Anions in seawater - Determination of Anions in Natural and Treated Waters - page 389

hydrochloride under acidic conditions to form an azo dye which is measured
spectrophotometrically. The reliability and precision of the procedure were tested and
found to be satisfactory for routine analyses provided that standards are prepared using
water of an appropriate salinity. Samples taken at the mouth of an estuary require
standards prepared in synthetic seawater while samples taken at the tidal limit of the
estuary require standards prepared using deionised water. At sampling points between
these two extremes there will be an error of up to 10% unless the salinity of the standards
is adjusted accordingly. In a modification of the method nitrate is reduced to nitrite in a
micro cadmium/copper reduction column and total nitrite estimated. The nitrate content is
then obtained by difference.
Matsunanga et al. [138] have also described a similar procedure for the determination
of nitrite in seawater. To 500ml of sample is added 10ml 1% solution of sulphanilamide
in 2M hydrochloric acid and 5ml 0.1% aqueous N-1-naphthylethylene-diamine
hydrochloride. After 10min 5ml 0.5% aq. dodecylbenzenesulphonate is added and the
mixture extracted for 2min with 50ml carbon tetrachloride; 75ml acetone and 10ml 0.1M
hydrochloric acid are added to the separated organic layer and the azo-dye back-extracted
into the acid. The extinction of the acid layer is measured at 543nm in a 2cm cell. The
sensitivity for 0.001 extinction in 1cm cells is 0.7ng atom nitrite-N per litre.
Gianguzza and Orecchio [139] have carried out comparative trials of various methods
for estimating nitrites in seawaters. These workers compared a method using sulphanilic
acid/alpha-naphthylamine complexes, with a method using sulphanilamide/N(1-naphthyl)
ethylenediamine complexes for the determination of nitrites in saline waters. The second
method has the greater sensitivity and lower detection limits. The former method is
subject to interference from chlorides and this interference can be completely eliminated
by the coupling diazotisation procedure of the latter method.
Anion exchange resins have been used to determine extremely low concentrations of
nitrite down to nanomoles in seawater. Wada and Hattori [140] formed an azo dye from
the nitrite using N-1 naphthylethylene diamine dihydrochloride and then adsorbed the
dye in an anion exchange resin. The dye is then eluted from the column with 60% acetic
acid and evaluated spectrophotometrically at 550nm.
3.21.2 Flow injection analysis
Fogg et al. [141] used flow injection voltammetry to reduce nitrite at a glassy carbon
electrode in acidic bromide or chloride medium to determine nitrite in sea water.
3.22 Nitrate and nitrite
3.22.1 Spectrophotometric methods
Bajic and Jaselskis [142] described a spectrophotometric method for the determination of
nitrate and nitrite in seawater. It included the reduction of nitrate and nitrite to
hydroxylamine by the zinc amalgam redactor (Jones reductor) at pH 3.4 and reoxidation
of the product with iron(III) in the presence of Ferrozine. Interference by high levels of