Amino acid effect on aspirin stability in propylene glycol.

Temperature stability studies were conducted on 0.36 M (6.5% W/V) aspirin solutions including either 0.02 M L-methionine or 0.02 M histidine in propylene glycol. Aspirin was determined spectrophoto-fluorometrically as salicylic acid content at 412 nm. A 0.36 M aspirin in polyethylene glycol 400 solution was studied concurrently. Aspirin degradation rate constants, k, obtained from semilogarithmic plots of percent drug remaining versus time at 30-70 +/- 0.5 degrees were used for preparing Arrhenius plots. Good correlation was seen between predicted aspirin stability and experimental k25 degrees values. L-Methionine and histidine markedly reduced aspirin stability.