Question

The test for sulfate used in lab may give a false positive. Even though there is no sulfate, the precipitate may form. However, the test for phosphate does not give a false positive. Explain why the test for phosphate does NOT give a false positive, whereas the sulfate test may give a false positive.

Sulfate tests:

Na2SO4(aq) + BaCl2(aq) --> BaSO4(S)2NaCl(aq) ---True positive

2Na3PO4(aq) + 3BaCl2(aq) --> Ba3(PO4)2(S) + 6NaCl(aq) ---False positive

Phosphate tests:

2Na3PO4(aq) + 3Mg(NO3)2(aq) --> Mg3(PO4)2(S) + 6NaNO3(aq)

Na2SO4(aq) + Mg(NO3)2(aq) --> MgSO4(aq) + 2NaNO3(aq)

Answer 1

Solution:

The sulfate gives false positive test because in the presence of phosphate, when sulfate is treated with BaCl2 solution, then both phosphate and sulfate forms the white colored precipitate which is indistinguishable.

BaSO4 ====== white precipitate

Ba3 (PO4)2 === white precipitate

Phosphate does not give false positive test because when sulphate and phosphate react with magnesium nitrate, then phosphate forms a light blue precipitate while sulfate forms a colorless precipitate. Thus, both precipitates can be easily distinguishable.

Mg3 (PO4)2 = === light blue precipitate

MgSO4 ======== colorless precipitate