1. Allport NL, Brockshop JE (1963) Colorimetric analysis, 2nd edn. Chapman and Hall, London, pp 239?240
2. Amlathe S, Upadhyay S, Gupta VK (1987) Analyst 112: 1463?1465
3. Baveja AK, Gupta VK (1984) J Ind Chem Soc LXI:348?351
4. Calabrese EJ (1978) Pollutants of high risk groups. Wiley Interscience, New York, p 8
5. Ettinger MB, Ruchhoff CC (1948) Anal Chem 20:1191?1196