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.
Illustration of the pH scale. pH < 7 : acidic medium, pH = 7 : neutral medium, pH > 7 : basic medium

The orders of magnitude of pH

Orders of magnitude of pH. Food and household products: lemon juice 2, coca cola 2.5, orange juice 3.5, beer 4.5, cheese 5.5, milk 6.2, water 7, egg white 7.8, borax 9, antacid 10. Chemicals: sulfuric acid 0.2, hydrochloric acid 1, acetic acid 3, cyanic acid 5, sodium hydrogen carbonate 8, potatium acetate 9, amonic acid 10, amonic acid sol 1.7% 11, calcium carbonate 12.5, caustic soda 14.
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