Here's a breakdown of the method:
1. Principle:
The method is based on the reaction between chloride ions and mercuric ions, which is represented by the following equation:
```
Hg2+ + 2Cl- → HgCl2 (s)
```
The reaction is quantitative and proceeds until all the chloride ions are consumed.
2. Procedure:
* Sample Preparation: The water sample is typically treated with a buffer solution to maintain a constant pH.
* Titration: A known volume of the water sample is titrated with a standard solution of mercuric nitrate (Hg(NO3)2).
* Indicator: A diphenylcarbazone indicator is used to detect the endpoint of the titration. This indicator forms a purple complex with excess mercuric ions, signifying the endpoint.
* Endpoint: The endpoint is reached when the addition of a single drop of mercuric nitrate solution causes a permanent faint purple color in the solution.
* Calculation: The chloride concentration in the sample is then calculated based on the volume of mercuric nitrate solution used in the titration.
3. Advantages:
* Simple and straightforward
* Relatively inexpensive
* Can be performed in the field
4. Disadvantages:
* Interference from other ions: Other ions, such as bromide, iodide, and sulfide, can interfere with the determination of chloride by reacting with mercuric ions.
* Susceptibility to pH changes: The method is sensitive to pH changes, and a buffer solution is needed to maintain a constant pH.
* Not suitable for very low chloride concentrations: The method is not as sensitive as other methods and may not be suitable for very low chloride concentrations.
5. Alternatives:
* Ion-selective electrode method: This method is more sensitive and less prone to interference than the mercuric nitrate method.
* Spectrophotometric method: This method involves reacting the chloride ions with a reagent to form a colored complex, which is then measured spectrophotometrically.
In summary:
The mercuric nitrate method is a simple and relatively inexpensive method for determining chloride concentration in water. However, it is susceptible to interference and pH changes, and it is not suitable for very low chloride concentrations. Alternative methods, such as the ion-selective electrode method or the spectrophotometric method, may be more suitable in some cases.
