In this article we will discuss about the gravimetric method for estimation of sulphate in plants.
Principle:
Sulphate is precipitated as barium sulphate in hydrochloric acid medium by the addition of barium chloride solution. The reaction is carried out near the boiling temperature. The precipitate is filtered and washed to remove the chlorides, then dried or ignited and weighed as BaSO4.
Requirements:
(a) Reagents:
1. Methyl red indicator:
Dissolve 100 mg methyl red sodium salt in dist. water to prepare 100 ml of solution.
2. Hydrochloric acid:
50% (V/V).
3. Barium chloride solution:
Dissolve 100 gm. BaCl2. 2H2O in distilled water to prepare 1 lit of solution. Filter the solution through a filter paper before use.
4. Silver nitrate:
Nitric acid reagent — Dissolve 8.5 gms of AgNO3 and 0.5 ml conc. HNO3 in distilled water to prepare 500 ml reagent.
(b) Glass-wares and instruments:
Conical flasks, beakers, pipettes, filter papers, balance with weight box, hot air oven.
(c) Sulphate solution or sample water or soil sample etc.
Test Procedure:
1. 100 ml of sulphate solution is taken in a conical flask and a few drops of methyl red is added after adjustment of pH at 4.5-5.0 by HCl soln. (indicator colour changed to orange).
2. The solution is boiled and warm BaCl2 soln. is added in excess slowly until precipitation is complete.
3. The precipitate is heated at 80-90°C for at least 2 hours or more.
4. The precipitate is filtered through filter paper (What-man No. 42) and washed by warm dist. water until the precipitate is free from chloride. This is determined by testing the washed material with AgNO3 soln.
5. The precipitate is dried on filter paper and ignited in a crucible at 800°C for 1 hr. — then cooled and weighed.
6. The sulphate content is calculated by the formula:
SO4mg/lit=mgmBaSO4 × 4111.5/ml sample solution