Why is the acid–base indicator methyl red added to the solution?
Adv.
The indicator’s color change, which occurs at a pH of approximately 6.3, indicates when there is sufficient NH3 to neutralize the HCl added at the beginning of the procedure. The amount of NH3 is crucial to this procedure. If we add insufficient NH3, then the solution is too acidic, which increases the precipitate’s solubility and leads to a negative determinate error. If we add too much NH3, the precipitate may contain traces of Mg(NH4)4(PO4)2, which, on drying, forms Mg(PO3)2 instead of Mg2P2O7. This increases the mass of the ignited precipitate, giving a positive determinate error. After adding enough NH3 to neutralize the HCl, we add the additional 5 mL of NH3 to quantitatively precipitate MgNH4PO4 • 6H2O.