Limit test for sulphates is based on the principal of precipitation of sulfate ions by barium chloride. It is done to check the extent of sulfate impurities in officially prepared products.
Principle:
The test depends on the interaction between the sulfate ions with the barium chloride in the presence of hydrochloric acid.
BaCl2 + SO42———————> BaSO4 + Chlorides (Test sample)
The above reaction leads to the formation of a precipitate. But, the precipitate is too small, and hence it appears as turbidity if mixed well. The extent of turbidity depends on the concentration of sulfate ions in the sample.
The turbidity produced by test sample is compared with of standard solution again dark background.
The procedure of limit test for sulfate
Dissolve 2gm of the test sample in 10ml of distilled water in a Nessler’s tube. Add 2mlof dilute hydrochloric acid to the solution. Dilute to 45 ml with distilled water and add 5ml of barium sulfate reagent. Stir the solution with a glass rod and allow to stand for 5 minutes. Prepare the standard solution using 2gm of sodium chloride. Compare the turbidity produced with a standard sample solution.
Experimental technique
Follow the steps mentioned in the table for experiment
| Steps | Test cylinder | Standard cylinder |
| 1 | Take the given sample in a nessler’s cylinder and dissolve it in 10ml of distilled water | Take 1ml of standard sulfate solution in the nessler’s cylinder and dissolve it in 10ml of distilled water. |
| 2 | Add 2ml of dilute Nitric acid | Add 2ml of dilute nitric acid |
| 3 | Add 5ml of Barium sulfate solution and stir the solution with glass rod | Add 5ml of Barium sulfate solution and stir the solution with glass rod |
| 4 | Allow the mixed solution to stand for 5 minutes. | Allow the solution to stand for 5 minutes. |
| 5 | Observe the cylinder turbidity against black background | Observe the turbidity of solution against black background |
Precautions:
1. Add nitric acid to prevent reaction of other ins with barium.
2. Use distilled water to prevent impurities in the reaction
3. Do not mix the two solutions with the same stirrer as it can lead to wrong results.
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