9.5 Titration

This topic explains acid-base titration — the process of determining the concentration of an unknown acid or base using a solution of known concentration. It covers the procedure, the titration equation, and the selection of indicators.

Definition

Titration: A laboratory method used to determine the concentration of an unknown acid or base by reacting it with a solution of known concentration (standard solution).

Endpoint: The point at which the indicator changes colour, indicating that the reaction is complete.

Equivalence Point: The point at which the acid and base have completely neutralized each other in stoichiometric amounts.

Main Idea

Acid-base titrations are conducted using burettes and volumetric pipettes. Generally, acid solution is placed in the burette. A fixed volume of base is placed into a conical flask along with a few drops of the suitable acid-base indicator. Acid from the burette is added to the base until the indicator changes colour. This change of colour indicates the end point of titration. In the neutralization of a strong acid with a strong base, phenolphthalein is used as an indicator. It imparts pink colour to the base solution. At the end point, solution just becomes colourless. The molarity of the acid solution under test is determined with the help of the following equation.

Core Concept

Titration Equation:

M₁ V₁n₁ = M₂ V₂n₂

• M₁ = Molarity of the base
• V₁ = Volume of the base taken in flask
• M₂ = Molarity of the acid
• V₂ = Volume of acid used from the burette
• n₁ = No. of moles of base
• n₂ = No. of moles of acid

Knowing the five parameters, the sixth can be calculated.

Detailed Explanation

Procedure of Titration

• Acid-base titrations are conducted using burettes and volumetric pipettes.
• Generally, acid solution is placed in the burette.
• A fixed volume of base is placed into a conical flask along with a few drops of the suitable acid-base indicator.
• Acid from the burette is added to the base until the indicator changes colour.
• This change of colour indicates the end point of titration.
• In the neutralization of a strong acid with a strong base, phenolphthalein is used as an indicator.
• It imparts pink colour to the base solution.
• At the end point, solution just becomes colourless.
• The molarity of the acid solution under test is determined with the help of the following equation.

Titration Equation

M₁ V₁n₁ = M₂ V₂n₂

• M₁ = Molarity of the base
• V₁ = Volume of the base taken in flask
• M₂ = Molarity of the acid
• V₂ = Volume of acid used from the burette
• n₁ = No. of moles of base
• n₂ = No. of moles of acid

Do You Know?

Sherbet sweets contain a mixture of citric acid, baking soda and icing sugar. When eaten, the acid dissolves and reacts with baking soda. The sherbet fizzes on the tongue as carbon dioxide gas is produced by the neutralisation reaction.

Concept Assessment Exercise 9.3

In a titration, it is found that 25 cm³ of 0.12 M NaOH is neutralized with 30 cm³ of HCl of unknown concentration. Calculate concentration and strength of HCl solution.

Solution:

Reaction: HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O

• M₁ = 0.12 M (NaOH)
• V₁ = 25 cm³
• n₁ = 1
• V₂ = 30 cm³
• n₂ = 1
• M₂ = ?

Formula: M₁ V₁n₁ = M₂ V₂n₂

Substitution: 0.12 × 251 = M₂ × 301

Calculation: M₂ = 0.12 × 2530 M₂ = 0.1 M

Final Answer: Molarity of HCl solution = 0.1 M

Strength of HCl solution: Strength = Molarity × Molar mass = 0.1 × 36.5 = 3.65 g dm^-3