1. Horowitz W (ed) (1983) Official methods of analysis of the Association of Official Analytical Chemists, 13th edn. AOAC, Washington, p 82
2. ASTM (1982) Annual book of ASTM standards, part 30. ASTM Philadelphia, p 292
3. Charlot G (1974) Chimie analytique quantitative, 6th edn. Masson, Paris, p 389
4. Application 918-301, Radiometer, Copenhagen
5. Application Bulletin 8, Metrohm, Herisau