Question

Explain why the anionic solution need to be acidified during identification of anions

Answer 1

Test for halide : Add AgNO3. Gives precipitate of silver halide if halide anion present.

To test for halide (Cl-, Br-, I-) ions, the sample is acidified (with dilute HNO3) before adding AgNO3 for testing. This is because , if carbonate ion is present (CO3 2-) , then it will form a precipitate of silver carbonate ( Ag2CO3) with AgNO3 , and so, a positive test will be obtained even if halide anions are not present. But, if acid is added, then the acid H+ reacts and removes the carbonate ions :

2H+ + CO3 2- = CO2 + H2O .

CO2 is evolved as a gas, and carbonates are removed from solution.

Test for sulfate : Add BaCl2. Gives white precipitate ( of BaSO4) if sulfate anion present.

When testing for sulfates:

If sulfites are present in solution, BaSO3 precipitate will form after adding BaCl2 , and so, a positive test is shown even if no sulfate anion is present. But this BaSO3 is soluble in acids. So, if the sample is acidified, then BaSO3 won't precipitate.. (the acid can also be added after adding BaCl2 to distinguish sulfite and sulfate)