Anions in seawater - Determination of Anions in Natural and Treated Waters - page 379

column is left in contact with 1mol L −1 ammonium carbonate for 3h, after which three
5ml fractions are collected. Finally, molybdenum is determined by atomic absorption at
313.2nm with use of the hot-graphite-rod technique. At the 10mg L −1 level the standard

deviation was 0.13µg.

3.19.2 Determination and preconcentration
The determination and preconcentration of molybdate is discussed in section 15.3.1.

3.20 Nitrate

3.20.1 Spectrophotometric methods
Spencer and Brewer [115] have reviewed methods for the determination of nitrate in
seawater. Classical methods for determining low concentrations of nitrate in seawater use
reduction to nitrite with cadmium/copper [116] or zinc powder [117] followed by
conversion to an azo dye using N -1-naphthylethylenediamine dihydrochloride and
spectrophotometric evaluation. Malhotra and Zanoni [118] and Lambert and Du Bois
[119] have discussed the interference by chloride in reduction-azo dye methods for the
determination of nitrate.
Jones [120] has described a cadmium reduction method involving shaking the sample
with spongy cadmium as an alternative to the use of cadmium columns. Shaking is
carried out for a period of 90min followed by colorimetric determination of the nitrite
formed using sulphanilanide- N -(phthaloyl) ethylene diamine dihydrochloride colour
reaction. This technique has advantages over using a cadmium column in that
sample/cadmium contact time can be more consistently controlled for a large series of
samples and that problems of progressive deterioration in reduction capacity met in
column methods are avoided. Recoveries of nitrate-spiked non saline water samples with
up to 12 parts per thousand salinity were 94-106%.
One of the most appealing features of this method employing a shaker instead of a
cadmium column is that the cadmium-sample contact time for a number of samples can
be consistently controlled simultaneously. Samples can be shaken in covered flasks, but
racks of screw-capped centrifuge tubes are handled more efficiently between work bench
and shaker table. Shaking for 60-90min gave complete reduction at concentrations
between 1 and 100µmol L −1 nitrate. There was slight but inconsistent evidence of
reduction of nitrite when 1µmol L − 1 samples were shaken for longer than 60min.

Table 3.8 Absorbance and coefficients of variation (CV) for potassium nitrate standards
( n =3)

µmol L −1 Nitrate

Optical path (cm) Mean absorbance %CV

0.0 10 0.050 14