Question

1. Why is the acid, HCl, added to the sodium sulfate solution?

2. Why is the Na2SO4 solution boiled prior to the addition of BaCl?

3. Why is the solution kept hot after the BaCl2 has been added?

4. What is the process termed "digestion" as it applies to the precipitated crystals?

Answer 1

The questions are based on gravimetric determination of sulfate and Barium.

Answer 1

HCl is address to the sodium sulfate solution so as to hinder the precipitation of various carbonate, phosphate ions present as the impurities. HCl will keep them in the form of weak acids and hence these will not react with barium to form compounds. Moreover these anions will volatilize on boiling in acidic medium.

Answer 2

Boiling of sodium sulfate solution is to ensure that the volatile impurities of carbonate and phosphates have been removed prior to addition of barium chloride.

Answer 3

Solution is kept hot after addition of BaCl2 so as to break the bonds between Ba2+ and Cl-, thereby easily releasing Cl- ions from BaCl2 and freeing Ba2+ to form BaSO4.

Answer 4

Process of digestion is to heat the freshly formed precipitates and its solution at high temperatures.

This causes the precipitates to grow bigger and cleaner along with the formation of mother liquor.

This process is also called as precipitate or Ostwald ripening. In this, at microscale, the formed precipitates dissolve and then reform larger precipitates.