You can use a flame test to help identify the composition of a sample. The test is used to identify metal ions (and certain other ions) based on the characteristic emission spectrum of the elements. The test is performed by dipping a wire or wooden splint into a sample solution or coating it with the powdered metal salt. The color of a gas flame is observed as the sample is heated. If a wooden splint is used, it's necessary to wave the sample through the flame to avoid setting the wood on fire. The color of the flame is compared against the flame colors known to be associated with the metals. If a wire is used, it is cleaned between tests by dipping it in hydrochloric acid, followed by a rinse in distilled water.
Flame Colors of Metals
- magenta: lithium
- lilac: potassium
- azure blue: selenium
- blue: arsenic, cesium, copper(I), indium, lead
- blue-green: copper(II) halide, zinc
- pale blue-green: phosphorus
- green: copper(II) non-halide, thallium
- bright green: boron
- pale to apple green: barium
- pale green: antimony, tellurium
- yellowish-green: manganese(II), molybdenum
- intense yellow: sodium
- gold: iron
- orange to red: calcium
- red: rubidium
- crimson: strontium
- bright white: magnesium
Notes about the Flame Test
The flame test is easy to perform and does not require special equipment, but there are drawbacks to using the test. The test is intended to help identify a pure sample; any impurities from other metals will affect the results. Sodium is a common contaminant of many metal compounds, plus it burns brightly enough that it can mask the colors of other components of a sample. Sometimes the test is performed by viewing the flame through blue cobalt glass to strip the yellow color from the flame.
The flame test generally can't be used to detect low concentrations of metal in a sample. Some metals produce similar emission spectra (for example, it may be difficult to distinguish between the green flame from thallium and the bright green flame from boron). The test cannot be used to distinguish between all metals, so while it has some value as a qualitative analytical technique, it must be used in conjunction with other methods to identify a sample.