Capillary electrophoretic determination of degradation products of nitrilotriacetic acid used as a complexing agent in a desulphuration process

Abstract

Capillary electrophoresis (CE) was assessed for the determination of the expected degradation products of nitrilotriacetic acid (NTA), viz., imiodiacetic acid (IDA), dimethyliminoacetic acid (DMIA), N-methylglycine (NMG), glycine (GLY), acetate, formate and ammonium, in solutions containing iron(III)-NTA complexes which were used for the desulphuration of industrial gases. Owing to the wide range of absolute mobilities and acidic constants of these analytes, the analytical requirements were best met with three different sets of operating conditions, all carried out with indirect absorbance detection. The more mobile ions (formate, acetate and NTA) were obtained without interference with other process-relevant anions using a buffer of pH 6 and an electroosmotic flow modifier under reversed polarity. Using normal polarity and no modifier, the less anionic species (GLY, NMG, DMIA) were determined at pH 9.5, while the major degradation product, IDA, was best determined at pH 7. Ammonium can be detected in the last two cases. The cumbersome presence of iron (III) ions was avoided by either precipitation (potassium hydroxide) or complexation (ethylenediaminetetraacetic acid). Special attention was paid to method validation. The analyses were fast (<6 min) and exhibited low relative standard deviations for migration times (<0.1%) and peak areas (<5%). The methods were applied to monitor the process of desulphuration. The results are well correlated with those obtained by ion chromatography (IC), but CE appeared simpler and more versatile and cost effective.