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 reag ent (CIT) and thus interfere with the indophenol blue spectrophotometric me thod for ammonia measurement by altering the final pH of the color solution . The complex system, possibly Mg(CIT)(-) + OH- double left right arrow Mg( OH)(CIT)(2-), shows buffer characteristics and can be identified by acid-ba se titration. A maximum capacity of ca. 40 mu eq pH(-1) ml(-1) was found at pH 10.2 when [Mg2+] was 0.045 M. To compensate for this capacity. extra al kali 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 t o be corrected empirically either for pH. [Mg2+] or salinity. (C) 2001 Else vier Science B.V. All rights reserved.