Question

Normal calomel electrode has e.m.f. of $0.67$ V at $25$${}^{o}C$. Calculate pH of solution. The oxidation potential of calomel electrode on $H$ scale is $0.28V$ (nearest integer).

Answer 1

First calculate the potential of hydrogen electrode.

$E=E_{calomel}-E_{HE}$

$0.67=0.28-E_{HE}$

$E_{HE}=-0.39V$

Now calculate the hydrogen ion concentration.

$E_{HE}=E_H^0-\frac{0.0592}{n}log\frac{P_{H2}}{[H^{+}{]}^2}$

$-0.39=0-\frac{0.0592}{2}log\frac{1}{[H^{+}{]}^2}$

$\left[H^{+}\right]=2.51\times {10}^{-7}M$

Hence, the pH of the solution is $pH=-log\left[H^{+}\right]=-log2.5\times {10}^{-7}=6.6\approx 7$