Question

$K_{3}Fe{\left(CN\right)}_6$ is used as an external indicator for ferrous sulphate vs $K_{2}C{r}_2{O}_7$ titrations.

• A. True
• B. False

Answer 1

The correct option is A True

Indicator : Postassium ferricyanide solution $\left[K_{3}Fe\right(CN{)}_6]$

Principle: Potassium dichromate oxidises ferrous sulphate present in Mohr's salt into ferri sulphate in presence of dilute sulphuric acid. In this titration, potassium ferricyanide is used as an external indicator which gives a greenish-blue colour due to the formation of Ferro ferricyanide.

$C{r}_2{O}_7^{2-}+14H^{+}+6e^{-}⟶2C{r}^{+3}+7H_{2}O$
$[F{e}^{+2}⟶F{e}^{+3}+e^{-}]\times 6$
$C{r}_2{O}_7^{2-}+14H^{+}+6F{e}^{+2}⟶2C{r}^{+3}+7H_{2}O+6F{e}^{+3}$
$3F{e}^{+2}+2\left[Fe\right(CN{)}_6{]}^{-3}⟶\underset{\underset{\left(\text{Blue complex}\right)}{\left(\text{Ferro ferricyonise}\right)}}{F{e}_3\left[Fe\right(CN{)}_6{]}_2}$