Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 280

Procedure 2
Se(-II, 0, IV) determination is carried out as follows. To the pretreated sample solution
(500ml) 25ml of concentrated hydrochloric acid and 2ml of bromine solution (3%) is
added and the solution mixed for 5min at room temperature. By this treatment Se(-II, 0)
is oxidised completely to the quadrivalent state but Se(VI) is not reduced. 2ml of
hydroxylamine hydrochloride (1mol L −1 ) is added to the solution to reduce the excess
bromine to bromide. The solution is washed with 25ml of toluene. The solution is treated
with 4, 6-dibromopiazaselenol and the analysis continued as above in procedure 1.
Calculate selenium (-II, 0) from the difference between the peak heights of the
piazeselenol by the two procedures 1 and 2.
Procedure 3
Se(-II, O, IV, VI) determination is carried out as follows. To the pretreated sample water
(500ml) concentrated hydrochloric acid (50ml), 47% hydrobromic acid (25ml) and 3%
bromine solution (0.5ml) is added, and the solution boiled gently for 15min. By this
treatment Se(-II, O) is oxidised to Se(IV) and Se(VI) is reduced to Se(IV). Thus all
selenium is now in the quadrivalent state. After the solution is cooled to room
temperature, 2ml 1mol L −1 hydroxylamine hydrochloride is added to reduce the excess
bromine. The solution is washed with 25ml of toluene and total selenium determined by
the same method as procedure 1. Calculate selenium(VI) by deducting Se(-II, O, IV)
from Se(-II, O, IV, VI).
Distilled water containing 4.6ng of Se 0, 4.8ng of Se(IV) and 5.9ng of Se(VI) were
analysed by procedures 1, 2 and 3 and good recoveries were found (Table 2.45).
As an example, the gas chromatograms of selenium in river water by procedures 2 and
3 are shown in Fig. 2.47(a) (b) respectively. Selenium(VI)