1. pH Dependence:
* The Mohr method relies on the formation of a red-brown precipitate of silver chromate (Ag2CrO4) as the endpoint indicator.
* This precipitate forms only when the solution has a specific pH range of 6.5 to 10.5.
* If the pH is too low, the silver ions will react with hydroxide ions to form silver hydroxide, which is insoluble.
* If the pH is too high, chromate ions will react with hydrogen ions to form chromic acid, which is yellow.
2. Interference from Other Anions:
* The Mohr method is not suitable for solutions containing anions that form insoluble silver salts, such as bromide (Br-), iodide (I-), sulfide (S2-), or cyanide (CN-). These anions will react with silver ions before the chromate ions, interfering with the endpoint determination.
* Other interfering ions include phosphate (PO43-) and arsenate (AsO43-) which can form soluble silver complexes, interfering with the titration.
3. Sensitivity:
* The Mohr method is generally considered to have moderate sensitivity.
* It is not suitable for determining very low concentrations of chloride.
* The detection limit is typically around 5-10 ppm chloride.
4. Color Blindness:
* The endpoint detection in the Mohr method relies on the color change from yellow to red-brown.
* This method can be challenging for individuals with color blindness.
5. Sample Type:
* The Mohr method is generally suitable for clear, colorless solutions.
* It may be difficult to use with samples that are colored or contain suspended particles.
To improve the reliability of the Mohr method, it is important to:
* Adjust the pH of the sample solution to the optimal range.
* Use a blank titration to correct for any background interference.
* Ensure that the solution is clear and colorless.
* Use a standardized silver nitrate solution.
Alternatives to the Mohr Method:
* Volhard method: A titrimetric method that uses a back titration with thiocyanate ions. This method is more sensitive than the Mohr method and is less affected by pH changes.
* Fajans method: A titrimetric method that uses an adsorption indicator. This method is suitable for determining chloride in a variety of samples, including those containing interfering ions.
In summary, the Mohr method is a useful technique for determining chloride ions in solutions with moderate concentrations and in the absence of interfering ions. It is important to be aware of its limitations and to take appropriate precautions when using this method.
