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3. V. A. Devyatnin, Methods of Chemical Analysis in Vitamin Production [in Russian], Moscow (1964), p. 79.
4. E. Bala et al., Bull. Soc. Chim. Er., No. 6, 1813 (1966).
5. S. R. Stevens and G. A. Stein, J. Am. Chem. Soc.,62, 1045 (1940).