Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 756

with an end-window Geiger-Muller tube. The irradiated standards are dissolved in water
together with potassium perrhenate. At a level of 0.057µg L −1 rhenium the coefficient of
variation was ±7%.
15.3.3 Phosphate
Mullins [98] has described a procedure for determining the concentrations of dissolved
chromium species in sea water. Chromium (III) and chromium(VI) separated by
coprecipitation with hydrated iron(III) oxide and total dissolved chromium are
determined separately by conversion to chromium(VI), extraction with ammonium
pyrrolidinediethyl-dithiocarbamate into a methyl isobutyl ketone and determination by
atomic absorption spectroscopy The detection limit is 40ng L −1 Cr. The dissolved
chromium not amenable to separation and direct extraction is calculated by difference. In
the waters investigated, total concentrations were relatively high (1-5µg L −1 ) with
chromium(VI) the predominant species in all areas sampled with one exception, where
organically bound chromium was the major species. A standard contact time of 4h was
found to be necessary for the quantitative coprecipitation of chromium on ferric oxide.
The r.s.d values for the determination of chromium(III), chromium(VI) and total
dissolved chromium were generally 10.0, 5.0, and 5.0% respectively. From these results,
the r.s.d for the calculated concentration of the bound species was 20%.
The sorption of hexavalent chromium (chromate) onto hydrous ferric oxide can be
explained in terms of ligand exchange, hydroxyl ions being released from the surface of
the hydrated oxide particles. For magnetic particles, however, the sorption effect was
explained by the reduction of traces of hexavalent chromium at the magnetite/water
interface, as a result of which a very fine ferric hydroxide/chromium hydroxide coating
was produced which exhibited further reduction of hexavalent chromium.
Nakayama et al. [99] have described a method for the determination of chromium(III),
chromium (VI) and organically bound chromium in sea water. They found that sea water
in the sea of Japan contained about 9× 10 −9 mol L −1 dissolved chromium. This is shown
to be divided as about 15% inorganic Cr(III), about 25% inorganic Cr(IV), and about
60% organically bound chromium. These workers studied the coprecipitation behaviours
of chromium species with hydrated iron(III) and bismuth oxides.
The collection behaviour of chromium species was examined as follows. Sea water
(400ml), spiked with 10 −8 mol L −1 Cr(III), Cr(VI) and Cr(III) organic complexes labelled
with 51 Cr, was adjusted to the desired pH with hydrochloric acid or sodium hydroxide.
An appropriate amount of hydrated iron(III) or bismuth oxide was added; the oxide
precipitates were prepared separately and washed thoroughly with distilled water before
use [100]. After about 24h, the samples were filtered on 0.4µm Nuclepore filters. The
separated precipitates were dissolved with hydrochloric acid and the solutions thus
obtained were used for γ-activity measurements. In the examination of solvent extraction,
chromium was measured by using 51 Cr, while iron and bismuth were measured by
electrothermal atomic absorption spectrometry The decomposition of organic complexes
and other procedures were also examined by electrothermal atomic absorption
spectrometry.
The percentage collection of Cr(III) with hydrated iron(III) oxide may decrease