The stability of quinapril hydrochloride--a mixture of amorphous and crystalline forms (QHCl-AC)--in solid phase.

The first-order rate constants and thermodynamic parameters (Ea [kJ x mole(-1)] = 139.9 for RH=0% and 133.6 for RH=76.4%; deltaH(++)[kJ x mole(-1) = 137.4 for RH=0% and 131.1 for RH=76.4%; delta(S(++) [J x (K(-1) x mole(-1)) = 35.4 for RH=0% and -207.8 for RH=76.4% for the degradation of quinapril hydrochloride--a mixture of amorphous and crystalline forms (QHCl-AC) in solid state were calculated. The effect of humidity on the stahility of QHCl-AC in the humidity range 25.0% to 76% at 363K is described by the equation ln ki = ax + b = (0.058 +/- 0.0086) x RH% - (14.19 +/- 0.50). Mechanism of degradation of QHCl-AC was investigated at 363 K (relative humidity 76.4%) and 373 K (relative humidity 0%).