Hydrogen-ion activity pH cf. also Chapter

In chemically pure water, water molecules split up to form H3O+ (hydronium ion) and OH (hydroxyl ion) according to the following formula  

In its old form (merely taking into consideration the proton activity H+) this reads  

The hydrogen ion is carrier of the acid reaction. The pH is used as measuring unit this is defined according to the following equation 

The concentration of hydrogen ions in chemically pure, neutral water is 1 10 g/1, corresponding to the dissociation constant of the water = 10 at 25 °C deducible from the principle of mass action  

The pH is determined electrometrically by measuring the difference in potential between the measuring electrode (glass electrode) and the reference electrode with known potential (saturated calomel electrode used instead of the normal hydrogen electrode). 

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