The determination of nitrate is essentially similar to that of nitrite except for incorporating a
pre-reduction step. Morris and Riley [21] reported
that cadmium filings are the best for quantitative
reduction. We used a column of 1.5 cm diameter,
however, the effects of the height of cadmium filings
packed into it, the pH of the sample solution, and
the flow rate affecting the reduction process have
been re-investigated. The absorbance of the azo dye
sorbed into the foam phase increased on increasing
the height of the cadmium layer from 5 cm up to
10 cm at which a maximum value is reached. Any
further increase in the height of the cadmium filings
was accompanied by a sharp decrease in absorbance,
probably attributable to a further reduction of nitrite.
Optimum reduction was attainable on adjusting the
pH of the sample solution to 10. At these optimum
values for column length and pH, flow rates as fast
as 4 ml min −1 proved adequate for quantitative reduction which is a considerable improvement when
compared with the procedure of Morris and Riley
[21]