Mohr's method is a widely used precipitation titration technique for the determination of chloride ions in a solution. It is based on the formation of a silver chloride precipitate when a silver nitrate solution is added to a chloride-containing solution. The titration is carried out using a standard solution of silver nitrate, and the end point is detected visually or with the help of an indicator.
Here's a detailed discussion of Mohr's method of precipitation titration:
Principle:-
Mohr's method is based on the principle of selective precipitation. In this method, a known volume of the chloride-containing solution is titrated with a standard solution of silver nitrate (AgNO3). Silver chloride (AgCl) is formed as a white precipitate when silver ions react with chloride ions present in the solution. The reaction is as follows:
Ag+ (aq) + Cl- (aq) → AgCl (s)
The silver nitrate solution is added gradually until all the chloride ions in the sample solution have reacted to form silver chloride precipitate. At the end point, the addition of silver nitrate results in the formation of a slight excess of silver ions that react with an indicator or excess chloride ions to form a colored compound.
Procedure:
Preparation of Sample Solution: A known volume of the chloride-containing solution is taken in a titration flask. It is usually advisable to add a few drops of concentrated nitric acid (HNO3) to ensure the complete conversion of other chloride compounds to chloride ions.
Addition of Indicator: A suitable indicator is added to the sample solution. The commonly used indicator in Mohr's method is potassium chromate (K2CrO4). It forms a red-colored compound with excess silver ions.
Titration: The standardized silver nitrate solution is added from a burette to the sample solution while continuously stirring. As silver nitrate is added, silver chloride precipitate starts to form. Initially, the precipitate dissolves as it is redissolved by excess chloride ions. As the titration progresses, the chloride ions are gradually consumed, and at the end point, the addition of silver nitrate results in the formation of a slight excess of silver ions.
Detection of End Point: The end point is detected by the appearance of a reddish-brown color due to the reaction between excess silver ions and the indicator (potassium chromate). The color change is due to the formation of silver chromate (Ag2CrO4). The first permanent reddish-brown tinge indicates the end point.
Calculation: The volume of silver nitrate solution used is noted. From the known concentration of the silver nitrate solution, the amount of silver ions used in the titration can be determined. This is then used to calculate the concentration of chloride ions present in the original sample solution.
Advantages and Limitations:
- Mohr's method is simple and economical, requiring only a few pieces of equipment.
- It provides reliable results for the determination of chloride ions in a wide range of concentrations.
- The method is relatively fast and can be performed with reasonable accuracy.
- Mohr's method is limited to the determination of chloride ions only and may not be suitable for other ions or complex mixtures.
- It requires careful observation and subjective judgment to detect the end point, which can introduce some errors.
- Interference from other ions, such as bromide and iodide, can affect the accuracy of the results. Therefore, proper precautions and selective masking agents may be required.
