Mg is precipitated and estimated gravimetrically as: 

${\mathrm{Mg}}^{2+}+{\mathrm{HPO}}_4^{2-}+{\mathrm{NH}}_4^{\oplus }+\stackrel{\ominus }{\mathrm{OH}}⟶\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}$

A cold acid solution of magnesium salt is treated with an excess of diammonium hydrogen phosphate and then excess of hydrogen phosphate and then excess of ammonia solution is added to precipitate out magnesium ammonium phosphate hexahydrate, $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4\cdot 6{\mathrm{H}}_2\mathrm{O}$. This precipitate is then filtered, washed and ignited (1000-1100°C) to magnesium pyrophosphate (Mg₂P₂O₇) and weighed as such.

$2{\mathrm{MgNH}}_4{\mathrm{PO}}_4 \stackrel{1000-1100°\mathrm{c}}{\to } {\mathrm{Mg}}_2{\mathrm{P}}_2{\mathrm{O}}_7+2{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}$