1. Groebel, W.: Dtsch. Lebensmittel-Rdsch.61, 209 (1965).
2. Official Methods of Analysis, 9th Ed., Association of Official Agricultural Chemists, Washington, D. C. 1960, S. 397.
3. Whittle, F.: Analyst69, 45 (1944).
4. The United States Pharmacopeia Sixteenth Revision (U.S.P. XVI) Washington, D. C. 1960, S.633.