Precipitation
A number of parameters have been studied with
a view to provide quantitative precipitation,
namely, the order of addition of reagents, the
effect of the amount of Na-DTC, the optimum
pH, etc.
It was established that the reagents should be
added in the following order: nitric acid to adjust
the required pH, Na-DTC, and H2O2.
The effect of the amount of Na-DTC on the
precipitation and subsequent ICP-AES analysis
was studied according to the procedure described
in Section 2.2.1.1. The concentration of all the
elements studied was kept constant at 2 mg l1.
The concentration of Na-DTC was varied in the
range 0.009–21.6 g l1. The results obtained are
shown in Fig. 1a and b. The retention versus the
Na-DTC concentration shows that the quantitative
sorption of all the elements studied was
achieved at 5.4 mg ml1 Na-DTC. Further increasing
the Na-DTC concentration did not significantly
affect the retention—R exceeded 95%.
The effect of pH is demonstrated in Fig. 2a and
b. No co-precipitation could be achieved at a pH
below 1.7 since Na-DTC was decomposed in the
acid medium. The best retention with good reproducibility
of the results was observed at pH 5–7
for all the elements studied. Apart from the title
elements the retention of As and Mo has been
investigated as well. Experiments revealed that
Mo(VI) was quantitatively sorbed at pH 1.7 (90%
retention). Our results are in good agreement with
Ref. [12], where R94% was achieved for Mo at
pH1.5. The retention of As(III) did not exceed
2.4% over the whole pH interval studied.
The effect of time on co-precipitation was studied.
The precipitate was allowed to stand from 1
up to 48 h. It was established that 5 h was
sufficient to produce results of good precision.
The effect of the sample volume has been studied
as well. The results obtained are listed in
Table 2. It is seen that the elements studied can be
successfully concentrated by a factor of 8–80
depending on the sample volume. This concentration
factor is sufficient to ensure the analysis of
the elements in real water samples.