Acid Base Titration - Titration Curves, Equivalence Point & Indicators of Acid Base Titration (2024)

An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration.

Table of Content

• What is Acid-Base Titration?
• Recommended Videos
• Theory
• Key Terms
• Types
• Titration Curve
• Indicator
• Solved Example
• Frequently Asked Questions – FAQs

What is Acid-Base Titration?

An acid-base titration is an experimental technique used to acquire information about a solution containing an acid or base.

Hundreds of compounds both organic and inorganic can be determined by a titration based on their acidic or basic properties. Acid is titrated with a base and base is titrated with an acid. The endpoint is usually detected by adding an indicator.

Also read practise questions:Acid Base Titrations Questions

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Learning Objective

To determine the concentration of an acid or base by measuring the volume of titrant (of known concentration) that reacts with it according to a stoichiometric proton-transfer reaction.

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.

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-His 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.

Titration Curve & Equivalence Point

In a titration, the equivalence point is the point at which exactly the same number of moles of hydroxide ions have been added as there are moles of hydrogen ions. In a titration, if the base is added from the burette and the acid has been accurately measured into a flask. The shape of each titration curve is typical for the type of acid-base titration.

The pH does not change in a regular manner as the acid is added. Each curve has horizontal sections where a lot of bases can be added without changing the pH much. There is also a very steep portion of each curve except for weak acid and the weak base where a single drop of base changes the pH by several units. There is a large change of pH at the equivalence point even though this is not centred on pH 7. This is relevant to the choice of indicators for each type of titration.

Choice of Indicators

Acid-base indicators are substances which change colour or develop turbidity at a certain pH. They locate equivalence point and also measure pH. They are themselves acids or bases are soluble, stable and show strong colour changes. They are organic in nature.

A resonance of electron isomerism is responsible for colour change. Various indicators have different ionization constants and therefore they show a change in colour at different pH intervals.

Acid-base indicators can be broadly classified into three groups.

• The phthaleins and sulphophthaleins (eg; Phenolphthalein)
• Azo indicators (eg; Methyl orange)
• Triphenylmethane indicators (eg; Malachite green)

The two common indicators used in acid-base titration is Phenolphthalein and methyl orange. In the four types of acid-base titrations, the base is being added to the acid in each case. A graph is shown below where pH against the volume of base added is considered. The pH range over which the two indicators change colour. The indicator must change within the vertical portion of the pH curve.

The Choice of indicators based on the type of titration is tabulated below.

Recommended Video

What is an Indicator? What Are Acid-Base Indicators?

Solved Example

Problem:

A 1.2gm sample of a mixture of (Na₂CO₃ + NaHCO₃) is dissolved and titrated with 0.5N HCl. With phenolphthalein, the endpoint is at 15ml while after further addition of methyl orange a second endpoint is at 22ml. Calculate the percentage composition of the mixture.

Solution:

15 + 15 = 30ml acid is necessary to neutralize Na₂CO₃ completely.

Total volume needed = 15 + 22 = 37ml

(37-30) = 7ml acid is needed for neutralizing NaHCO₃

Therefore, Na₂CO₃ composition (%) is

= [(30 x 0.5 x 0.053)/1.2] x 100 = 66.25%

= (7 x 0.5 x 0.042 x 100) / 1.2 = 24.50% NaHCO₃