Question

$K_3\left[Fe\right(CN{)}_6]$ is used as an external indicator in the dichromate estimation of $F{e}^{2+}$. The following change is observed:

• A. colourless to blue solution
• B. blue to red solution
• C. colourless to red solution
• D. blue to colourless solution

Answer 1

The correct option is D blue to colourless solution

$F{e}^{2+}$ ions react with potassium ferricyanide to form a blue precipitate of Turnbull's blue.
$F{e}^{2+}+\left[Fe\right(CN{)}_6{]}^{3-}\to Fe\left[Fe\right(CN{)}_6{]}^{-}$
Hence, the end point will be blue to colourless.