6.3.1 Preparation by Titration (Acid and Alkali)

Introduction

Titration is a method used to prepare soluble salts when both the acid and the base (alkali) are soluble in water. Since both reactants are soluble, we cannot simply add one in excess and filter it off — instead, we must use exact volumes so that the acid and alkali completely neutralize each other.

This section explains how sodium chloride (NaCl) is prepared from hydrochloric acid (HCl) and sodium hydroxide (NaOH) by titration.

Definition

Preparation by Titration: Preparation by titration is a method of preparing a soluble salt by reacting an acid with a soluble base (alkali) using exact volumes so that complete neutralization occurs.

In simple words: Titration is used when both the acid and the base are soluble. We use an indicator to find the exact volume needed for complete neutralization, then repeat without the indicator to get the pure salt.

Main Idea

• Titration is used when the base (alkali) is soluble.
• An indicator is used to find the end point (the point at which neutralization is complete).
• The experiment is repeated without the indicator to obtain the pure salt solution.
• The salt is then separated and purified by crystallization.

Core Concept

Key point: Titration is used when both reactants are soluble, so we need an indicator to find the exact volume for complete neutralization.

Detailed Explanation

Activity 6.1: Preparation of Sodium Chloride (NaCl)

Reaction:

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

Materials required:

• Dilute solutions of sodium hydroxide and hydrochloric acid
• Phenolphthalein indicator
• Burette
• Pipette
• Conical flask
• Funnel
• Evaporating dish
• Burette stand, tripod stand, and heating source

Procedure

• Measure exactly 25\ cm³ of HCl solution using a pipette and transfer it to a conical flask.
• Add a few drops of phenolphthalein as an indicator to the acid.
• Fill a burette with NaOH solution using a funnel.
• Slowly add the NaOH from the burette to the HCl in the conical flask, swirling the flask continuously until the indicator changes colour from colourless to light pink, indicating neutralization is complete.

Separation

Once the end point is reached, indicating that the acid and alkali have completely reacted, note the volume of NaOH used.

Crystallization

• Evaporate the water from the resulting NaCl solution gently to obtain a hot saturated solution.
• Allow the solution to cool and crystallize.
• Filter to collect the crystals.

Key point: The salt is obtained by evaporating the water to form a saturated solution, then cooling to form crystals.

Summary Table

Intermediate Level

Why titration is needed:

• When both the acid and the base are soluble, we cannot add one in excess and filter it off.
• Instead, we must use exact volumes so that both reactants completely neutralize each other.
• An indicator is used to find the exact volume needed.

Steps:

• Measure a known volume of acid using a pipette.
• Add a few drops of indicator.
• Fill a burette with the alkali solution.
• Slowly add the alkali to the acid until the indicator changes colour.
• Note the volume of alkali used.
• Repeat the experiment without indicator to obtain the pure salt solution.
• Evaporate the water to obtain a hot saturated solution.
• Allow to cool and crystallize.
• Filter to collect the crystals.

Key point: Titration is used when both reactants are soluble. The indicator helps find the end point, and the experiment is repeated without the indicator to get the pure salt.

Important Points

• - Titration is used when the base (alkali) is soluble.
• - An indicator (e.g., phenolphthalein) is used to find the end point.
• - The end point is the point at which neutralization is complete.
• - The experiment is repeated without the indicator to obtain the pure salt solution.
• - The salt is then obtained by crystallization.
• - Example: Preparation of NaCl from HCl and NaOH.
• - Reaction: HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)
• - Materials: Burette, pipette, conical flask, phenolphthalein, etc.
• - Steps: Measure acid, add indicator, titrate with alkali, note volume, repeat without indicator, evaporate, crystallize, filter.

Remember

• - Titration is used when both reactants are soluble.
• - Indicator is used to find the end point.
• - End point = neutralization complete.
• - Repeat without indicator for pure salt.
• - Salt obtained by crystallization.
• - Example: NaCl from HCl and NaOH.
• - Reaction: HCl + NaOH → NaCl + H₂O.
• - Materials: Burette, pipette, conical flask, phenolphthalein.

Common Mistakes

Quick Revision

Preparation by Titration (Acid and Alkali):

Activity 6.1: Preparation of sodium chloride (NaCl) from hydrochloric acid (HCl) and sodium hydroxide (NaOH) by titration.

• HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

Materials required:

• Dilute solutions of sodium hydroxide and hydrochloric acid
• Phenolphthalein indicator
• Burette
• Pipette
• Conical flask
• Funnel
• Evaporating dish
• Burette stand, tripod stand, and heating source

Procedure:

• Measure exactly 25\ cm³ of HCl solution using a pipette and transfer it to a conical flask.
• Add a few drops of phenolphthalein as an indicator to the acid.
• Fill a burette with NaOH solution using a funnel.
• Slowly add the NaOH from the burette to the HCl in the conical flask, swirling the flask continuously until the indicator changes colour from colourless to light pink, indicating neutralization is complete.

• Separation:
• Once the endpoint is reached, indicating that the acid and alkali have completely reacted, note the volume of NaOH used.

Crystallization:

• Evaporate the water from the resulting NaCl solution gently to obtain a hot saturated solution.
• Allow the solution to cool and crystallize, then filter to collect the crystals.

60-Second Revision

• Titration used when base is soluble.
• Indicator used to find end point.
• End point = neutralization complete.
• Repeat without indicator for pure salt.
• Salt obtained by crystallization.
• Example: NaCl from HCl and NaOH.
• Reaction: HCl + NaOH → NaCl + H₂O.
• Materials: Burette, pipette, conical flask, phenolphthalein.
• Steps: measure acid → add indicator → titrate → note volume → repeat → evaporate → crystallize → filter.
• ⚠️ Warning: the indicator is used only to find the end point — the experiment must be repeated without the indicator to obtain the pure salt.