Anions in seawater - Determination of Anions in Natural and Treated Waters - page 390

nitrite could be eliminated with azide treatment. Levels of nitrate of 0.1mg L−1 could be
detected with a precision of 3% in the presence of large amounts of nitrite and chloride.
Nagashima et al. [143] used second derivative spectrometry to determine nitrate and
nitrite in seawater.
Samples of artificial seawater, pure water and chlorinated water were pumped through
a column containing cadmium/copper powder which converted nitrate to nitrite. The
produced and existing nitrite were reduced to nitrogen monoxide and the mixture fed
through a gas-liquid separator. Evolved nitrogen monoxide was purged by nitrogen into a
heated optical cell where the second derivative absorbance was recorded at 214nm.
Various experimental conditions were investigated. A reducing agent of 0.13M sodium
iodide/13M phosphoric acid, a mixing coil 5m long operated at 50°C and a separator
50cm long with outer tube heated to 90°C were selected. Nitrite was determined directly
and nitrate indirectly by measuring differences in nitrite with and without the converter.
The Department of the Environment UK [144] has described a number of alternative
methods for the determination of total oxidised nitrogen (nitrate plus nitrite) in aqueous
solution, while specific methods for nitrate and nitrite are also included. Among the
methods for total oxidised nitrogen, one is based on the use of Devarda's alloy for
reduction of nitrate to ammonia, and another uses copperised cadmium wire for reducing
nitrate to nitrite, which is determined spectrophotometrically. Nitrate may also be
determined spectrophotometrically after complex formation with sulphosalicylic acid, or
following reduction to ammonia, the ammonia is eliminated by distillation and
determined titrimetrically. Other methods include direct nitrate determination by
ultraviolet spectrophotometry, measurements being made at 210nm, and the use of a
nitrate-selective electrode. Details of the scope, limits of detection and preferred
applications of the methods are given in each case.
Various other workers have discussed spectrophotometric methods for the
determination of nitrate and nitrite [145-152].
3.22.2 Ultraviolet spectroscopy
Suzuki et al. [153] determined total nitrogen in seawater by oxidising the sample with
potassium persulphate, boiling with sulphuric acid and potassium permanganate at pH 1.3
to remove bromide and bromate, removal of the excess potassium permanganate and
manganese dioxide with sodium thiosulphate, and measuring the absorbance of the
treated solution at 220nm.
Nakamura and Namiki [154] have determined total nitrogen in seawater by ultraviolet
spectrophotometry and correcting for background absorption due to bromide ion.
3.22.3 Continuous flow analysis
Workers at the Department of the Environment UK [155] have described continuous flow
methods for the determination of total oxidised nitrogen and nitrite in seawater. Limits of
detection are 1.3µg L−1 (total oxidised nitrogen) and 0.26µg L −1 (nitrite). Within-batch
standard deviations for total oxidised nitrogen range from 0.28µg L−1 to 17.5µg L−1at
the total oxidised nitrogen level to 0.96µg L−1 at the 560µg L−1total oxidised nitrogen