1. E. B. Sandell, Colorimetric Determination of Traces of Metals, New York: Interscience. 1959.
2. A. Moulin, Bull. soc. chim. France31, 295 (1904).
3. K. L. Cheng, Talanta14, 875 (1967).
4. R. Ishida, Hokkaido Kyoiku Diagaku Kiya, Sec.24, 17, 94 (1967).
5. A. R. Selmer-Olsen, Analyt. Chim. Acta26, 482 (1962).