3.7. Effect of interference Since ammonia was evolved from aqueous ammonium under alkali condition at 70 ◦ C, interference from some nitrogen- containing species such as amino acids might occur. Therefore, 2.0 M of ammonium ion was spiked into an aqueous solution con- taining 2.0 mM alanine (1000 times of ammonium ion) to examine the prospective interference from amino acids in the determi- nation of ammonium ion with the present method. Recovery of ammonium was found to be 97% and interference was not found. In the present method, the evolution of ammonium from sample solution was in a mild base condition (0.0476 M NaOH in the sam- ple solution), it was expected not to release any ammonia from amino acid in the presented experimental conditions. Moreover, OPA derivatization reagent was used along with sodium sulfite (as reductant) which is highly selective for the ammonium deriviti- zation [31,34,40,41]. The conjugation of NH3 -OPA-sulfite as being an efficient and reliable derivatization reagent for the selective and sensitive measurement of ammonium in real environmental water samples. This is attributed to the addition of sulfite which contributes to eliminate possible interferences from amino acids, amines, humic acids and other salts. Therefore the present study is much valid for ammonia determination in the real environmental water samples.