Mohr Titration
Definition and meaning of Mohr Titration in chemistry.
Mohr titration is a classical quantitative method used to precisely determine the concentration of chloride or bromide ions. It achieves this by using a standardized solution of silver nitrate as the main titrant. The technique relies on a visual color change to signal the end of the chemical reaction.
In more detail
This laboratory technique involves slowly adding a standard solution of silver nitrate to an unknown sample. The silver ions immediately react with the halide ions to precipitate a solid white silver halide. A small amount of potassium chromate is always added to the flask as a chemical indicator.
Once all the target chloride or bromide ions have completely reacted, the chemistry rapidly shifts. The very first excess drop of silver nitrate reacts with the chromate ions instead. This secondary reaction forms a distinct reddish-brown precipitate known as silver chromate.
This sudden color change clearly signals the visual endpoint of the titration process. The entire process must be carefully carried out in a neutral or slightly alkaline solution. Maintaining the correct pH ensures strict accuracy during the laboratory measurement.
If the solution is too acidic, the chromate indicator will not work properly. If the liquid is too basic, silver hydroxide will wrongly precipitate out. A common misconception is that the red solid forms continuously throughout the whole experiment.
In reality, the white silver chloride forms first because it is less soluble. The red silver chromate only appears after every single chloride ion is entirely gone.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Titrant | Silver nitrate (AgNO3) |
| Target Ions | Chloride or bromide |
| Indicator | Potassium chromate (K2CrO4) |
| Endpoint Signal | Reddish-brown precipitate |
A scientist can determine the exact salinity of a brackish water sample by titrating its dissolved chloride. They place the salty water in a flask and add the yellow potassium chromate indicator. They slowly drip silver nitrate into the flask from a glass burette. White silver chloride clouds the water at first. Suddenly, a distinct reddish-brown color appears and refuses to fade. This visual endpoint tells the scientist how much silver nitrate was used. They use this volume to calculate the exact amount of salt in the water.
Frequently asked questions
Why must the Mohr titration strictly be performed in a neutral solution?
If the solution is strongly acidic, the chromate indicator converts to dichromate and fails to form the red precipitate. If it is too basic, solid silver hydroxide accidentally precipitates instead.
What is the specific endpoint color change?
The liquid in the flask starts with a bright yellow tint from the indicator. The analytical endpoint is distinctively marked by the sudden permanent appearance of a reddish-brown solid.
Can the Mohr method be used to measure iodine levels?
No, it does not work well for iodide ions. The silver iodide precipitate strongly traps the chromate indicator, which makes it nearly impossible to see the final color change.