Kinetics of talampicillin decomposition in solutions.

Talampicillin stability in aqueous solutions was studied in a broad range of pH values using as medium solutions of hydrochloric acid (pH 0.4-1.8), phosphate buffers (pH 2.05-3.13 and 6.03-8.04), acetate buffer (pH 3.87-5.28) and borate buffer (pH 8.90-9.10) as well as sodium hydroxide solution (pH 11.48). For the determination of talampicillin concentration changes in kinetic studies, two methods were used: iodometric and spectrophotometric in UV (lambda(max) = 254.5 nm). The catalytic velocity constants (k(H+), k(x), k(o)) were established, the log k-pH profile (35 degrees C) was found, thermodynamic parameters were calculated of the hydrolysis reaction of the beta-lactam bond (k(H+): E(A)= 67.9 kJ mol(-1), delta S = -92.4 J K(-1) mol(-1), delta G = 92.6 kJ mol(-1); k(x): E(A) = 31.8 kJ mol(-1), delta S = -347.1 J K(-1) mol(-1), delta G = 131.1 kJ mol(-1); k(o), pH = 5.28: E(A) = 98.0 kJ mol(-1), delta S = -50.3 J K(-1) mol(-1), delta G = 110.3 kJ mol(-1) at 20 degrees C), and the stability of the lactone bond was studied in the medium with the highest stability of beta-lactam bond of talampicillin (pH 5.28: k(o): E(A)= 32.5 kJ mol(-1), delta S = -220.5 J K(-1) mol(-1), delta G = 94.7 kJ mol(-1) in 20 degrees C), at controlled ionic strength (mu = 0.5 mol l(-1)).