Analysis of effluent from caustic treatment plants: Improvement of accuracy in the simultaneous potentiometric determination of sulfide and mercaptide ions

Abstract

Potentiometric titration using silver nitrate for determination of sulfide and mercaptide ions, as defined by the UOP Method 209, has been widely used for quality control of Spent Caustic Neutralization Unit in petroleum industries. However, it has been found this procedure does not provide accurate results for samples taken from units based on Wet Air Oxidation (WAO) technology. Therefore, possible sources of error, such as concominant species, between-analytes interference and titration solvent, were investigated and adjustments in the method were proposed. Aqueous media should be used to prepare the titration solution instead of the recommended alcoholic solvent which was found to be related to the quite low analyte recoveries in WAO unit samples. The modified method can be successfully applied to samples with different chemical compositions which is the main advantage over the reference procedure. Limit of detection and relative standard deviation of 0,5 mg.L-1 and 5,2% for sulfide and 0,6 mg.L-1 and 5,5% for mercaptide were achieved.