Sulphate

This method is used for the determination of sulfate in waters and wastewaters. The EPA range of this method is 3.0 to 300 mg/L. However, this method is also applicable to other ranges.

Summary of Method

In the pH range of 2.5 to 3.0, sulfate ions react with a barium-methylthymol blue complex (Ba-MTB) to form barium sulfate and free methylthylmol blue (MTB). The analytical stream is then made highly basic (pH 12.5-13.0). At this pH, the absorbance maximum for the Ba-MTB complex is 610 nm while that of free MTB is 460 nm. Given that the molar concentrations of barium (Ba) and the MTB are approximately equal, and that the maximum sulfate concentration to be measured does not exceed the concentration of the Ba-MTB complex, the sulfate concentration is directly proportional to the free MTB concentration measured at 460 nm as shown in the following equation.

BaMTB^-4 + SO₄^{-2 pH 2.5-3.0} > BaSO₄ ↓ + MTB^-6

The sulfate calibration curve is nonlinear. Colovos et al1 have attributed this to two factors: formation of a binuclear barium-MTB complex (Ba₂-MTB), and impurities in commercially available MTB dye.

Limitations

Refrigerate samples at 2-8°C. Analyze the samples as soon as possible after collection. The EPA allows a maximum holding time of 28 days.

Multivalent cations, such as calcium, magnesium and aluminum that would otherwise interfere, are removed with an ion exchange column. Samples with pH values less than 2 must be neutralized to prevent the elution of cations from the ion exchange resin. Turbid samples must be filtered or centrifuged prior to analysis.

Availability