Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 757

considerably in the neutral pH range when organic materials capable of combining with
Cr(III), such as citric acid and certain amino acids, are added to the sea water [101].
Moreover, synthesized organic Cr(III) complexes are scarcely collected with hydrated
iron(III) oxide over a wide pH range [101]. As it was not known what kind of organic
matter acts as the major ligand for chromium in sea water Nakayama et al. [99] used
EDTA and 8-quinolinol-5-sulphonic to examine the collection chromium species,
because these ligands form quite stable water-soluble complexes with Cr(III) although
they are not actually present in sea water. Both these Cr(III) chelates are stable in sea
water at pH 8.1 and are hardly collected with either of the hydrated oxides. The organic
chromium species were then decomposed to inorganic chromium(III) and chromium(VI)
species by boiling with 1g ammonium persulphate per 400ml sea water and acidified to
0.1mol L −1 of acid with hydrochloric acid. Iron and bismuth, which would interfere in
atomic absorption spectrometry, were 99.9% removed by extraction from 2mol L −1
hydrochloric acid solution with a p -xylene solution of 5% tri-iso-octylamine. Cr(III)
remained almost almost quantitatively in the aqueous phase in the concentration range
10 −9 -10 −6 mol L −1 whether or not iron or bismuth was present. However, as about 95% of
Cr(VI) was extracted by the same method, samples which may contain Cr(VI) should be
treated with ascorbic acid before extraction so as to reduce Cr(VI) to Cr(III).
When the residue obtained by the evaporation of the aqueous phase after the extraction
was dissolved in 0.1mol L −1 nitric acid and the resulting solution was used for
electrothermal atomic absorption spectrometry, a negative interference, which was
seemingly due to residual organic matter, was observed. This interference was
successfully removed by digesting the residue on a hot plate with 1ml of concentrated
hydrochloric acid and 3ml of concentrated nitric acid. This process had the advantage that
the interference of chloride in the atomic absorption spectroscopy was eliminated during
the heating with nitric acid.
Boniforti et al. [102] compared several preconcentration methods in the determination
of metals in sea water. A comparison was made of ammonium
pyrrolidinedithiocarbamate-8-quinolinol complexation followed by extraction with
methyl isobutyl ketone or Freon-113, coprecipitation with magnesium hydroxide or iron
(II) hydroxide or chelating by batch treatment with Chelex-100 for the determination of
chromium. Atomic absorption spectrometry and inductively coupled plasma atomic
emission spectrometry were used for analysis. Interferences, recovery, precision,
accuracy, and detection limits were compared. The Chelex-100 resin method was most
suitable for the preconcentration of all determinands except chromium, whereas
preconcentration of Cr(III) and Cr(V) was achieved only by coprecipitation with iron(II)
hydroxide.
The preconcentration of phosphates in seawater is also discussed in section 15.1.8.
References
1 Howard, A.G., Volkan, M. and Ataman, Y. Analyst (London) , 112 , 159 (1987).
2 Fung, Y.S. and Dao, K.I. Analytica Chimica Acta , 309 , 173 (1995).
3 Yoshimura, K., Kariya, R. and Torntani, T. Analytica Chimica Acta , 109 , 115 (1979).
4 Jun, Z., Oshima, M. and Motomizu, S. Analyst (London) , 113 , 1631 (1988).