1. A.S.T.M. D 460-41, Section 42.
2. Silverman, Louis, Ind. Eng. Chem., Anal. Ed.,13, 524 (1941).
3. Smith, G. Frederick, “Mixed Perchloric, Sulfuric and Phosphoric Acids and Their Applications in Analysis.” G. F. Smith Chem. Co., Columbus, Ohio, 1942, Second edition, Page 10.