Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 290

Reaction of sodium iodate in the percolate with sodium iodide in hydrochloric acid
medium leads to the production of an amount of iodine equivalent to the sulphate content
of the sample. Iodine is titrated with 0.05N sodium thiosulphate to the starch end-point.
In the determination of 250mg L −1 sulphate in non saline water by this method, 250mg
L −1 of sodium, potassium, calcium, chloride or nitrate did not interfere. Phosphate
(100mg L −1 ) or bicarbonate (200mg L −1 ) caused an error of +10%.
Some of the chromophoric reagents that have been used to estimate sulphate are
reviewed in Table 2.47.
Reijnders et al. [722] have described the following batchwise photometric method for
the determination of sulphate in water. In this method metal ions are removed from the
sample by ion exchange and the sulphate reacts with the barium complex of
dimethylsulphonazo(III) in an ethanolic medium. The optical absorbance of the complex
indicates the sulphate concentration.
The results of determinations of pure sulphate solutions, containing 1-60µmol L −1
sulphate are presented in Table 2.48. Table 2.49 shows the effect of different amounts of
interfering substances on the results of sulphate standards. The lower limit of
determination, defined as the concentration at which the coefficient of variance is 24%,
was 2.6µmol L −1

Table 2.49 Effect of different amounts of interfering substances on the results of sulphate
standards

Sulphate given
µmol L −1

Interference Sulphate found

µmol L −1

0 1 2.6

0 2 4

0 3 8

50 1 51

50 2 53

50 3 53

100 1 100

100 2 100

100 3 102

Key to interference:
1 No interferences
2 Sulphate and interference as specified in 3 in 1:1 dilution
3 Sulphate and 0.01mol L −1 NaCl, 0.00 15mol KCl, 0.01mol L −1 L −1 NaHCO 3 , 0.05mol L −1 I Fe
(NO 3 ) 3 , 0.01mol L −1 Ca(NO 3 ) 2