Investigation of the photochemical and thermal degradation of aqueous nitroprusside solutions using liquid chromatography

Abstract Nitroprusside is in aqueous solutions sensitive to light. A rapid ion-pair reversed phase Chromatographic method was employed for selective determination of nitroprusside in photodegraded solutions. Solutions of nitroprusside (50 μg/ml) in water, normal saline, 5% dextrose and autoclaved 5% dextrose were subjected to 350 nm light and to irradiation of normal daylight fluorescent tubes, respectively. Almost identical, non-linear decay curves were obtained for nitroprusside in these solutions in 350 nm light and under normal daylight fluorescent tubes, respectively. Addition of citric acid or disodium edetate (0.25 mg/100 ml) to non-sterilized and autoclaved 5% dextrose admixtures did not improve the stability of nitroprusside. A 10-fold concentration of citric acid promoted the degradation. By contrast the addition of cyanocobalamin (1 mg/100 ml) to 5% dextrose solutions resulted in a significantly slower rate of degradation, particularly upon exposure to 350 nm light because of the high molar absorptivity of cyanocobalamin at that wavelength. The non-linearity of the nitroprusside decay curves, i.e. an initial rapid loss followed by a phase of slower decomposition, can be at least in part explained by the occurrence of a screening effect caused by the gradually developing yellow colour of the solutions due to formation of pentacyanoaquoferrate(III). Secondly, back-formation of nitroprusside from the degradation products pentacyanoaquoferrate(II) and nitrite comes into play. Regeneration of nitroprusside was found to be most effective in the pH range 3.5–5.5.