An automated spectrophotometric method utilizing the Technicon Autoanalyzer has been developed to determine nitrogen in fertilizer digestates in the range from 5 to 15 ppm nitrogen at a rate of 40 analyses per hour. NH4 is determined by reacting it with phenol and hypochlorite in an alkaline medium to form a blue compound closely related to indole phenol. Optimum reagent concentrations for the colored reaction were determined, and several ions were investigated to determine if they interfere with the reaction. Comparison of results obtained by the automated method and the comprehensive nitrogen method in primary standard grade NH4SO4 and KN03, Magruder check samples, and several commercial fertilizers show that the automated method is accurate and precise. The automated method results in considerable saving of analytical time, especially in laboratories analyzing large numbers of samples
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Gehrke et al. (1968) studied this question.