In the washing of precipitates, the wash liquid should contain a...

The addition of a common ion to the wash liquid suppresses the ionization (or dissociation) of the sparingly soluble precipitate due to the common ion effect.

According to Le Chatelier's principle, the presence of the common ion shifts the solubility equilibrium towards the undissociated solid precipitate.

This decreases the solubility of the precipitate, preventing its loss during the washing process.

Therefore, the wash liquid should contain a common ion for suppressing the ionization and decreasing the solubility.

Answer: suppressing the ionisation and decreasing the solubility