| Qualitative Analysis - Flame Tests | |
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What is the flame test?
The flame test is used to visually determine the identity of an unknown metal of an ionic salt based on the characteristic color the salt turns the flame of a bunsen burner.
How is the test performed?
First, you need a clean wire loop! Platinum or nickel-chromium loops are most common. They may be cleaned by dipping in hydrochloric or nitric acid, followed by rinsing with distilled or deionized water. Test the cleanliness of the loop by inserting it into a bunsen burner flame. If a burst of color is produced, the loop was not sufficiently clean. Ideally, a separate loop is used for each sample to be tested, but a loop may be carefully cleaned between tests.
The clean loop is dipped in either a powder or solution of an ionic (metal) salt. The loop with sample is placed in the clear or blue part of the flame and the resulting color is observed.
What metals do colors indicate?
| Color | Metal |
|---|---|
| Red | Carmine: Lithium compounds. Masked by barium or sodium. Scarlet or Crimson: Strontium compounds. Masked by barium. Yellow-Red: Calcium compounds. Masked by barium. |
| Yellow | Sodium compounds, even in trace amounts. A yellow flame is not indicative of sodium unless it persists and is not intensified by addition of 1% NaCl to the dry compound. |
| White | White-Green: Zinc |
| Green | Emerald: Copper compounds, other than halides. Thallium. Blue-Green: Phosphates, when moistened with H2SO4 or B2O3. Faint Green: Antimony and NH4 compounds. Yellow-Green: Barium, molybdenum. |
| Blue | Azure: Lead, selenium, bismuth, CuCl2 and other copper compounds moistened with hydrochloric acid. Light Blue: Arsenic and come of its compounds. Greenish Blue: CuBr2, antimony |
| Violet | Potassium compounds other than borates, phosphates, and silicates. Masked by sodium or lithium. Purple-Red: Potassium, rubudium, and/or cesium in the presence of sodium when viewed through a blue glass. |
What are the limitations of this test?
The value of the flame test is limited by interference from other brighter colors and by ambiguities where certain different metals cause the same flame color. Sodium, in particular, is present in most compounds and will color the flame. Sometimes a colored glass is used to filter out light from one metal. Cobalt glass is often used to filter out the yellow of sodium.
Primary Reference: Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.
