Quinhydrone electrode

The quinhydrone electrode may be used to measure the hydrogen ion concentration (pH) of a solution containing an acidic substance. 
A platinum wire electrode is immersed in a saturated aqueous solution of quinhydrone, in which there is the following equilibrium 
The potential difference between the platinum electrode and a reference electrode is dependent on the activity, formula_1, of hydrogen ions in the solution. 
The quinhydrone electrode provides an alternative to the most commonly used glass electrode. however, it is not reliable above pH 8 and cannot be used with solutions that contain a strong oxidizing or reducing agent.