Stability and degradation of sodium nitroprusside
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The primary aim of the investigation was to formulate a stable sodium nitroprusside (Np) injection. However, during the investigation a number of inexplicable phenomena were observed, which led us to examine some degradation processes. Reproducible determination of the concentration OfNp, aquopentacyanoferrate (Apcf) (II) and (III), iron-cyanide or cyanide in degraded solutions appeared to be impossible. For this reason we used the absorbance ratio, ΔE, at 394 nm, the ratio E394/E440 and the resultant colours to study decomposition. The absorbance at 394 nm increases upon controlled irradiation and is accelerated by irradiation of light of greater intensity. This again is accompanied by a relatively rapid decrease in pH value. The addition of citrate, phosphate and edetate does not produce a significant difference of ΔE. Changes were observed in the ratio E394/E440 and resultant colours. These differences are probably due to chelate formation or iron ions, which delay the formation of Prussian blue, or produce other modifications of Prussian blue, or retard the conversion ofApcf (II) IntoApcf (III. The influence of irradiation appears to be dependent upon the concentration ofNp.
IfNp solutions are stored protected from light, no changes are detected in either the spectra or the colour. The longer TheNp solution is irradiated, the more changes occur after storage in the dark. This means that in the presence of degradation products, reaction of these products can continue.
Autoclaving of aNp solution for long periods results in a change of spectrum and the pH value. Addition of 0.05% citric acid has a favourable effect on these changes. Sterilisation by autoclaving at 120°C for a 20 minute period is justifiable with the addition of citric acid, provided that the solution is prepared under exclusion of light and air. To obtain an impression of the way in WhichNp degrades, the individual reaction products were also irradiated in the presence and absence of additives. From these results a degradation scheme ofNp upon irradiation was produced.
KeywordsCitric Acid Sodium Nitroprusside Prussian Blue Edetate Pharmaceutisch Weekblad Scientific Edition
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