Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 295

2.93.5 Nephelometry
2-aminoperimidine hydrochloride yields a microcrystalline precipitate of the
corresponding sulphate with solutions containing down to 0.05mg L −1 sulphate [752]. For
the determination of sulphate, 0.5% aqueous reagent is added to the test solution, the
mixture is diluted and a nephelometer reading is made after 5 to 10min and compared
with the readings obtained with use of standard potassium sulphate solution. Many other
anions also yield precipitates but, with the exception of iodide, fluoride, silicofluoride
and phosphate, at least 10-fold amounts of most common anions are tolerated.
2.93.6 Flow injection analysis
Sulphate determination is one of the routine analyses encountered frequently in analytical
laboratories. Besides the technique of ion chromatography [753,754] methods applicable
to the analysis of sulphate are indirect. The analytical procedures usually include the
precipitation of sulphate ions with barium or lead ions, followed by the turbidimetric,
spectrophotometric or potentiometric determination of the precipitates formed or the
remaining barium or lead ions in solution [755-768]. Among these methods, the
potentiometric method has attracted continuous attention due to its convenience and ease
of operation. Recently, the flow injection technique has been coupled with the lead ion
selective electrode and used for the determination of sulphate (see section 2.93.6).
Kondo et al. [769] determined sulphate ion in river water by flow injection analysis at
a rate of 30 samples per h. The method was applied to samples whose sulphate contents
were typically less than 30mg L −1 . The reagent solution contains dimethylsulphonazo(II),
barium chloride, potassium nitrate and chloroacetate buffer in 70% (v/v) ethanol, and is
saturated with barium sulphate. The aqueous carrier stream is also saturated with barium
sulphate. The sample is filtered and treated with Amberlite IR120 B cation exchanger
before injection into the carrier stream, and the decoloration of the barium-
dimethylsulphonazo(III) complex by sulphate is measured at 662nm. The calibration
graph is linear over the range 0-30mg L −1 for sulphate in water.
A diagram of the flow system is shown in Fig. 2.49.
The absorption spectra of DMSA-III and its barium chelate are shown in Fig. 2.50. The
absorption maxima are 586nm and 660nm for DMSA-III and its chelate, respectively.
Measurement at 662nm provides the largest difference for the sulphate determination.
Maximum constant absorbance is achieved between pH 3.5 and 5.5, so the
monochloroacetate buffer (pH 4.2) is again appropriate.
A calibration run obtained with 0mg L −1 sulphate standards had good reproducibility.
Linear calibration graphs can be obtained up to 30mg L −1 sulphate, with a correlation
coefficient of 0.999.
The effects of coexisting ions are listed in Table 2.51. Calcium ion in river wate r
interferes with the determination. To prevent this, samples are