1. P. Senise, unpublished studies.
2. F. Feigl, Spot tests, Vol. I, 4th ed. Amsterdam: Elsevier. 1954. p. 285.
3. F. Feigl, Chemistry of Specific, Selective and Sensitive Reactions. New York: Academic Press. 1949. p. 135.
4. F. Feigl andE. Fränkel, Ber. dtsch. chem. Ges.65, 545 (1932).