Potassium Flame Test

Identifying potassium by its lilac/violet flame

Flame Tests Beginner (Middle School) 20 min ~$6.00

Objective

Observe the lilac/violet flame color produced by potassium ions, and learn to use cobalt blue glass to distinguish it from sodium contamination.

Background

Potassium produces a pale lilac or violet flame when heated. The emission occurs at 766.5 nm and 769.9 nm (near-infrared/deep red doublet). Because the flame is quite faint and easily overwhelmed by sodium's intense yellow, cobalt blue glass is used as a filter to block sodium emission and reveal potassium's true color.

Safety Warnings

  • Wear safety goggles at all times
  • Keep hair tied back near open flames
  • Handle HCl with care

Required PPE

goggles lab_coat

Materials

  • Potassium chloride (KCl) (1 g)
  • Hydrochloric acid (HCl) (10 mL)
    1M concentration
  • Cobalt blue glass (1 piece)
    For filtering sodium light

Equipment

Bunsen burner Nichrome wire loop Watch glass Forceps

Procedure

1

Clean the nichrome wire thoroughly by dipping in HCl and heating.

2 min
2

Place potassium chloride on the watch glass and collect a sample on the clean wire.

1 min
3

Hold the sample in the Bunsen burner flame. Observe with the naked eye first.

2 min
4

Now observe the flame through cobalt blue glass to filter out any sodium contamination.

2 min
5

Record the difference in appearance with and without the cobalt blue glass filter.

2 min

Expected Results

Without the filter, you may see a faint lilac color possibly masked by yellow from sodium impurities. Through the cobalt blue glass, a clear violet/lilac flame should be visible as the glass absorbs the yellow sodium emission.

Cleanup

Clean the wire loop with HCl. Return cobalt blue glass to storage.

Details

Category
Flame Tests
Difficulty
Beginner (Middle School)
Duration
20 min
Est. Cost
$6.00
Steps
5
Materials
3

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Frequently Asked Questions

What is the objective of Potassium Flame Test?
Observe the lilac/violet flame color produced by potassium ions, and learn to use cobalt blue glass to distinguish it from sodium contamination.
How difficult is Potassium Flame Test?
This experiment is rated as Beginner (Middle School). It takes approximately 20 minutes to complete.
What safety precautions are needed for Potassium Flame Test?
Key safety precautions include: Wear safety goggles at all times; Keep hair tied back near open flames; Handle HCl with care.
What materials are needed for Potassium Flame Test?
The main materials required are: Potassium chloride (KCl), Hydrochloric acid (HCl), Cobalt blue glass.
What results should I expect from Potassium Flame Test?
Without the filter, you may see a faint lilac color possibly masked by yellow from sodium impurities. Through the cobalt blue glass, a clear violet/lilac flame should be visible as the glass absorbs the yellow sodium emission.