Spectrophotometric determination of ammonia using ninhydrin assay and kinetic studies

Abstract

A spectrophotometric method for the determination of ammonia in aqueous media wasinvestigated. This newly developed method for the reaction of ammonia with ninhydrin foundto be simple, accurate and precise. A deep red color product was formed in the presence ofsodium hydroxide at wavelength of 508 nm. The effects of variables such as temperature,time, acidity and reagents concentration have been evaluated to select the appropriateconditions for this reaction. Beer's law is obeyed; the calibration curve is linear over the rangeof 2×10‐4‐1.4×10‐3 M of ammonia with regression coefficient (r2) of 0.994 (n = 7). Thecalculated molar absorptivity and Sandell's sensitivity values are 1.069×103 L/mol.cm and0.015 μg/cm2, respectively. The limits of detection (LOD) and quantification (LOQ) arecalculated to be 1.2×10‐5 and 3.6×10‐5 M, respectively. The intra‐day accuracy expressed asrelative error was less than 0.5% with precision (RSD) ranging from 0.49 to 1.55%. The inter‐day accuracy ranged (RSD) from 0.49 to 1.90 % with a relative error was less than 1.50 %. Thestoichiometry of this reaction was determinedto be of 1:2 between ammonia and ninhydrinwith first order reaction.