Magnesium is precipitated from its salt solution as only magnesium ammonium phosphate by adding disodium hydrogen phosphate solution in absence of ammonium chloride and aqueous ammonia.

Explanation

Precipitation is carried out in presence of ammonium chloride and aqueous ammonia as they prevent precipitation of magnesium hydroxide.
`Mg^(2+)+NH_(3)+HPO_(4)^(2-)toMg(NH_(4))PO_(4)darr`(white)