Spectrophotometrie, \(p\) H-titration, and paper electrophoreais studies of the ferric-phosphate system indi­cate presence of the unique complex of the composition FeHPO4+ below \(p\) H 2. The thermodynamic stability constants of FeHPO4+, determined from \(p\) H-titration and spectrophotometric studies, are 4.2X 1010 and 7.04 X 1010 ). respectively, as 25°. The ∆F, ,∆H, and ∆S values of the system, Fe3+ + HPO42 \(\rightleftharpoons\) FeHPO44+ determined from spectrophotometric measutements at

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... °, are respectively —14.85 kcal./mole —3.77 kcal./ mole, and 37 e. u.