Chemistry Reference
In-Depth Information
8.6
Chromate (dichromate)
8.6.1
Spectrophotometric methods
Wei
et al.
[17] described a Spectrophotometric method for the determination of
chromium(VI) in electroplating waste waters and total chromium(III) and (VI) in
industrial waste waters using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol. This was
found to be the most sensitive reagent for chromium. The complex formed was very
stable.
In this method a portion of sample containing not more than 15µg of chromium is
transferred into a 100ml flask. 2ml of acetate buffer (pH 4.7), 1ml of 10% w/v
hydroxylamine hydrochloride solution, and 2ml of 2(5-bromo-2-pyridylazo)-5-
diethylaminophenol solution are added and 5ml water added. The flask is fitted with a
stopper which carries a funnel. The flask is placed on an electric plate having a surface
temperature of 100-105°C. The contents of the flask are heated until the volume of the
solution is reduced to 3-5ml. The walls of the flask are rinsed with 5ml of ethanol and
reflux continued for 40min. The flask is removed from the hot plate, and after cooling to
room temperature 5ml of ethanol is added to dissolve the reagent and reaction product.
The solution is transferred to a 25ml volumetric flask and made up to the mark with
distilled water. The absorbance of the coloured solution is measured at 600nm, using a
reagent blank as reference.
The effect of foreign ions on the determination of 10µg of chromium(III) was
examined. The following ions (when present in the amounts in mg shown in brackets) do
not interfere:
Equal amounts of Fe
3+
, Cu
2+
, Co
2+
, Ni
2+
, Ga
3+
and Tl
3+
produce positive errors.
Table 8.8 shows the results of determination of chromium by the above method in
comparison with the diphenylcarbazide method, o-toluidine [18],
o
-nitrophenylfluorone
acid [19], and 2(5-bromo-2-pyridylazo)-5-diethylaminophenol have been used as
chromogenic reagents in the determination of hexavalent chromium (chromate) in waste
waters. Qi and Zhu [19] have reacted chromium(VI) with o-nitrophenylfluorene in the
presence of acetyltrimethyl ammonium bromide to form a purple red complex at pH 4.7-
6.6 by heating at 50°C for 10min. The wavelength of maximal absorbance was 582nm.
Beer's law is obeyed up to 0.2µg L
−1
chromium(VI). Interference due to copper(II), iron
(III) and aluminium(III) was eliminated by the addition of a masking agent containing
potassium fluoride, trans-1, 2-diaminocyclohexane tetraacetic acid and potassium sodium
tartrate.
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