Anions in seawater - Determination of Anions in Natural and Treated Waters - page 391

level. Within-batch standard deviations for nitrite range from 0.056µg L−1at the 3.5µg
L − 1nitrite level to 0.042µg L−1 at the 70µg L −1 nitrite level.
Reduction of nitrate to nitrite is achieved by the use of a mixing coil containing
copperised cadmium wire; spectrophotometric evaluation of the produced nitrite is
achieved by the sulphanilamide N-1 naphthylethylene diamine hydrochloride system.
3.22.4 Flow injection analysis
Flow injection analysis has been adapted to automatic air-segmented continuous flow
systems, eg the Technicon AutoAnalyzer system. Several reducing agents such as zinc
[146], cadmium [147], amalgamated [148, 149] or copperised cadmium [150,151] have
been investigated. Reductor columns are, however, difficult to operate in an air-
segmented stream. This problem can be avoided by using the continuous flow injection
technique developed by Ruzicka and Hansen [132,156].
The flow injection technique is based on three main principles: sample injection,
reproducible timing and controlled dispersion [132]. The dispersion can be described as
limited, medium or large; in a colorimetric system based on a reaction between the
sample and a suitable reagent, a medium dispersion is preferred. Thus in the flow
injection determination of nitrate, the reductor column should not excessively increase
the dispersion. In a copperised cadmium reductor, more than 90% of the total nitrate is
reduced within 1-2s with minimum risk of further reduction of nitrite [157].
Consequently, the reductor can be made very small which results in a minimal increase of
dispersion.
In this development of a flow injection method for the determination of nitrate and
nitrite, Anderson [130] chose the Shinn [158] method of reduction of nitrate to nitrite
because of its high sensitivity and relative freedom from interferences. Anderson [130]
used the flow injection principle in the photometric determination of nitrite and nitrate
with
Fig. 3.11 Flow diagram for the colorimetric determination of nitrite and nitrate.