Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 515

7.25 Sulphate
7.25.1 Nephelometric method
Regnet and Quentin [78] describe a nephelometric method for determining up to 50mg
L −1 sulphate in potable waters in which a suspension of barium sulphate is produced by
addition of a specially prepared barium chloride reagent, and the turbidity measured at
490nm after a reaction time of 45min. The calibration curve can be considered as made
up of two linear portions of different slope, which intersect at about 30mg L −1 sulphate
ion. The procedures for calibration and determination, taking about 2h, are outlined.
Other ionic constituents do not interfere at concentrations normally encountered,
although nitrate can cause discrepancies if present in excess of 100mg L −1 .
7.25.2 Turbidimetric method
Van Staden [79] reported the application of an active pre-valve carbon filter, for the
turbidimetric determination of sulphate in potable waters. The filter is incorporated into
an automatic flow injection system, between the sampler and the sampling valve system,
to remove suspended and coloured materials which interfere with the spectrophotometric
determination of turbidity. The method has been applied to the analysis of sulphate at
concentrations up to 200mg L −1 at a sampling rate of up to 60 samples per h, with a
coefficient of variation less than 1%. The accuracy is similar to that of a standard
automated segmented method and standard flow injection methods.
Krug [80] and Van Staden et al. [79,81] have also described a turbidimetric method in
which the sample is injected into an inert carrier stream which is mixed with barium
chloride solution to form a barium sulphate suspension. Intermittent pumping of a
disodium ethylenediamine tetraacetate wash solution is used to inhibit a precipitation of
barium sulphate in the lines of the system.
7.25.3 Continuous flow analysis
Archer [82] has described a method in which sulphate is precipitated as 2-
aminoperimidine sulphate under acidic conditions, the excess of 2-aminoperimidine is
diazotized and coupled with ethanolamine to give a red coloured solution the intensity of
which is inversely proportional to the sulphate concentration. Down to 2mg L −1 sulphate
can be determined by this method with a standard deviation of 0.5mg L −1 at the 32mg
L −1 sulphate level and 3.4mg L −1 at the 96mg L −1 sulphate level. The layout of the
apparatus required to perform this analysis is illustrated in Fig. 7.13.
7.25.4 Flow injection analysis
Koch [83] modified the standard ASTN 15/84 (Tecator) flow injection method for the
determination of sulphate in potable water by adding methyl thymol blue to barium ions
in an alkaline alcoholic solution to