Flame Test Introduction
The flame test is used to visually
determine the identity of an unknown metal or metalloid ion based on the
characteristic color the salt turns the flame of a Bunsen burner. The
heat of the flame excites the electrons of the metals ions, causing them
to emit visible light. Every element has a signature emission spectrum
that can be used to differentiate between one element and another.
How to Do the Flame Test
Classic Wire Loop MethodFirst,
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 gas flame.
If a burst of color is produced, the loop is not sufficiently clean. The loop must be 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.
Wooden Splint or Cotton Swab MethodWooden
splints or cotton swabs offer an inexpensive alternative to wire loops.
To use wooden splints, soak them overnight in distilled water. Pour out
the water and rinse the splints with clean water, being careful to
avoid contaminating the water with sodium (as from sweat on your hands).
Take a damp splint or cotton swab that has been moistened in water, dip
it in the sample to be tested, and wave the splint or swab through the
flame.
Do not hold the sample in the flame as this would cause the splint or swab to ignite. Use a new splint or swab for each test.
How to Interpret Flame Test Results
The sample is identified by comparing the observed flame color against known values from a table or chart.
RedCarmine to Magenta: Lithium compounds.
Masked by barium or sodium.
Scarlet or Crimson: Strontium compounds. Masked by barium.
Red: Rubidium (unfiltered flame)
Yellow-Red: Calcium compounds. Masked by barium.
YellowGold: Iron
Intense
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.
WhiteBright White: Magnesium
White-Green: Zinc
GreenEmerald: Copper compounds, other than halides. Thallium.
Bright Green: Boron
Blue-Green: Phosphates, when moistened with H
2SO
4 or B
2O
3.
Faint Green: Antimony and NH
4 compounds.
Yellow-Green: Barium, manganese(II), molybdenum.
BlueAzure: Lead, selenium, bismuth, cesium, copper(I), CuCl
2 and other copper compounds moistened with hydrochloric acid, indium, lead.
Light Blue: Arsenic and come of its compounds.
Greenish Blue: CuBr
2, antimony
PurpleViolet: Potassium compounds other than borates, phosphates, and silicates. Masked by sodium or lithium.
Lilac to Purple-Red: Potassium, rubidium, and/or cesium in the presence of sodium when viewed through a blue glass.
Limitations of the Flame Test
- The test cannot detect low concentrations of most ions.
- The
brightness of the signal varies from one sample to another. For
example, the yellow emission from sodium is much brighter than the red
emission from the same amount of lithium.
- Impurities or
contaminants affect the test results. Sodium, in particular, is present
in most compounds and will color the flame. Sometimes a blue glass is
used to filter out the yellow of sodium.
- The test cannot
differentiate between all elements. Several metals produce the same
flame color. Some compounds do not change the color of the flame at all.
Primary Reference: Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.
Flame Test Colors
| Symbol | Element | Color |
| As | Arsenic | Blue |
| B | Boron | Bright green |
| Ba | Barium | Pale/Yellowish Green |
| Ca | Calcium | Orange to red |
| Cs | Cesium | Blue |
| Cu(I | Copper(I) | Blue |
| Cu(II) | Copper(II) non-halide | Green |
| Cu(II) | Copper(II) halide | Blue-green |
| Fe | Iron | Gold |
| In | Indium | Blue |
| K | Potassium | Lilac to red |
| Li | Lithium | Magenta to carmine |
| Mg | Magnesium | Bright white |
| Mn(II) | Manganese(II) | Yellowish green |
| Mo | Molybdenum | Yellowish green |
| Na | Sodium | Intense yellow |
| P | Phosphorus | Pale bluish green |
| Pb | Lead | Blue |
| Rb | Rubidium | Red to purple-red |
| Sb | Antimony | Pale green |
| Se | Selenium | Azure blue |
| Sr | Strontium | Crimson |
| Te | Tellurium | Pale green |
| Tl | Thallium | Pure green |
| Zn | Zinc | Bluish green to whitish green |