Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 755

octadecylamine for tap water samples, hexa-decyltrimethylammonium bromide for non
saline waters, and both hexadecyltrimethylammonium bromide and octadecylamine for
sea water samples. Differential pulse polarography using the catalytic wave caused by
molybdenum(VI) in nitrate medium was applied to the preconcentrate. Good
reproducibility was obtained with mean values of 0.7µg L −1 and 5.7µg L −1 for
molybdenum(VI) in tap water and sea water, respectively.
15.3 Seawater
15.3.1 Molybdate
Shriadah et al. [43] determined molybdenum(VI) in seawater spectro-photometrically
after enrichment as the Tiron complex on a thin layer of anion exchange resin. There
were no interferences from trace elements or major constituents of sea water except for
chromium and vanadium. These were reduced by the addition of ascorbic acid. The
concentration of dissolved molybdenum (VI) determined in Japanese sea water was
11.5µg L −1 with a relative standard deviation of 1.1%.
Kuroda et al. [96] preconcentrated trace amounts of molybdenum from acidified sea
water on a strongly basic anion exchange resin (Bio-Rad Ag 1-X8 in the chloride form)
by treating the water with sodium azide. Molybdenum(VI) complexes with azide were
stripped from the resin by elution with ammonium chloride/ammonium hydroxide
solution (2mol L −1 /mol L −1 ). Relative standard deviations of better than 8% at levels of
10µg L −1 were attained for sea water using graphite furnace atomic absorption
spectrometry.
15.3.2 Perrhenate
Matthews and Riley [97] have described the following procedure for determining down
to 0.06µg L −1 perrhenate in sea water. From 6 to 8µg L −1 rhenium, was found in Atlantic
sea water. The rhenium in a 15L sample of sea water, acidified with hydrochloric acid, is
concentrated by adsorption on a column of De-Acidite FF anion-exchange resin (Cl −
form), followed by elution with 4mol L −1 nitric acid and evaporation of the eluate. The
residue (0.2ml), together with standard and blanks, is irradiated in a thermal neutron flux
of at least 3×10 12 neutrons cm −2 s −1 for at least 50h. After a decay period of 2 days, the
sample solution and blank are treated with potassium perrhenate as carrier and evaporated
to dryness with a slight excess of sodium hydroxide. Each residue is dissolved in 5mol
L −1 sodium hydroxide. Hydroxylammonium chloride is added (to reduce Tc(VIII)) which
arises as 99m Tc from activation of molybdenum present in the samples, and the Re(VII) is
extracted selectively with ethyl methyl ketone. The extracts are evaporated, the residue is
dissolved in formic acid:hydrochloric acid (19:1), the rhenium is adsorbed on a column of
Dowex 1 and the column is washed with the same acid mixture followed by water and
0.5mol L −1 hydrochloric acid; the rhenium is eluted at 0°C with acetone:hydrochloric
acid (19:1) and is finally isolated by precipitation as tetraphenylarsonium perrhenate. The
precipitate is weighed to determine the chemical yield and the 186 Re activity is counted