Experiment Overview

A strong acid strong base titration is used to determine the concentration of an unknown acidic or basic solution by neutralizing it with a solution of known concentration. The titration reaches an equivalence point at pH 7, where the number of moles of acid equals the number of moles of base.

What This Experiment Demonstrates

• Neutralization reactions between strong acids and strong bases
• Use of indicators to determine titration endpoints
• Stoichiometric relationships in acid base reactions

Apparatus Required

• Burette
• Pipette
• Erlenmeyer (conical) flask
• Beaker
• Funnel
• Distilled water

Chemicals Required

• Standard sodium hydroxide solution 0.1 M
• Hydrochloric acid solution of unknown concentration
• Indicator (phenolphthalein or methyl orange)

Theory

In a strong acid strong base titration, a neutralization reaction occurs between hydrogen ions from the acid and hydroxide ions from the base. This produces water and a salt.

The general neutralization reaction is:

\[\ce{H^+ + OH^- -> H2O}\]

For the titration of hydrochloric acid with sodium hydroxide, the overall reaction is:

\[\ce{HCl + NaOH -> NaCl + H2O}\]

The equivalence point occurs when the number of moles of acid equals the number of moles of base. For strong acid strong base titrations, this point occurs at approximately pH 7.

Phenolphthalein is commonly used as an indicator because its color change occurs around pH 8.2 to 10, which coincides with the steep pH change near the equivalence point. The indicator changes from colorless to faint pink.

Method

1. Rinse and fill the burette with standard sodium hydroxide solution 0.1 M.
2. Record the initial burette reading.
3. Pipette a known volume of hydrochloric acid solution into a conical flask.
4. Add 2 to 3 drops of phenolphthalein indicator.
5. Place the flask under the burette and begin adding sodium hydroxide slowly while swirling the flask.
6. As the endpoint approaches, add the base dropwise until a faint pink color persists.
7. Record the final burette reading.
8. Repeat the titration until concordant readings are obtained.

Observations

Initially the solution in the flask is colorless. As sodium hydroxide is added, the solution eventually turns a faint pink color due to the phenolphthalein indicator. This color persists for about 30 seconds and indicates that the endpoint has been reached.

Calculations

• Determine the volume of sodium hydroxide used.
• Volume of NaOH used = Final burette reading − Initial burette reading.
• Use the titration formula:

\[\text{M}_1 \text{V}_1 = \text{M}_2 \text{V}_2\]

• \(\text{M}_1\) represents the unknown concentration of the acid.
• \(\text{M}_2\) represents the known concentration of sodium hydroxide.

Safety Precautions

• Wear safety goggles and gloves.
• Handle acids and bases carefully to avoid spills.
• Rinse any chemical spills immediately with water.
• Always read the burette at eye level to avoid parallax error.

Result

At the endpoint, the solution turns a faint pink color due to the phenolphthalein indicator. This persistent color indicates that neutralization between the acid and base has occurred and allows the concentration of the unknown solution to be calculated.