1. I. M. Kolthoff and P. J. Elving:Treatise on Analytical Chemistry, part II, vol. 2, pp. 286, 295, Interscience Pub., New York, N.Y., 1962.
2. Scott’s Standard Methods of Chemical Analysis, N. H. Furman, ed., 6th ed., vol. 1, p. 553, D. Van Nostrand, Inc., New York, N.Y., 1962.
3. F. D. Snell and C. T. Snell:Colorimetric Methods of Analysis, vol. HA, 4th ed., pp. 231–35, D. Van Nostrand Co., Inc., New York, N.Y., 1959.
4. G. Chariot and D. Bezier:Quantitative Inorganic Analysis, pp. 453–54, John Wiley and Sons, New York, N.Y., 1957.
5. S.Standard Methods for the Examination of Waterand Waste Water, 13th edn., pp. 187-92, APHA, Washington, D.C., 1970.