Quinhydrone Electrode and pH Determination

Quinhydrone Electrode

Quinone + Hydroquinone = Quinhydrone
The foloowing half cell reaction takes place between quinone and hydroquinone in the presence of H⁺ ion-

This electrode is easy to set up and small amount of solution required for measurement. It gives accurate pH between 0 to 9. In alkaline solution, the pH values are not accurate due to ionization and aerial oxidation of QH₂.

pH Determination

For pH determination, it is kept in the solution whose pH is to be determined and connected to a calomel electrode. Hence the cell is represented as-
(−),Pt,Hg|Hg₂Cl₂(s),KCl sat. solution || Solution of unknown pH,Q|QH₂Pt(+)
Therefore, the emf of cell is given by-
E_cell = − 0.2415 − (−0.69994 + 0.0591 pH)
or, E_cell = − 0.2415 + 0.69994 − 0.0591 pH
or, 0.0591 pH = 0.4579 − E_cell
or, pH = (0.4579 − E_cell)/0.0591
By Knowing the value of emf of the cell, pH of the solution can easily be calculated.

Glass Electrode for pH Measurement