1 Sulfate
1.1 Barium sulfate turbidimetry
1.1.1 Scope
This standard specifies the determination of sulfate in drinking water and its source water by barium sulfate turbidimetry.
The method is applicable to the determination of soluble sulfate in drinking water and its source water.
The minimum detectable mass of this method is 0.25mg; if 50mL of water sample is taken for determination, the minimum detectable mass concentration is 5.0mg/L.
This method is applicable to the determination of water sample with sulfate concentration below 40mg/L. The stirring speed, time, temperature and reagent adding method will influence the determination results of turbidimetry; therefore, the operating conditions shall be strictly controlled to be consistent.
1.1.2 Principle
Sulfate and barium ions in water generate barium sulfate precipitation, forming turbidity with the turbidity degree proportional to sulfate content in water sample.
1.1.3 Reagents
1.1.3.1 Standard solution of sulfate [ρ(SO2- 4)=1mg/mL]: weigh 1.4786g of anhydrous sodium sulfate (Na2SO4) or 1.8141g of anhydrous potassium sulfate (K2SO4), dissolve it in pure water and scale the volume to 1000mL.
1.1.3.2 Stabilizer solution: weigh 75g of sodium chloride (NaCl) and dissolve it into 300mL pure water; add 30mL of hydrochloric acid (ρ20=1.19g/mL), 50mL of glycerinum (glycerine) and 100mL of ethanol [φ(C2H5OH)=95%], and mix them well.
1.1.3.3 Barium chloride crystal (BaCl2•2H2O), 20 mesh~30 mesh.
1.1.4 Instruments
1.1.4.1 Electromagnetic stirrer.
1.1.4.2 Turbidimeter or spectrophotometer.
1.1.5 Analytical procedure
1.1.5.1 Pipet 50mL of water sample in a 100mL beaker; if the sulfate concentration in water sample exceeds 40mg/L, take appropriate amount of water sample and dilute it to 50mL.
1.1.5.2 Add 2.5mL of stabilizer solution (1.1.3.2) and adjust the speed of electromagnetic stirrer so that the solution does not spill at stirring and 0.2g of barium chloride crystal (1.1.3.3) can dissolve within 10s~30s. Fix this condition which applies in the same batch of determination.
1.1.5.3 Add 0mL, 0.25mL, 0.50mL, 1.00mL, 1.50mL, and 2.00mL of standard solution of sulfate (1.1.3.1) respectively in six 100mL beakers of the same type. Then add pure water in each beaker to 50mL, thus to make the sulfate concentration be 0mg/L, 5.0mg/L, 10.0mg/L, 20.0mg/L, 30.0mg/L and 40.0mg/L (calculated by SO2- 4) respectively.
1.1.5.4 Take another 50mL of water sample to be under the same condition with the standard series. Add 2.5mL of stabilizer solution (1.1.3.2) in the water sample and standard series respectively; then add 0.2g of barium chloride crystal (1.1.3.3) after the stirring speed gets stable; start timing immediately and stir for 60s±5s. For each beaker, start timing when adding barium chloride crystal; at exact 10min, measure the absorbance with 3cm cuvette at 420nm wavelength by taking pure water as reference. Or determine the turbidity with turbidimeter.
1.1.5.5 Draw the working curve and search out the sulfate mass in sample from the curve.
1.1.6 Calculation
The mass concentration of sulfate (SO2- 4) in water sample is calculated according to Formula (1):
