The qualitative test for hydrogen cyanide has already been referred to on p. 533. Quantitatively it is best determined gravimetrically: About 1 g. of oil is accurately weighed and dissolved in 10 to 20 times its volume of alcohol. To this solution 10 cc. of alcoholic ammonia free from chlorine are added to decompose the phenylhydroxyacetonitrile since otherwise only a part of the hydrocyanic acid is determined.1) After standing for a short time an aqueous solution of silver nitrate is added and the mixture acidulated with nitric acid.'2) After the solution has become clear, the silver cyanide is is collected on a filter, that has been previously dried and weighed, is carefully washed with water and dried at 100° until of constant weight. If s represents the number of grams of oil used, and a the number of grams of silver cyanide obtained, the percentage of hydrogen cyanide in the oil results from the following formula:

1) Kremers and Schreiner, Pharm. Review 14 (1896), 196.

2) If much hydrogen cyanide be present, the addition of silver nitrate to the ammoniacal solution soon causes the precipitation of a finely crystalline precipitate of silver cyanide ammonia, NH3AgCN. In order to prevent this the nitric acid should be added immediately after the addition of the silver nitrate solution.

Hydrocyanic Acid Assay 233

More convenient, but less accurate, because the end reaction is ascertainable with difficulty and uncertainty, is the titrimetric method according to Vielhaber.1) Liebigs' method, which is frequently applied to bitter almond water, is not at all applicable to the oil.

The assay is carried out according to the following directions: About 1 g. of oil is weighed accurately and shaken with 10 cc. of water. In order to split up the cyanhydrin, some freshly precipitated magnesium hydroxide is added, then 2 to 3 drops of a 10 p. c. potassium chromate solution as indicator. While constantly shaking - in order to insure the intimate contact of the oil with the mixture - the titration is carried out by slowly

N adding N/10 silver nitrate solution until the red color of the silver chromate indicates the end of the reaction. Every cc. of N/10 silver nitrate solution corresponds to 0,0027018 g. of hydrocyanic acid. The percentage can be computed with the aid of the following formula:Hydrocyanic Acid Assay 234

N/10 in which b represents the number of cc. of N/10 silver nitrate solution and s the number of grams of oil used.

For practical purposes the titrimetric method may suffice. It has been adopted by the U. S. Pharmacopoeia for the hydrocyanic acid assay of bitter almond oil. The U. S. P. gives 0,002684 as factor. The difference is due to the fact that the factor given above is based on 0 = 16 whereas the factor of the U. S. P. is based on H=1.

Reference should here also be made to a critical discussion by Runne'-) of all gravimetric and volumetric methods for the determination of hydrocyanic acid in bitter almond water.

1) Arch, der Pharm. 213 (1878), 408.

2) Apotheker Ztg. 24 (1909), 288, 297, 306, 314, 325, 333, 344, 356.