1. Official Methods of Analysis of the Association of Official Agricultural Chemists, by William Horwitz, 3rd Edition 1965, P. O. Box, Washington.
2. M. Hangos-Mahr und E. Pungor, Hung. Sci. Instrum.45, 9 (1979).
3. T. N. Tschekirda und G. W. Fliantikova, Khim. Techn. Wody3, 6, 521 (1981).
4. D. S. Taylor, J. Assoc. Off. Agric. Chem.33, 132 (1950).
5. K. Lang, Z. analyt. Chem.163, 241 (1954).