Formation Constants of Chromium(III) Complexes with Phthalic Acid and the Jumping Property of the Solid Complex

Applying the continuous variation method to a mixed solution of the hexaaquochromium(III) perchlorate and phthalic acid, the complex formed in the aqueous solution was found to have mole ratios of chromium and ligand material of 1 : 1 and 1 : 3 at pH 1–2 and 3–4 respectively. The formation constants of the chromium(III) complexes with phthalic acid were determined by the pH method. The step-by-step formation constants were found to be log k1=5.52, log k2=4.48, and log k3=2.48 respectively. The solid chromium(III) complex with phthalic acid was also prepared. Though it was found to be an amorphous solid by X-ray diffraction measurement, the composition was close to K3[Cr3(ph)3(OH)6(H2O)6] (ph=C8H4O42−). A curious phenomenon was observed in the moist air; the substance in the dried state jumped, probably due to the absorption of water vapor.