Acid-Base Titrations - Identifying the End-Point

This page details how to identify the end-point of an acid-base titration using an indicator. Titrations are a fundamental analytical technique used to determine the concentration of a solution. An acid-base titration involves reacting a solution of known concentration (the titrant) with a solution of unknown concentration (the analyte) until the reaction is complete.

What is a Titration?

A titration is a laboratory procedure used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. The reaction between the analyte and the titrant is carefully monitored until the reaction is complete, which is signaled by a change in color – the end-point.

Indicators

An indicator is a substance that changes color depending on the pH of the solution. Indicators are used in titrations to visually signal the end-point of the reaction. The indicator should change color close to the equivalence point of the titration.

How Indicators Work

Indicators exist in two different forms: their acid form and their base form. These forms have different colors. The relative amounts of each form depend on the pH of the solution. When the pH changes, the ratio of the acid and base forms shifts, resulting in a color change.

Identifying the End-Point

The end-point of a titration is the point at which the reaction between the acid and base is complete. Ideally, the indicator should change color very close to the equivalence point, which is the point where the moles of acid are equal to the moles of base.

Steps to Identify the End-Point

1. Carefully add the titrant (solution of known concentration) to the analyte (solution of unknown concentration) while stirring continuously.
2. Add the indicator to the analyte solution before starting the titration.
3. As the titrant is added, the pH of the solution will change.
4. The indicator will change color when the pH reaches the equivalence point.
5. Stop the titration when the color change is stable and persists for at least 30 seconds.

Choosing the Right Indicator

The choice of indicator depends on the strength of the acid and base being titrated. A good indicator should have a pH range of color change that overlaps with the pH at the equivalence point of the reaction.

Indicator	pH Range of Color Change	Suitable Titration
Methyl Orange	3.1 - 4.4	Strong Acid - Strong Base
Phenolphthalein	8.3 - 10.0	Weak Acid - Strong Base; Strong Base - Weak Acid
Bromothymol Blue	6.0 - 7.6	Weak Acid - Weak Base

Important Considerations

Consideration	Details
Stirring	Continuous and gentle stirring is essential to ensure thorough mixing of the solution and uniform distribution of the indicator.
Adding Titrant	The titrant should be added slowly, especially near the end-point, to avoid overshooting the end-point.
Observation	Observe the solution from directly above to ensure accurate color detection.
Stability	Wait for the color change to become stable before recording the end-point.

By carefully following these steps and understanding the principles of indicator use, you can accurately identify the end-point of an acid-base titration.