Anions in estuary and coastal waters - Determination of Anions in Natural and Treated Waters - page 425

(b) 0.63ng Se(IV) and 63ng Pb: (c) 0.63ng Se(IV), 63ng Pb and 63ng Cu; (d)
0.63ng Se(IV) and 63ng As; (3) 0.63ng Se(IV), 63ng As and 63ng
Cu: (f) 0.63ng Se(IV) and 63ngTe; (g) 0.63ng Se(IV), 63ng Te and
63ng Cu; (h) temperature increase. Gas flow rate 500ml Ar min−1
Source: Reproduced with permission from Springer Verlag [4]
the analysis of estuarine waters permitted the separate determination of both selenium
(IV) and selenium(VI) since the tellurium coprecipitation method did not differentiate
between the two species.
Most cations and anions have interferences on atomisation of selenium. Fig. 4.1 shows
the effect of sodium chloride on selenium. The effects of 100-fold amounts of lead,
arsenic and tellurium on atomisation of selenium are shown in Fig. 4.2. Samples were
checked for background effects and reagent blank. Bismuth and antimony also depressed
the selenium absorption.
Arsenic lowered somewhat the peak temperature in atomisation profile for selenium.
Copper tends to suppress the interferences of diverse elements on atomisation of
selenium. However, the interferences from large concentrations of diverse elements and
matrices were not improved even in the presence of copper at the atomisation step.
Therefore, the separation of selenium from matrices was recommended.
Selenium is extracted as diethyldithiocarhamate complex from the solution containing
citrate and EDTA [5]. Ohta and Suzuki [6] found that only a few elements, such as
copper, bismuth, arsenic, antimony and tellurium, are also extracted together with
selenium. They examined this for effects of 100-fold amounts of elements co-extracted
with the selenium diethyldithiocarbamate complex. An appreciable improvement of
interferences from diverse elements was observed in the presence of copper. Silver
depressed the selenium absorption in the case of atomisation of diethyldithiocarbamate
complex, but the interference of silver was suppressed in the presence of copper. The
atomisation profile from diethyldithiocarbamate complex was identical with that from
selenide.
No selenium(VI) is extractable from citrate buffered solution containing EDTA and
copper. Therefore, tetra- and hexavalent selenium could be determined separately.
The recovery of selenium was satisfactory. The forms of selenium in waters are known
to be selenite and selenate [7]. Selenium occurs in non saline water at concentrations
ranging from less than 0.0002µg L−1 t o greater than 50µg L−1 . Therefore, a large sample
size is necessary for analysis at lower concentration level.
4.5 Sulphate
4.5.1 Spectrophotometric method
The determination of sulphate is discussed under multianion analysis in section 4.6.1.