Acid composition calorimetric measurements

By means of calorimetric measurements reflecting the dependence of the stability constants of metal complexes on the composition of the solvent mixture, Gergely and Kiss have shown that the stability increase resulting from a decrease in the relative permittivity shows up primarily in an increase in the — dff values, whereas the dS values decrease or remain unchanged compared with those measured in aqueous medium. A comparison of the AH values for aliphatic and aromatic amino acid complexes of copper(II) has revealed that the change in the medium acts to a lesser extent on the AH values of the aromatic amino acid complexes. This is probably due to the presence of the hydrophobic phenyl group in the aromatic ligands, as a consequence of which the extent of hydration even in aqueous medium is lower than in the case of alanine. 

Due to its vital functions related to water balance and the excretion of metabolic end products, the composition and pH of urine can vary widely (which is in contrast to the homeostatic internal environment provided by most other body fluids). Although the solubilities of uric acid (the end product of purine metabolism in humans), xanthine (an intermediate product of purine metabolism) and L-cystine (the least soluble amino acid) strongly depend on pH, accurate measurements performed by our group (Fig. 1) have shown that they hardly depend on the nature and concentration of urinary constituents [36-41], including organic urine components such as urea and creatinine [38]. The solubility of cystine in real urine was found to be comparable to that in synthetic solutions [39]. For xanthine [40] and uric acid [42], thermodynamic consistency of solubility and calorimetric data has been demonstrated. 

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