How do you use a pH meter?

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.
e.g. Lactic acidification in cheese factories.

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:

pH = –log [H3O+]

When the H3O+ ion concentration changes by a factor of 10, the pH changes by 1 unit.



What is pH neutral?


Not only acids and bases dissociate to form hydronium or hydroxyl ions, pure water also dissociates to form these ions:

2 H2O ⇔ H3O+ + OH

Kw is the dissociation constant of this reaction, also called the self-ionization 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 10–7 mol/L, i.e. pH 7.
 


The orders of magnitude of pH

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