Investigation of complex equilibria in solutions of p-aminohippuric acid and tin (II) ion in perchlorate medium

The complex equilibrium in tin (II)-p-aminohippuric acid (PAH) solutions in perchlorate medium was investigated. The interaction between Sn (II)-ion and PAH was studied with the purpose to establish whether PAH could form complex with, Sn (II)-ion during the labeling with technetium-99m. HPLC, spectrophotometric and pH-metric methods were used. Dissociation constants of PAH, thermodynamic parameters of PAH dissociation, as well as stability constant of tin (II)-PAH were determined. The values of thermodynamic dissociation constants of PAH were 4.25, 4.19 and 4.15 at 295, 302 and 310 K respectively. The calculated thermodynamic values DeltaGdegrees, DeltaHdegrees and DeltaSdegrees, were -24.01 KJ mol(-1), -12.44 KJ mol(-1) and 39.22 Jmol(-1) K-1 at 295 K respectively. Sn (II)-ion exhibits only a weak interaction with PAH in the pH range 2.0 divided by 3.7, e.g. in the Sn-PAH solution a weak complex with high degree of dissociation was obtained. The treatment of spectral data referring to Sn-PAH solutions indicates the formation of a mononuclear complex [SnPAH](+). Mathematical analysis of experimental data indicated that the best fit of the potentiometric data was achieved by assuming the formation of a mixed hydrolytic complex [Sn(OH)PAH]. The calculated value of the stability constant was: log beta(1,-1.1) = 0.95 +/- 0.11 at 298 K