pH and buffering capacity problems involved in the determination of ammonia in saline water using the indophenol blue spectrophotometric method

Magnesium ions in brackish water or sea water complex with the citrate reagent (CIT) and thus interfere with the indophenol blue spectrophotometric method for ammonia measurement by altering the final pH of the color solution. The complex system, possibly Mg(CIT) −+OH −⇔Mg(OH)(CIT) 2−, shows buffer characteristics and can be identified by acid–base titration. A maximum capacity of ca. 40 μeq pH −1 ml −1 was found at pH 10.2 when [Mg 2+] was 0.045 M. To compensate for this capacity, extra alkali reagent should be added to sea water samples for the optimal IPB color reaction. For samples with a wide salinity range, the results still need to be corrected empirically either for pH, [Mg 2+] or salinity.