This method was proposed by J. L. Turner and R. C. Holmes in America in 1914. It depends on the fact that arsenic acid of about 85 per cent. strength combines with cineol to form a solid compound in the same way as does phosphoric acid. The method is now official in the United States Pharmacopoeia, where the process is fully described and directions given for carrying it out.

It has been known for some time that cineol combines with arsenic acid in this way, and a patent was taken out in Germany in 1901, Ger. Pat. No. 132,606 (1901), and also in the United States of America, U.S. Patent No. 705,545 (1902), covering the production of cineol from this compound. The method was probably first applied for investigation purposes by Thorns and Molle for the separation of cineol from the oil of Bay Laurel. (Arch, der Pharm. 242, 1904, p. 172.)

We do not think that arsenic acid can claim superiority over phosphoric acid for the quantitative determination of cineol in Eucalyptus oils. We have not been successful in obtaining concordant results with the same oil when using the arsenic acid method quantitatively, and for that reason have ceased to use the process.