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Flame Test Introduction

Flame Test Introduction

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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 converts the metal ions into atoms which become excited and emit visible light. The characteristic emission spectra can be used to differentiate between some elements.

How to Do the Flame Test

Classic Wire Loop Method
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 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 Method
Wooden 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 the Results

The sample is identified by comparing the observed flame color against known values from a table or chart.

Red
Carmine 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.

Yellow
Gold: 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.

White
Bright White: Magnesium
White-Green: Zinc

Green
Emerald: Copper compounds, other than halides. Thallium.
Bright Green: Boron
Blue-Green: Phosphates, when moistened with H2SO4 or B2O3.
Faint Green: Antimony and NH4 compounds.
Yellow-Green: Barium, manganese(II), molybdenum.

Blue
Azure: Lead, selenium, bismuth, cesium, copper(I), CuCl2 and other copper compounds moistened with hydrochloric acid, indium, lead.
Light Blue: Arsenic and come of its compounds.
Greenish Blue: CuBr2, antimony

Purple
Violet: 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

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Color

Element

Symbol

Blue

Arsenic

As

Bright green

Boron

B

Pale/Yellowish Green

Barium

Ba

Orange to red

Calcium

Ca

Blue

Cesium

Cs

Blue          

Copper(I)

Cu(I

Green        

Copper(II)   non-halide

Cu(II)

Blue-green

Copper(II)   halide

Cu(II)

Gold

Iron

Fe

Blue

Indium

In

Lilac to red

Potassium

K

Magenta to carmine

Lithium

Li

Bright white

Magnesium

Mg

Yellowish green

Manganese(II)

Mn(II)

Yellowish green

Molybdenum

Mo

Intense yellow

Sodium

Na

Pale bluish green

Phosphorus

P

Blue

Lead

Pb

Red to purple-red

Rubidium

Rb

Pale green

Antimony

Sb

Azure blue

Selenium

Se

Crimson

Strontium

Sr

Pale green

Tellurium

Te

Pure green

Thallium

Tl

Bluish green to whitish green

Zinc

Zn