Initial Colour:
At the beginning of the titration, the solution contains potassium dichromate, which has a bright orange colour. This colour is due to the presence of dichromate ions (Cr2O7^2-) in the solution.
Colour Change:
As you start adding the salt solution (containing chloride ions, Cl-) to the potassium dichromate solution, a reaction takes place between the chloride ions and the dichromate ions. This reaction produces silver chloride (AgCl) precipitate and chromate ions (CrO4^2-).
Ag+ (from salt solution) + Cr2O7^2- (from potassium dichromate) -> AgCl (precipitate) + 2CrO4^2-
As a result of this reaction, the orange colour of the dichromate ions starts to fade, and a yellow colour begins to appear. This colour change is due to the formation of chromate ions (CrO4^2-), which have a yellow colour.
Endpoint Colour Change:
The titration continues until all the dichromate ions have reacted with the chloride ions. At this point, the solution will have a pure yellow colour, indicating the endpoint of the titration. This colour change serves as a visual signal that the reaction is complete and allows you to determine the concentration of the salt solution accurately.
In summary, the colour change from orange to yellow during the titration of salt against potassium dichromate occurs due to the formation of silver chloride precipitate and the conversion of dichromate ions (orange) to chromate ions (yellow). This colour change helps indicate the endpoint of the titration.
