1. Cs. Várhelyi and E. Kékedy, Studia Universitatis Babes-Bólyai, Ser. Chem., 1 (1962) 15.
2. F. Lindstrom and B. G. Stephens, Anal. Chem., 34 (1962) 993.
3. S. Mizuta, Denki Kadaku oyobi Kogyo Bitsuri Kogaku, 41 (1979) 33.
4. Z. Contarz and A. Gorski, Rocz. Chem., 48 (1974) 2091.
5. Ito Naotsugu, Obata Kenzaburo and Yoshitome Hiroshi Kagaku Kogaku Ronbunshu, 7 (1981) 377.