Anions in seawater - Determination of Anions in Natural and Treated Waters - page 382

detergents, phenols and other ultraviolet absorbing substances. The method incorporates
three features designed to reduce interferences:

(1) Humic acid interference is reduced by carrying out measurements at 225nm, a higher
wavelength than that used by previous workers (210-220nm).
(2) Removal of inorganic interferences, particularly the removal of bromide interference
in seawater by diluting the sample five-fold, the removal of nitrite by the addition of
sulphamic acid and the removal of metals by passage through Amberlite IR 120 cation
exchange resin.
(3) Removal of ultraviolet absorbing organics by passage through a specific ion
exchange resin such as Amberlite XAD-2.

Table 3.10 compares nitrate determinations in the presence of various interfering
substances for this method and for two alternate methods—phenoldisulphonic acid and
ion selective electrode methods. In general, the method proposed by Brown and Bellinger
[127] is less subject to interference.
The speed of the nitrate selective ion electrode makes its use potentially ideal for
nitrate determinations on a large number of samples. However, the results from adding
various interfering substances (Table 3.10) seem to cast some doubt upon the values
obtained in the presence of chloride and bicarbonate for, although the results are precise,
they are not accurate—approximately 20-30% high.

3.20.3 Chemiluminescence method

Chemiluminescent techniques have been used to determine nanomolar quantities of
nitrate and nitrite in seawater [128,129]. This method depends on the selective reduction
of these species to nitric oxide which is then determined by its chemiluminescent reaction
with ozone, using a commercial nitrogen oxides analyser. The necessary equipment is
compact and sufficiently sturdy to allow shipboard use. A precision of ±2nmol L −1 is
claimed and an analytical range of 2nm mol L−1 with analytical rates of 10-12 samples
hourly.
In this method [128] nitrate, nitrate plus nitrite or nitrite alone are selectively reduced
to nitric oxide which is swept from the sample in a helium carrier gas flow. Nitric oxide
is allowed to react with ozone in a nitrogen oxides analyser where it forms excited
nitrogen dioxide. The

Table 3.10 The effects of various interferences on the determination of nitrates by three
methods. For all determinations (with the exception of non saline river waters)
the concentration of NO 3 N was 1.125mg L −1 . All concentrations are given in
mg L −1

Interference Method

Phenoldisulphonic add

Uv method Selective ion electrode

Uncorrected Corrected Uncorrected Corrected Uncorrected Corrected