Acid-Base Titration

Learning Objective

Acid-Base Titration is a way to find out how much acid or base is in a substance. You do this by adding a known amount of another substance (called titrant) to it until the reaction is complete. This helps you figure out the concentration of the acid or base.

Theory

An acid-base titration involves strong or weak acids or bases. Specifically, an acid-base titration can be used to figure out the following.

1. The concentration of an acid or base
2. Whether an unknown acid or base is strong or weak.
3. pK_a of an unknown acid or pK_b of the unknown base.
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Let us consider acid-base reaction which is proceeding with a proton acceptor. In water, the proton is usually solvated as H₃O⁺. H₂O is added to the base to lose (OH^–) or gain (H₃O⁺). Acid-base reactions are reversible.

The reactions are shown below.

HA + H₂O → H₃O⁺ + A^– (acid)

B^– + H₂O → BH + OH^– (base)

Here [A^–] is the conjugate base, B-H is conjugate acid. Thus we say

Acid + Base ⇋ Conjugate base + Conjugate acid

Hence

It is possible to give an expression for [H⁺] in terms of K_A, K_B and K_w for a combination of various types of strong and weak acids or bases.

Key Terms

1. Titration – A process where a solution of known strength is added to a certain volume of a treated sample containing an indicator.
2. Titrant – A solution of known strength of concentration used in the titration.
3. Titrand – The titrand is any solution to which the titrant is added and which contains the ion or species being determined.
4. Titration curve – A plot of pH Vs millilitres of titrant showing the manner in which pH changes Vs millilitres of titrant during an acid-base titration.
5. Equivalence point – The point at which just an adequate reagent is added to react completely with a substance.
6. Buffer solution – A solution that resists changes in pH even when a strong acid or base is added or when it is diluted with water

Types of Acid-Base Titration

The types and examples of strong/weak acids and bases are tabulated below.

S.NoTypesExamples1.Strong acid-strong base Hydrochloric acid and sodium hydroxide2.Weak acid-strong base Ethanoic acid and sodium hydroxide3.Strong acid-weak base Hydrochloric acid and ammonia4.Weak acid-weak base Ethanoic and ammonia

Titration Curve & Equivalence Point

The equivalence point in a titration is the point at which exactly the same number of moles of hydroxide ions have been added as there are moles of hydrogen ions. This is the point at which the acid and base have completely reacted and the solution is neutral. The shape of the titration curve depends on the type of acid-base titration being performed. For example, in a strong acid-strong base titration, the titration curve will have a steep rise at the beginning, followed by a gradual rise to the equivalence point, and then a steep drop off to the endpoint.

In contrast, in a weak acid-strong base titration, the titration curve will have a gradual rise to the equivalence point, followed by a gradual drop off to the endpoint.

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