Acid-Base Titration with Phenolphthalein

Determining the concentration of an acid using a standard base

Titrations Intermediate (High School) 45 min ~$15.00

Objective

Determine the concentration of an unknown hydrochloric acid solution by titrating it with a standardized sodium hydroxide solution, using phenolphthalein as an indicator.

Background

This classic acid-base titration demonstrates stoichiometric neutralization. HCl reacts with NaOH in a 1:1 molar ratio. Phenolphthalein turns pink above pH 8.2, providing a clear endpoint. Accurate burette technique and multiple trials are essential for reliable results.

Safety Warnings

  • NaOH is corrosive — avoid skin and eye contact
  • HCl is corrosive — handle with care
  • Wear safety goggles throughout
  • Wash any spills immediately with water

Required PPE

goggles gloves lab_coat

Materials

  • Hydrochloric acid (HCl) (100 mL)
    Unknown concentration
  • Sodium hydroxide (NaOH) (100 mL)
    0.1M standardized
  • Phenolphthalein indicator (5 mL)
    1% in ethanol
  • Distilled water (500 mL)
    For rinsing

Equipment

50 mL burette Burette stand and clamp 250 mL Erlenmeyer flask 25 mL pipette Pipette filler White tile Wash bottle

Procedure

1

Rinse the burette with a small amount of NaOH solution, then fill it to the 0 mL mark. Record the exact starting volume.

5 min Handle NaOH carefully
2

Using the pipette and filler, transfer exactly 25.0 mL of the unknown HCl solution into the Erlenmeyer flask.

3 min
3

Add 2-3 drops of phenolphthalein indicator to the flask. The solution should remain colorless (acidic).

1 min
4

Place the flask on the white tile under the burette. Begin adding NaOH slowly while swirling the flask continuously.

5 min
5

As the endpoint approaches (pink color lingers longer), add NaOH drop by drop until a permanent pale pink color persists for 30 seconds.

5 min
6

Record the final burette reading. Calculate the volume of NaOH used.

2 min
7

Repeat the titration at least two more times for concordant results (within 0.10 mL).

15 min
8

Calculate the concentration of HCl using: C(HCl) = C(NaOH) x V(NaOH) / V(HCl).

5 min

Expected Results

The endpoint should show a clear transition from colorless to a persistent pale pink. Concordant titre values (within 0.10 mL) indicate good technique. The calculated concentration should match the expected value within experimental error.

Cleanup

Drain the burette and rinse with distilled water. Neutralize waste solutions before disposal. Rinse all glassware thoroughly.

Details

Category
Titrations
Difficulty
Intermediate (High School)
Duration
45 min
Est. Cost
$15.00
Steps
8
Materials
4

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Frequently Asked Questions

What is the objective of Acid-Base Titration with Phenolphthalein?
Determine the concentration of an unknown hydrochloric acid solution by titrating it with a standardized sodium hydroxide solution, using phenolphthalein as an indicator.
How difficult is Acid-Base Titration with Phenolphthalein?
This experiment is rated as Intermediate (High School). It takes approximately 45 minutes to complete.
What safety precautions are needed for Acid-Base Titration with Phenolphthalein?
Key safety precautions include: NaOH is corrosive — avoid skin and eye contact; HCl is corrosive — handle with care; Wear safety goggles throughout.
What materials are needed for Acid-Base Titration with Phenolphthalein?
The main materials required are: Hydrochloric acid (HCl), Sodium hydroxide (NaOH), Phenolphthalein indicator, Distilled water.
What results should I expect from Acid-Base Titration with Phenolphthalein?
The endpoint should show a clear transition from colorless to a persistent pale pink. Concordant titre values (within 0.10 mL) indicate good technique. The calculated concentration should match the expected value within experimental error.