Here's why:
* The Mohr method relies on the formation of a colored precipitate. The method uses silver nitrate (AgNO₃) as the titrant. Chloride ions (Cl⁻) react with silver ions (Ag⁺) to form a white precipitate of silver chloride (AgCl).
* Bromide ions also react with silver ions. Bromide ions (Br⁻) react with silver ions to form a pale yellow precipitate of silver bromide (AgBr).
* The precipitates form at different points in the titration. Silver chloride precipitates at a lower concentration of silver ions than silver bromide. This means that silver bromide will start to precipitate before all the chloride has reacted.
* The color change is obscured. The presence of the pale yellow silver bromide precipitate will mask the color change of the silver chromate (Ag₂CrO₄) indicator, which is used to signal the endpoint of the titration.
In summary, the Mohr method is not suitable for determining chloride and bromide simultaneously because the precipitates formed interfere with each other and obscure the endpoint of the titration.
Instead, other methods like potentiometric titration or ion chromatography are more appropriate for determining chloride and bromide simultaneously.
