10.01.2 Flame Tests & Flame Emission Spectroscopy
Syllabus
Flame tests can be used to identify some metal ions (cations).
Lithium, Sodium, Potassium, Calcium and Copper compounds produce distinctive colours in flame tests:
Lithium compounds result in a crimson flame
Sodium compounds result in a yellow flame
Potassium compounds result in a lilac flame
Calcium compounds result in an orange-red flame
Copper compounds result in a green flame.
If a sample containing a mixture of ions is used some flame colours can be masked.
Elements and compounds can be detected and identified using instrumental methods. Instrumental methods are accurate, sensitive and rapid.
Students should be able to state advantages of instrumental methods compared with the chemical tests in this specification.
Flame emission spectroscopy is an example of an instrumental method used to analyse metal ions in solutions. The sample is put into a flame and the light given out is passed through a spectroscope.
The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.
Students should be able to interpret an instrumental result given appropriate data in chart or tabular form, when accompanied by a reference set in the same form, limited to flame emission spectroscopy.
What does this mean?
What's the point of flame tests?
The majority of solids are white powders and difficult to distinguish between just by looking.
Simple tests are needed to quickly tell us what they are made-up of.
Flame tests can give us a good indication of what metal ion it contains.
You should know the colours associated with four metal ions as well as the method used to carry out flame tests.
Method
Dip a Nichrome wire in a concentrated solution of Hydrochloric acid - this cleans the wire and makes it damp. Dirty wires might colour the flame. Nichrome is used because it is an alloy with a high melting point that will not affect the colour of the Bunsen flame.
The slightly damp wire is then dipped into the unidentified powder.
A blue Bunsen flame is needed and the colour change notice when the powder is placed in it should be noted.
Results
Problems with Flame tests.
It can be difficult to see the right colour even with a pure sample - with a sample that contains more than one ion we would not know what colours were mixed together.
And there are over 80 metals, ions of which often give very similar flames.
A Flame-emission Spectrometer splits up the colours from a flame and records the exact wavelength of the colours in the flame.
The wavelengths that each element emit are well established.
More than a dozen metal ions make blue flames that a person couldn't tell apart - but a machine can spot the difference with ease.
And if the sample is mixed, it can tell you exactly which metal ions are mixed together.
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