Determination of ammonia in seawater by the indophenol-blue method: Evaluation of the ICES NUTS I/C 5 questionnaire

Abstract Replies to the questionnaire on the determination of ammonia, sent to participants in the Fifth ICES Intercomparison Exercise for nutrients in seawater, are evaluated. The conclusions provide some useful recommendations for analysts. Berthelot's reaction (formation of indophenol-blue) is the basis of the colorimetry used by the great majority of participants, and their individual methods are examined in detail. Application of the indophenol-blue method to seawater poses some specific problems, for example, precipitation, and variation of the pH of the matrix as a function of salinity. Basic principles are discussed and participants' procedures are compared with those most commonly used in the literature. Large disparities in reaction and operating conditions are evident between these many versions of what is nominally the same method. Various points of the reaction mechanism are examined in order to determine optimal ranges for reagent concentrations. Procedures for calibration, measuring the blank, and defining the concentration-zero are also evaluated, as the intercomparison results indicate that systematic (particularly relative) errors are widespread. The use of low-nutrient seawater as a calibration matrix is strongly recommended if errors such as those incurred by the use of demineralised water or artificial seawater are to be avoided. Sources of contamination are discussed, the majority of participants being aware that ambient laboratory air can be a major problem. In general, users of automated methods produce better results than users of manual methods, but this may simply reflect a tendency for the former to be the more experienced analysts. Although it was not possible consistently to relate performance in the intercomparison with methodology, it would appear that many procedures deviate from optimal operating conditions, while some are unnecessarily complicated and might benefit from simplification. Analysts are invited to re-examine their ammonia methods, with particular attention to suitability of reagents and reaction conditions, use of adequate blank and calibration procedures and matrices, correction for refraction effects and identification of local contamination sources.