DIFFERENTIAL CONDUCTIMETRY IN FLOW-INJECTION - DETERMINATION OF AMMONIA IN KJELDAHL DIGESTS

A differential conductivity meter has been constructed for use in flow injection (Fl) systems. The instrument employs a Wien bridge to generate a sinusoidal alternating electrical potential (2 V, 1.4 kHz) applied to a measurement bridge containing two twin conductance flow cells. The difference in the conductance between the cells is monitored. The instrument can be used to follow the small changes in conductance that occur in samples in which a high background ionic concentration is found. This facility was used for the determination of ammonia in Kjeldahl digests. The de-ionized water acceptor stream, previously used in the Fl system, was replaced by a 5 x 10(-4) mol dm-3 HCl solution and the decrease in the conductance caused by the reaction of hydrogen ions with ammonia was monitored. Linear calibration graphs were obtained in the range 0.5-25 mmol dm-3 of ammonium ion and samples can be processed at a rate of 90 h-1. The relative standard deviation for the peak heights of the Fl signals is 1 % or lower. The differential system was briefly studied for the determination of carbon dioxide by using NaOH solution as the acceptor stream.