Why is the solution acidified with HCl before adding the precipitant?

Adv.

Answer:

The HCl ensures that MgNH₄PO₄ • 6H₂O does not immediately precipitate when adding the precipitant. Because PO₄^3– is a weak base, the precipitate is soluble in a strongly acidic solution. If the precipitant is added under neutral or basic conditions (high RSS) the resulting precipitate consists of smaller, less pure particles. Increasing the pH by adding base allows the precipitate to form under more favorable (low RSS) conditions.

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