapidly as possible. Calculate the hydrogen cyanide content by means of the following formula :
Percentage of hydrogen cyanide = 0.2702a/s
where: a = number of cc. of 0.1 N silver nitrate required;
           s = weight of sample in grams.

125 Such a solution of ferrous sulfate always contains a small amount of ferric salt which is necessary for this reaction.
126 Twelfth Revision, 314. (Now official in "The National Formulary," Eighth Ed., 31 .)

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