What is the endpoint color change in the Mercuric Nitrate method?

In the Mercuric Nitrate method, which is commonly used in water analysis to determine the presence of chlorine, the endpoint is indicated by a distinct color change that takes place during titration. When mercuric nitrate is added to a solution containing chlorine, the endpoint is reached when the solution changes from blue to purple.

This color change occurs because the mercuric ion reacts with chloride ions to form a complex, and as the titration progresses, the color of the solution shifts due to the presence of the mercuric chloride complex. The initial blue color is indicative of the prior state of the solution before reaching the endpoint, while the transition to purple signifies that all available chloride has reacted and any excess mercuric nitrate is resulting in the formation of a colored complex.

This specific color change is crucial for analysts to accurately determine the concentration of chlorine in the sample being tested. Understanding this endpoint helps in ensuring proper analysis and contributes to the effective monitoring of water quality.