SE of soils containing ≈1.6–13.3 mg SO4-S/L are suited to turbidimetric analysis associated with the quantitative precipitation of barium sulfate [BaSO4; ASTM-Method D 516-07 (2008)]. For extracts with higher concentrations of SO4-S, gravimetric, IC or ICPAES procedures are preferred. Although generally reliable within the specified concentration range, the method can be affected adversely by interferences. Examples of interferences [ASTM-Method D 516-07 (2008)] include:
Where one or more of these interferences are anticipated, the best option (if available) is to choose an alternative method, such as ICPAES or IC.
In this method, SO42– is precipitated as BaSO4, following the addition to SE of barium chloride (BaCl2) in the presence of HCl (Steinbergs 1955). The precipitation of BaSO4 is performed under precise conditions in order to produce suspended crystals of uniform size. The turbidity (or absorbance) of the BaSO4 suspension is then measured and the SO4-S concentration determined turbidimetrically by comparison with known standard solutions.
Conditioning Reagent
Dissolve 75 g sodium chloride (NaCl) in 300 mL deionised water, add 30 mL of 10 M HCl and 100 mL 95% ethanol (CH3CH2OH). Combine with 50 mL glycerol (CH2OH.CHOH.CH2OH) and mix well.
Potassium Sulfate Solution
Dissolve 0.544 g potassium sulfate (K2SO4) and dilute to 1 L with deionised water.
Barium Chloride ‘Seed’ Suspension
Place 23.0 g barium chloride dihydrate (BaCl2.2H2O) in a 150 mL Erlenmeyer flask. Add, in the precise order, 4.0 mL K2SO4 Solution and 46.0 mL of deionised water. Dissolve the BaCl2 crystals by heating and allow to cool. Transfer to a test tube and allow the mixture to stand for 30 min. Pipette (while keeping the end of the pipette about 1 cm from the bottom of the tube and away from the sides) ≈48 mL of the suspension back into the Erlenmeyer flask. Discard the residue in the tube. Next add another 4.0 mL K2SO4 solution, shake for 1 min and allow to stand for 45 min. Transfer to test tube and pipette off the bulk of this suspension in the manner described above. Add a further 4.0 mL of K2SO4 solution, shake for 1 min and allow to stand for 1 h. Pipette off as above and discard the residue in the tube. Dilute the final suspension to 50 mL in a volumetric flask and store away from direct sunlight.
Sulfate-S Primary Standard
1 L contains 300 mg of SO4-S.
Dissolve 1.6306 g potassium sulfate (K2SO4; previously dried at 105°C for 4 h) in deionised water and make to 1.0 L in a volumetric flask. Store in borosilicate glass, preferably in the dark.
Sulfate Working Standards
Pipette 0, 0.5, 2.0, 5.0, 10.0 and 20 mL of SO4-S Primary Standard to separate 100 mL volumetric flasks and dilute to volume with deionised water. These standards contain 0, 1.5, 6.0, 15.0, 30.0 and 60.0 mg SO4-S/L.
Filter the SE (if necessary) through a Whatman No. 42 filter paper and transfer a 5.0 mL aliquot to a 25 mL volumetric flask. Add 2.5 mL of Conditioning Reagent to each sample and make to volume with deionised water. Transfer the solution to a 50 mL Erlenmeyer flask, add 1.0 mL of the Barium Chloride ‘Seed’ Suspension, and shake well. Allow to stand for 1 h. Pour some of the solution into the absorption cell of a spectrophotometer and measure absorbance at 420 nm (see Note 1).
Treat the SO4-S Working Standards and a deionised water blank in the same manner (including volumes) as described above, and construct a calibration curve of absorbance versus concentration of the standards. Estimate the SO4-S concentration in the sample by comparing the absorbance with the calibration curve: take account of any significant method blank.
Report SO4-S/SE (mg/L) with relevant method codes, or divide by 16.03 to convert to cmolcSO4-S/L.
1. To obtain reproducible results using this method, careful attention must be paid to uniform temperature, time and rate of stirring, and time of standing of the SE before measurement. If air temperature in the laboratory is known to fluctuate, use a water bath to maintain temperature to within ± 0.1°C. Note that prolonged mechanical stirring can heat up the aqueous mixture, as the electric motor in the stirrer can release heat.
2. If desired, the measurement of absorbance can be automated. An example is turbidimetric flow-injection analysis, as described by Morais et al. (2001).