Question

In the titration of ferrous ammonium sulphate and potassium dichromate, the external indicator used is:

• A. $KCNS$
• B. $N{H}_{4}CNS$
• C. $K_3\left[Fe\right(CN{)}_6]$
• D. $K_4\left[Fe\right(CN{)}_6]$

Answer 1

The correct option is C $K_3\left[Fe\right(CN{)}_6]$

The solution will turn yellow with addition of dichromate ions and when ferric ions are produced they will also impart a yellow color to the solution so there is no color change during the reaction and you need some other indicator

The use of potassium ferricyanide($K_{3}Fe(CN{)}_6$ as an external indicator is the best known example in the titration of iron with dichromate. As long as ferrous ion is present in the titration mixture, the indicator will turn blue on the transfer of solution. The appearance of blue colour is due to the formation of ferroferricyanide (Turnbull’s blue) according to flowing.

Hence option C is correct.