Determination of sulphite and hydrogen peroxide in pharmaceutical matrices via classical spectrophotometry and flow injection

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Abstract
Sulphite reacts quantitatively at room temperature with 5,5′-dithiobis(2-nitrobenzoic acid) to produce a thiol chromophore and can be determined by measuring the absorbance at 410 nm. Under similar conditions, hydrogen peroxide also reacts with sulphite and can be determined indirectly using the same approach. In the classical method, the reaction rate is matrix dependent and in this work the method was optimised in terms of reagent pH, analyte concentration and sample to reagent volume ratio. In the flow injection system, a similar kinetic effect is noted as a dependence of sensitivity on flow-rate. Both methods have a limit of determination of less than 1 p.p.m. The matrix interferences (cations) are controlled by using a citrate diluent. The reduced anions (sulphide and cyanide) cause a positive bias. In common pharmaceutical matrices, both methods are capable of determining both analytes with an accuracy of 100 ± 2% and an imprecision of 3% relative standard deviation (RSD) in the p.p.m. concentration range. However, the presence of more than 2.5% by mass of certain amino acids produces a minor positive bias.

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