1. Mahesh V. K., Goyal R. N., Gupta R., Sharma R. (1980) J. Indian Chem. Soc.57: 1037
2. Singh C. P. (1982) Acta Chim. Acad. Hung.110: 35
3. McDowell W., Weingarten R. (1969) Melliand Textiber50: 59
4. Ayers G. H., Narang B. D. (1961) Anal. Chim. Acta24: 241
5. Sandell E. B. (1959) Colourimetric Determination of Traces of Metals, 3rd edn. Interscience, New York