A simple accurate and convenient reductimetric titration method has been developed for the determination of two nitroso compounds namely: I -Nitroso 2 Napthol (α-Niraso β –Napthol) and 2-Nitraso 1-Napthol (β-Nitroso α- Napthol) using iron(II) as a reducing agent in a phosphoric acid medium. The method consists in titrating the solutions of these compounds against iron(II) in about 9 M (or above) phosphoric acid medium and in presence of a small amount of resorufin (RSF) which acts as a catalyst as well as redox indicator. The color transition of the indicator at the end- point (from red to bluish green) is sharp, reversible and no indicator correction need to be applied. The two nitroso compounds in the range 5 -20 mg have been determined with an accuracy of ± 0.06 %. The precision of the method is found by computing pooled standard deviation and 95% confidence limits. The two nitroso compounds are found to be reduced by iron(II) to their corresponding amino compounds each in a four electron reduction step. The indicator RSF is reduced to its bluish green semi quinone by iron(II) in one electron reduction step. The formal redox potentials of the two oxidant systems ( [nitroso compound]/[amino compound] couple) and the reductant system ( [ iron(III)] /[ iron(II)] couple ) and the transition potential of the indicator RSF have been measured in 9-12 M phosphoric acid medium. Based on these potentials data, the conditions needed in the titrations have been explained.
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