Determination of ammonia in seawater using catalytic cathodic stripping voltammetry

A procedure is presented for the determination of ammonia in seawater using adsorptive cathodic stripping voltammetry (CSV). The ammonia is reacted with formaldehyde (13% w/v) at pH 3.8 to form an imino compound (methylenimine) which is adsorbed on the hanging mercury drop electrode (HMDE). Subsequently, a differential pulse scan is made toward more negative potentials and the reduction current is measured. Optimized conditions include an adsorption potential of -0.85 V and an adsorption time of 60 s; the voltammetric scan was initiated from -0.6 V. The peak potential was at -0.91 V. The CSV response was linearly related to the ammonia concentration between 10 and 3000 nM. The limit of detection for ammonia was 4 nM. Comparative measurements of the concentration of ammonia in samples from the estuary of the River Mersey and in lake waters using the CSV method and conventional spectrophotometric technique showed good agreement between the two methods. The method was applied successfully to the determination of low ammonia levels in seawater from the water column of the northwest Mediterranean.