In the classic Schales-Schales method for chloride, what substance reacts with the indicator to produce a violet color?

In the classic Schales-Schales method for determining chloride concentration, the reaction involves the use of an indicator that forms a colored complex when it reacts with excess mercury ions (Hg²⁺). In this procedure, silver ions (which are added to precipitate chloride as silver chloride) play a crucial role, but it is the mercury ions that are responsible for the development of the characteristic violet color.

When chloride ions are present, they first react with silver ions to form a precipitate of silver chloride. Once all the chloride ions have been precipitated, any additional silver ions will remain in the solution. The addition of excess mercury ions later in the procedure results in the formation of a complex with the indicator, which is what imparts the violet color.

Therefore, it is the interaction between the indicator and the excess mercury ions that ultimately leads to the visible color change, confirming the endpoint of the titration and allowing for the quantification of chloride in the sample. This understanding highlights the importance of the role of mercury ions in this analytical method.