How to use a pH meter?

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 H3O+ ions, derived from the dissociation of molecules:

pH = –log [H3O+]

When the H3O+ 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

Kw 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 H3O+ and OH.
This is the case when concentrations of [H3O+] and [OH] are worth 10–7 mol/L, i.e. pH 7.
 


The order of magnitude of pH