Question

In the test for Cl1- and SO4 2- , explain why the addition of acid will dissolve or decompose other silver salts and barium salts, which otherwise might Interfere

Answer 1

Strong acids can dissolve the salts of weak acid. When we consider the different salts of silver:

Salts of silver with the conjugate bases of a weak acid are soluble in strong acidic solutions. Some of these salts are:

$2Ag^{+}(aq)+CO_{3}^{2-}(aq) --->Ag_{2}CO_{3}(s)$

$Ag^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq)-->AgC_{2}H_{3}O_{2}(aq)$

$2Ag^{+}(aq)+SO_{3}^{2-}(aq) --->Ag_{2}SO_{3}(s)$

Salts of silver with the conjugate bases of a strong acid are not affected by change in pH:

$Ag^{+}(aq)+Cl^{-}(aq) --->AgCl(s)$

$2Ag^{+}(aq)+SO_{4}^{2-}(aq) --->Ag_{2}SO_{4}(s)$

These two salts with Chloride and sulfate ions are not soluble in acidic solutions as the salts of silver with the conjugate bases of a strong acid are not soluble in acidic solutions, they remain unaffected by any change in pH.

So for salts of Ag and Ba with the conjugate bases of a weak acid, solubility is increased upon the addition of an acid. So, the interference from the ions of weak acids can be removed by decreasing the pH.