Why measure pH?
- To prepare a buffer solution at a controlled pH level necessary for carrying out chemical or biological experiments in optimal conditions.
- To monitor and direct a chemical reaction and/or the transformation process of food.
What is pH?
According to Sørensen, pH is defined as the negative logarithm of the concentration of hydronium ions, H3O+, resulting from the dissociation of molecules:
When the H3O+ ion concentration changes by a factor of 10, the pH changes by 1 unit.
pH = –log [H3O+]
What is pH neutral?
Not only acids and bases dissociate to form hydronium or hydroxyl ions, pure water also dissociates to form these ions:
Kw is the dissociation constant of this reaction, also called the self-ionization of water :
2 H2O ⇔ H3O+ + OH–
The solution is neutral for equal quantities of H3O+ and OH–.
Kw = [H3O+][OH–] = 10–14 mol/L (25 ºC)
This is the case when concentrations of [H3O+] and [OH–] are 10–7 mol/L, i.e. pH 7.