pH Indicator Rainbow

Mapping indicator color changes across the full pH range

Qualitative Analysis Beginner (Middle School) 45 min ~$8.00

Objective

Observe how different pH indicators change color at specific pH values and understand the concept of indicator transition ranges.

Background

pH indicators are weak acids or bases whose conjugate forms have different colors. Each indicator has a characteristic transition range where it changes color. By testing multiple indicators across a range of buffered solutions, students create a visual map of indicator behavior that reinforces concepts of acid-base equilibrium and the Henderson-Hasselbalch equation.

Safety Warnings

  • Indicators can stain skin and clothing
  • Extreme pH solutions (pH 1, 14) are corrosive
  • Avoid mixing different buffer solutions

Required PPE

goggles lab_coat

Materials

  • Buffer solutions (pH 1–14) (5 mL each)
    Pre-prepared set
  • Methyl orange indicator (5 mL)
    pH 3.1–4.4 range
  • Bromothymol blue indicator (5 mL)
    pH 6.0–7.6 range
  • Phenolphthalein indicator (5 mL)
    pH 8.2–10.0 range
  • Universal indicator (5 mL)
    Full pH range
  • Litmus solution (red and blue) (5 mL each)
    Simple acid/base test

Equipment

Spot plate or microwell plate Dropper pipettes (one per indicator) pH meter or pH paper White background paper

Procedure

1

Arrange the spot plate and add 10 drops of each buffer solution (pH 1 through 14) across each row.

10 min Handle pH 1 and 14 solutions carefully
2

Add 2 drops of methyl orange to the first row of buffer solutions. Record the color at each pH.

5 min
3

Add 2 drops of bromothymol blue to the second row. Record the color at each pH.

5 min
4

Add 2 drops of phenolphthalein to the third row. Record the color at each pH.

5 min
5

Add 2 drops of universal indicator to the fourth row. Observe the full rainbow of colors.

5 min
6

Create a color chart recording each indicator's color at every pH level. Identify each indicator's transition range.

15 min

Expected Results

Methyl orange: red (acidic) to yellow (basic) around pH 3–4. Bromothymol blue: yellow to blue around pH 6–7.6. Phenolphthalein: colorless to pink around pH 8–10. Universal indicator: red through violet across the full pH range.

Cleanup

Pour solutions into the appropriate waste container (acid or base). Rinse spot plates thoroughly with distilled water.

Details

Category
Qualitative Analysis
Difficulty
Beginner (Middle School)
Duration
45 min
Est. Cost
$8.00
Steps
6
Materials
6

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Frequently Asked Questions

What is the objective of pH Indicator Rainbow?
Observe how different pH indicators change color at specific pH values and understand the concept of indicator transition ranges.
How difficult is pH Indicator Rainbow?
This experiment is rated as Beginner (Middle School). It takes approximately 45 minutes to complete.
What safety precautions are needed for pH Indicator Rainbow?
Key safety precautions include: Indicators can stain skin and clothing; Extreme pH solutions (pH 1, 14) are corrosive; Avoid mixing different buffer solutions.
What materials are needed for pH Indicator Rainbow?
The main materials required are: Buffer solutions (pH 1–14), Methyl orange indicator, Bromothymol blue indicator, Phenolphthalein indicator, Universal indicator.
What results should I expect from pH Indicator Rainbow?
Methyl orange: red (acidic) to yellow (basic) around pH 3–4. Bromothymol blue: yellow to blue around pH 6–7.6. Phenolphthalein: colorless to pink around pH 8–10. Universal indicator: red through violet across the full pH range.