Anions in aqueous precipitation - Determination of Anions in Natural and Treated Waters - page 442

Table 5.7 Determination of sulphate in rainwater samples

mg L −1 sulphate

Sample no Flow injection

analysis

s %s Ion chromatography ∆ %∆

1 0.78 0.08 10.3 0.91 −0.13 −14.3

2 3.75 0.08 2.1 3.90 −0.15 −3.8

3 3.50 0.02 0.6 3.52 −0.02 −0.6

4 2.60 0.04 1.5 2.51 +0.09 +3.6

5 1.75 0.05 2.9 1.67 +0.08 +4.8

6 0.82 0.01 1.2 0.83 −0.01 −1.2

6A a

0.80 0.03 3.8

7 1.23 0.14 11.4 1.31 −0.08 −6.1

7A a

1.24 0.04 3.2

8 0.67 0.03 4.5 0.69 −0.02 −2.9

9 2.23 0.07 3.1 2.37 −0.15 −6.3

a Samples 6A and 7A were spiked with an additional 1.0mg L −1 calcium. Standard deviations are
based on N=3 except for sample 7A where N=6
Source: Reproduced with permission from the American Chemical Society [39]

The composition of reagent solution used will influence the applicability of the method. It
was observed that the absorbance decrease at 608µm due to methylthymol blue barium
complex was approximately three times greater than the corresponding absorbance
increase at 460nm due to uncomplexed methylthymol blue. Absorbance base line stability
measured in the absence of sulphate was essentially the same at both wavelengths.
Measurement of the absorbance decrease (∆A) at 608nm was therefore adopted in the
method.
In Table 5.7 are some sulphate determinations obtained by Madsen and Murphy [39]
on rainwater samples using their flow injection analysis system and by ion
chromatography are compared. Differences in results by the two procedures are
insignificant at the 95% confidence level.
Many cations interfere with the determination of sulphates using methylthymol blue.
Sample pretreatment with a cation exchange resin inthe H + form will eliminate this
interference. Rainwater samples typically contain 0.2mg L −1 calcium and 0.2mg L −1
magnesium. The addition of 1.0mg L −1 calcium to two different rainwater samples did
not alter results for sulphate by the flow injection method as can be seen from results
presented in Table 5.7.