Acid-base titration

In acid base titrations, a base is added to an acid successively (or vice versa) and the pH of the solution is noted after every addition. The plot of pH against the amount of base (or acid) added is called a titration curve. The shape of the curve depends upon the ionization constants of the acid and base used in the titration.

Titration of strong acid and strong base

When we add a solution of NaOH to the solution of HCl, the pH progressively increases.

This is because OH^- ions from the base will react with H⁺ ions of the acid to form water. This decreases the concentration of H⁺ ions and therefore pH increases. The pH of the solution increases only slightly in the beginning until just before the stoichiometric point, there is a sudden jump in pH. The pH continues to increase after the stoichiometric point but then levels off because of the presence of excess strong base in the solution. At the equivalence point the pH is 7, but has increased sharply from 3.5 to 11 just before this point. Any indicator, which has effective range between pH 3.5 to 11 may be used to detect the equivalence point. For example, phenolphthalein and methyl orange are good indicators in this range.

When we titrate the base (NaOH) with the continuous addition of acid (HCl), the curve obtained is exactly reversed. The pH decreases slowly in the beginning and near the equivalence point there is sudden fall through pH = 7 as the concentration of H⁺ ions increases sharply.

Fig: 8.2 - (a) Titration curve of a strong acid (HCl) with a strong base (NaOH) (b) Titration curve of a strong base (NaOH) with a strong acid (HCl)

Titration of weak acid and a strong base

This type of titration is carried out between acetic acid and sodium hydroxide.

The free H⁺ ion from the weak acid is neutralized by OH^- ions from the base and there is a small increase in pH. Around the equivalence point large increase in pH is observed. However, the slope of the curve is not as steep as in the case of strong base and strong acid titration curve. Sodium acetate formed after neutralization reacts with OH^- ions of water (hydrolysis) and so at the equivalence point the pH is not 7. The H⁺ ion concentration decreases and the pH is about 8.7 at the equivalence point. By the addition of small amount of NaOH, pH change is observed from about 8.0 to 10.0 near the equivalence point. It is therefore necessary to use an indicator with pH range on alkaline side. Phenolphthalein is commonly used. Methyl orange cannot be used because it would give inaccurate indication of equivalence point.

Fig: 8.3 - Titration curve of a weak acid and strong base

Titration of weak base and a strong acid

This type of titration is carried out between a weak base such as ammonium hydroxide and strong acid such as hydrochloric acid.

The equivalence point is below 7 because the salt (NH₄Cl) formed at the neutralization reacts with water to give H⁺ ions. The equivalence point lies at about pH 5.3. It is, therefore necessary to use an indicator with pH range slightly on the acidic side. Methyl orange can be used. Phenolphthalein is not suitable because its colour change occurs away from the equivalence point.

Fig: 8.4 - Titration curve of weak base and strong acid

Titration of weak acid and weak base

This type of titration is carried out between a weak acid such as acetic acid (CH₃COOH) and weak base such as ammonium hydroxide (NH₄OH).

There is no sharp change in the pH during the titration. Hence, no sharp equivalence point can be obtained with common indicators. However, a mixed indicator, which shows a sharp colour change over limited pH range may be used.

Fig: 8.5 - Titration curve of weak base and weak acid