Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 748

Zn Sea

water

Anion
exchange

1 M thiocyanate
0.1 M HCl

2mol L −1
HNO 3

- AAS [31]

Zn Sea

water

Polyacryl
amidoxime
resin

- 1:1

HCl:H 2 O

- AAS [32]

Various
radio
nucleides

Non
saline

Single
bead
anion
exchange

- - - Point source mass

spectrometry

[61]

22
elements

Non
saline

Anion
exchange

- - - NAA [62]

Source: Own files

(molybdophosphate) and its reduction product (so-called molybdenum blue) have been
used, as has the yellow vanadmolybdophosphoric acid. The protonated forms of these
species have also been extracted into organic solvents for spectrophotometry. It is well
known that molybdo-phosphate reacts with cationic dyes and organic bases to form ion
pairs.
Shida and Matsuo [66] reported a very sensitive spectrophotometric method based on
formation of the ion associate of molybdophosphate with methylene blue, flotation of this
ion-pair between the aqueous phase and cyclohexane-4-methylpentan-2-one and
displacement of methylene blue from the ion pair by tetradecyldimethylbenzylammonium
ion.
In an attempt to improve the extractability of ion pairs of molybdo-phosphate and thus
the sensitivity of the determination of phosphate, Motomizu et al. [67] examined several
cationic dyes and extracting solvents. They found that procedures based on ethyl violet
and a cyclo-hexane-4-methylpentan-2-one mixture enabled 10µg L −1 concentrations of

phosphate to be determined spectrophotometrically.
In the samples of river water and seawater, the phosphorus content is often in the µg
L −1 range. The procedure requires small volumes of sample (below 10ml) and very
simple vessels (25ml test tube) which are easily heated in order to hydrolyse any
condensed phosphate.
The absorption spectrum of the ion pair formed between molybdophosphate and ethyl
violet in the organic phase obtained by the procedure B is shown in Fig. 15.1. Maximum
absorbance occurs at 602nm where the absorbance of the reagent blank is about 0.1. The
calibration graphs obtained are linear in the range 0-0.6µg of phosphorus and the molar
absorptivity calculated from the slope of the curve was 2.7×10 5 L mol −1 cm −1 . Silicate,
vanadate and tungstate, which may react with molybdate to form heteropoly acids, do not
interfere with the determination of phosphorus even at the amounts of 5×10 −5 M, 10 −5 M
and 5×10 −6 M respectively. Arsenic(V) causes positive errors because it reacts with
molybdate to form the heteropoly acid, which is quantitatively extracted into the organic
phase. But in non saline waters such as river water and sea water, the arsenic content is
very much smaller than the phosphorus content. Tin(II) and (IV) ions at concentrations