Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 193

Fig. 2.24 Dependence on reduction time: (a) of absorbance of standards; (b) of
absorbance of sample; (c) of recovery of spiked sample
Source: Reproduced with permission from the Royal Society of
Chemistry [391]

Fig. 2.24a shows a graph of absorbance vs time for the standards of 0.5mg L −1 or nitrate-
nitrogen that were measured, at different reduction times, in the same batches as some
lake water samples. It was possible to calculate the confidence interval of each point
because the measurements were performed in triplicate. However, owing to the
confidence interval being fairly constant, and for purposes of clarity, only average values
are shown in the figure. The cadmium filings showed a consistent reduction efficiency
after 10min for both masses of cadmium used. The spongy cadmium, however, attained a
maximum reduction efficiency between 10 and 30min which then declined. The same
basic shape for the spongy cadmium results is shown in Fig. 2.24b, the graph of
absorbance vs time for a lake water sample, and reference to Fig. 2.24c shows that the
recoveries for spongy cadmium are near theoretical for reduction times in excess of
20min. Fig. 2.24b also shows that the reduction efficiency attained by cadmium filings in
a lake water sample is greatly reduced for short reduction times, although this effect is
much less pronounced when a large amount of cadmium is used for each reduction.
Because of the consistent efficiencies that were obtained for the standards (Fig. 2.24a),
the recoveries (Fig. 2.24c) have the same shape as the sample absorbances (Fig. 2.24b).
These results suggest the following. First, results obtained when using spongy
cadmium are probably less prone to interference although they are less precise. Second,
interferences in lake water slow down the reduction by cadmium filings. Third, the
interferences can be reduced by either increasing the time of reduction or increasing the
amount of cadmium filings used. An increase in the reduction time or in the amount

Table 2.20 Effect of other substances on the determination of nitrate-nitrogen.

Error b in determination of nitrate-

Other substances Concn of other Added as