Improvements to the ethylene blue method for the determination of hydrogen sulphide in air

Abstract

The Ethylene Blue (EB) method, used in the determination of H2S in various environments, has given cause for concern due to poor precision and varying sensitivity. The reason for this has been shown to be poor pH control in the solution at the time of reaction. Both the rate of formation of the blue colour and the final absorbance obtained are affected by the amount of sulphuric acid added with the reagent (p-diethylaminoaniline). Good precision can be attained if the volumes of reagents added are strictly controlled, but the method has been improved further by incorporating a pH 1.6 maleic acid buffer system in the reaction medium before the addition of reagents. The new method is tolerant to wide variations in the amounts and concentrations of absorbent and reagents and has a greatly improved precision (RSD = 1.7% compared with 6% for ten samples containing 6.5 µg of S with absorbances ranging from 0.387 to 0.412). Extraction of the EB into 10 ml of chloroform can improve the precision further (RSD = 0.643% for ten samples containing 6.2 µg of S with absorbances ranging from 0.549 to 0.557).