4) Method III : 26.025-26.030, Official Methods of Analysis of the Association of Official Analytical Chemists, 11th Ed., (ed. Willeam Horwitz), pp.430-431 (1970)
5) Method I : 26.015-26.020, Official Methods of Analysis of the Association of Official Analytical Chemists, 11th Ed., (ed.Willeam Horwitz), pp. 429-430 (1970)