Although ion chromatography is the method of choice for the determination of sulphate, spectrophotometric determination with barium perchlorate and Thorin will also give useful results, particularly if the determination is automated (Autoanalyser or FIA).
This method is applicable to the determination of sulphate in precipitation within the range 0.05 mg S/l to 4 mg S/l. Samples containing higher concentrations must be diluted prior to the analysis.
Ba(ClO4)2 is added in excess to precipitate the sulphate as barium sulphate in an organic solvent. The organic solvent will minimize the solubility product of barium sulphate.
The excess concentration of barium (II) ions in the solution is determined spectrophotometrically at 520 nm through the reaction with Thorin (the sodium salt of 4-(ortho-arsenophenyl-azo)-3-hydroxy-2,7-naphtalenedisulphonic acid).
Several organic solvents may be used. The most favourable calibration curve is obtained with dioxane.
All glassware should be of borosilicate and should be thoroughly rinsed in distilled water before use.
All chemicals, except Thorin, must be of recognized analytical grade. The water used for dilution and rinsing must be double distilled or deionized.
Prepare a series of standard solutions containing 0, 0.5, 1.0, 1.5 ..... 4 mg S/l by diluting 0, 1, 2, 3, ..... 8 ml of the sulphate standard solution in Section 4.2.1.5 (6) to 100 ml with water in volumetric flasks. Transfer 4 ml of each of these standard solutions to a test tube. Add 10 ml barium perchlorate reagent solution and 250 µl Thorin solution. Use a micro pipette for the Thorin solution. Mix thoroughly (do not use rubber stoppers!)
Transfer the solutions to optical cells. The spectrophotometer wavelength is set at 520 nm, and 0% transmission is adjusted according to the procedure in the manual of the photometer. Then gain and/or slit width is adjusted to give a reading of 0.80 absorbance units with the blank (0 mg S/l) in the sample compartment. Measure the absorbance of the solutions within 10 minutes after addition of the Thorin solution. This is especially important for low concentrations of sulphate and for the blank because the barium-Thorin compound may precipitate from the solution.
A calibration graph is constructed from the absorbance readings obtained from the standard solutions. The calibration curve is not linear below 0.5 mg S/l. This is suppressed by adding sulphate in a quantity corresponding to 0.5 mg S/l to all samples and blanks. The detection limit is then 0.05 mg S/l.
Cations are removed by treating the sample with a strongly acidic cation exchange resin.
Transfer 4 ml of the pre-treated sample to a test tube and proceed according to Section 5.2.1.6.
Determine the sulphur concentration of the sample from the absorbance reading by means of the calibration curve.
With suitable equipment, the barium perchlorate-Thorin method can be made automatic. This method is described in detail in the next Section.
Phosphate will interfere with this method.
Persson, G.A. (1966) Automatic colorimetric determination
of low concentrations of sulphate for measuring sulphur dioxide in ambient air.
Air Water Pollut., 10, 845-852.
This automatic method can be used to determine the concentration of sulphate in precipitation within the range 0.05 to 2.5 mg S/l.
The basis principle is the same as in the manual method above.
A known amount of (Ba(ClO4)2 is added in excess to the sample and the sulphate is precipitated as barium sulphate. The excess of barium ions reacts with the Thorin indicator to form a red compound. The concentration is determined colorimetrically at 520 nm.
Figure 4.2.1: Flow scheme for automatic spectrophotometric analysis of sulphate.
All chemicals, except Thorin, must be of recognized analytical grade. The water used must be double-distilled or deionized.
Prepare a series of standard solutions containing 0.0, 0.1, 0.5, 1.0 ..... 2.5 mg S/l by diluting 0, 2, 10, 20 ..... 50 ml of the standard sulphate solution (5) to 1000 ml with water in volumetric flasks.
Start the pump and check the flow, all connections, tubings and debubblers with water running through the instrument. Turn on the photometer and the recorder (paper speed 5 mm/min.). Connect the tubings to the reagents and check that the baseline is stable.
Fill the cups of the automatic sampler with samples and standard solutions. Sampling time is 130 seconds and rinsing time with water after each sample is 180 seconds. Start with the series of standard solutions, and run a series of standard solutions after every tenth sample.
When isopropanol is used as organic solvent, the tubings must be thoroughly cleaned after use. Flush water through the system until all the reagents are rinsed out, then run the cleaning solution for 5 minutes. Rinse again with water. Turn off the recorder, photometer, sampler and pump, and loosen the tubings in the pump so they are not stretched.
Prepare a calibration curve from each of the series of standard solutions by plotting the recorder response in mm (absorbance) against the concentration of the standards.
Convert the recorder response (absorbance) of the sample to mg S/l by means of the calibration curve obtained just before or after the sample.
The use of a transparent sheet with several vertical scales corresponding to the different responses from the standard solutions will save time when many samples are to be handled.
Henriksen, A. and Bergmann-Paulsen, I.M. (1974) An automatic method for determining sulphate in natural soft water and precipitation. Vatten, 2, 187-192.
Persson, G.A. (1966) Automatic colorimetric determination
of low concentrations of sulphate for measuring sulphur dioxide in ambient air.
Air Water Pollut. 10, 845-852.