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^–

K_w is the dissociation constant of this reaction, also called the self-ionization of water :

K_w = [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.