Why measure pH?
- To prepare a buffer solution at the controlled pH level necessary to carry out chemical or biological experiments in optimal conditions.
- To monitor and to drive a chemical reaction and/or the transformation process of food.
Ex: Lactic acidification in cheese factories.
What is pH?
According to Sørensen, pH is defined as being the negative logarithm of the concentration of hydronium H3
ions, derived from the dissociation of molecules:
pH = –log [H3O+]
When the H3
ion concentration changes by factor 10, the pH changes by 1 unit.
What is neutral pH?
Acids and bases are not the only ones to dissociate and form either hydronium or hydroxyl ions, pure water also dissociates and forms these ions:
2 H2O ⇔ H3O+ + OH–
is the dissociation constant of this reaction, also called the self-ionisation of water
Kw = [H3O+][OH–] = 10–14 mol/L (25 ºC)
The solution is neutral for equal quantities of H3
This is the case when concentrations of [H3
] and [OH–
] are worth 10–7
mol/L, i.e. pH 7.
The order of magnitude of pH