Anions in seawater - Determination of Anions in Natural and Treated Waters - page 412

methyl mercapton, carbon disulphide, dimethyl sulphide and dimethyldisulphide) in
water by gas chromatography using flame detection. Detection limits ranged from 0.2ng
L−1 (ass) for carbon disulphide to 0.6ng L−1 (ass) for methyl mercaptan. Hydrogen
sulphide was determined at the 1ng L −1 (ass) level.

3.31 Valerate

3.31.1 Ion chromatography
The application of this technique is discussed under multianion analysis in section 12.3.1.

3.32 Multianion analysis

3.32.1 Spectrophotometric methods, phosphate, arsenate, arsenite and
sulphide
Nasu and Kan [224] determined phosphate, arsenate and arsenite in non saline water by
flotation spectrophotometry and extraction indirect atomic absorption spectrometry using
malachite green as an ion-pair reagent. A floated (between aqueous and organic phase)
ion-pair of malachite green with molybdophosphate was dissolved by the addition of
methanol to the organic layer. Phosphate and arsenate were determined by measuring
absorbance of the organic phase. An oxidative (potassium dichromate) or a reductive
(sodium thiosulphate) reaction of arsenic was used for the determination of phosphate,
arsenate and arsenite. A positive interference effect was observed in the presence of large
amounts of silicon. This was overcome by acidification with concentrated hydrochloric
acid. The method was applied to samples of hot spring water, sea water and ground water
with almost complete recovery of added amounts (0.2-1.0µg per 5ml).
Johnson and Pilson [225] have described a spectrophotometric molybdenum blue
method for the determination of phosphate and arsenate and arsenite in estuary water and
sea water. A reducing reagent is used to lower the oxidation state of any arsenic present
to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will
not form the molybdenum complex. This results in an absorbance value for phosphate
only.

Table 3.17 Dependence of chloride content of wash liquid on chloride

Content of sample
Chloride content of wash mg L −1

Chloride content of sample mg L −1

0 0-1800

4850 1800-7200

10000 7200-12600