Studies of Metal Complexes of Radiobiological Interest. I. The Acidity Constants of Croconic Acid.

Abstract

Potentiometric and spectrophotometric methods were used for the determination of the dissociation constants of croconic acid at 25[degree]C. The potentiometric method gave only approximate values of pK1 0.3-0.5 and pK2 1.4-1.6 (2 M (H, Na)Cl) and pK1 ^ 0.6 and pK2 ^ 1.8 (0.1 M (H, Na) Cl). The ultraviolet spectra of croconic acid have been determined in mixed solutions of 2 M HC1 and 2 M NaCl. The absorption data were treated by two methods: (a) The graphical method for determining the four unknown parameters in eqn. (9) by testing reasonable values of the molar extinction coefficients of H2A and HA". The slope and intercept of the best straight lines corresponded to pKi = 0.2-0.3 and pK2 = 1.3-1.5. (b) The least squares technique using a high-speed digital computer for determining a set of parameters giving the minimum standard deviation. This minimum was obtained for pK1 = 0.32 and pK2 = 1.51. In addition the spectra for the H2A and HA" forms of croconic acid were obtained by this method.