1. C. J. van Nieuwenburg, Bull. soc. chim. France (5)20, C 63 (1953).
2. M. K. Zacherl, Mikrochim. Acta [Wien]1954, 719.
3. G. D. Beal, Analyt. Chemistry26, 792 (1954).
4. J. Gillis, Bull. soc. chim. France (5)20, C 91 (1953).
5. R. L�vy, Bull. soc. chim. France (5)19, 672 (1952).