Anions in aqueous precipitation - Determination of Anions in Natural and Treated Waters - page 439

Levels of this ion were 2-16 times greater in eastern North American precipitation than
in that from remote areas.

5.1.11.1 Spectrophotometric methods

Burns et al. [36] determined down to 2mg L −1 sulphate indirectly in rainwater by a
Spectrophotometric method using an excess of 2- aminopyridine hydrochloride which
forms an insoluble complex with sulphate. After titration the excess reagent is determined
spectrophotometrically at

Table 5.5 Comparison of the results of the examined methods in rainwater samples

Found µmol L −1 sulphate

Cont flow
photometry;
thorin
sd

Cont flow
photometry;
DMSA(III)
sd

Cont flow
photometry;
DMSA(III)
sd

Cont flow
titrimetry;
DMSA(III)
sd

63 2 65.0 0.6 61.9 0.7 64.0 0.5

85 3 81.0 1.0 79.2 0.8 83.2 0.7

60 2 61.0 0.7 61.5 0.9 61.2 0.6

DMSA III—Dimethylsulphonazo(III)
Source: Reproduced with permission from Springer Verlag [37]

305.5nm. Unfortunately, only up to 10mg L −1 iron can be tolerated by this method,
otherwise interferences are negligible.

5.1.11.2 Continuous flow analysis

Reijnders et al. [37] compared results obtained by the continuous flow method with those
obtained using a continuous flow apparatus and a flow through titrimeter. In each case
dimethyl-sulphonazo(III) was used as a chromogenic reagent. Results obtained by these
three procedures were compared with those based on the thorin-barium reaction in a
continuous flow analyser. The chemical system employed in the first three methods
mentioned above basically involves a complex of barium ions with the indicator
dimethylsulphonazo(III). Sulphate ions from the sample react with barium and set free
the uncomplexed dimethylsulphonazo(III). The absorption spectra of complexed and free
dimethylsulphonazo(III) are different.
Results obtained by the four methods are summarised in Table 5.5, which shows that
there is no significant differences between the results from the various methods.
Significantly higher sulphate values were returned by the thorin based
spectrophotometric method than by the three methods involving dimethylsulphonazo(III)
when the samples contained water matrix components (sodium chloride, potassium