Influence of the presence of surfactants and humic acid in waters on the indophenol-type reaction method for ammonium determination

This work has evaluated the influence of humic acid and/or surfactants in the quantification of ammonium in waters with the indophenol-type reaction method. Thymol has been employed with the colorimetric method for sample ammonium concentrations between 0.25 and 1 mg L−1. In addition, SPE/diffuse reflectance method has been used for quantification of ammonium at low concentrations (between 0.025 and 0.25 mg L−1). Matrix effect owing to humic substances were observed with the colorimetric method when the concentration was equal or higher than 25 mg L−1. Lower concentrations of humic compounds produced matrix effects with the more sensitive SPE/diffuse reflectance method. Generalized H-point standard addition method (GHPSAM) was employed for evaluating the effect of humic acids and for eliminating the bias error produced by humic acids. Real water samples of different types were analyzed and accurate results for ammonium concentration were obtained with both procedures in presence or absence of humic acids. Cationic and anionic surfactants inhibited the derivatization reaction at percentages up to 0.001% and 0.5%, respectively, and non-ionic surfactants at percentages of 5.5%