In the group VI test, Mg^2+ with Na2HPO4 in presence of NH4Cl and excess NH3 gives a white crystalline precipitate of

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In the group VI test, $Mg X^{2 +}$ with $Na X_{2} HPO X_{4}$ in presence of $NH X_{4} Cl$ and excess $NH X_{3}$ gives a white crystalline precipitate of

Mg X^{2 +} + HPO X_{4} X^{2 -} + NH X_{3} Mg (NH X_{4}) PO X_{4}

. The

NH X_{4} Cl

prevents formation of

Mg (OH) X_{2}

, so the double phosphate forms.