flame emission spectroscopy

Chemistry only

You may remember an experiment in your science lessons where you passed white light through a glass prism and the white light was split up into a spectrum of colours: red, orange, yellow, green, blue, indigo, and violet. Remember from your physics lessons that red light is long wavelength light with the wavelength getting smaller as you go towards the violet end of the spectrum.

refraction of light using a prism- dispersion prism

The light emitted from the Sun would produce a spectrum similar to the one above, it would show a continuous band of colours with all the wavelengths of light in the visible spectrum being produced. It is called a continuous spectrum and all the colour are shown below. When sunlight is passed through a prism the light is split up to give the continuous spectrum of colours shown below.

continuous spectrum of colour

The following experiment was set-up, it looks similar to the first image above but this time the white light is shone through a glass window in a lamp containing hot sodium gas.

sodium lamp and spectra

The spectra produced this time is shown below, it is called an absorption spectra.

absorption spetrum of sodium

The spectrum produced this time has several very noticeable dark lines in it? What could be causing these dark lines? Something has absorbed certain wavelengths of light from the continuous spectrum to leave these dark gaps, but what? What has happened is that the electrons in the sodium atoms/ions will absorb certain wavelengths of light and be promoted to different energy levels within the atom, the black lines in the spectrum above correspond to the wavelengths of light that have been absorbed by the electrons inside the sodium ionS. A spectrometer or spectroscope is an instrument that when light is shone into one end , it will produce a line spectra at the other.

If the experiment above is reversed and the light from the hot sodium gas inside the lamp is shone through a spectroscope then the line spectra below is obtained. It has a black background with a lots of coloured lines. These lines correspond to wavelengths of light that are emitted when electrons inside the sodium atom/ion drop from one energy level or shell to another. This is called an emission spectrum for an element. emission spectra of sodium

This particular line spectra is unique for a sodium ion. Other atoms and ions will produce different spectra, the pattern of lines produced will depend on the electron arrangement within the atom or ion and on its charge. Since no two atoms or ions have identical electron arrangements then each element or ion has its own characteristic line spectra, it acts like a fingerprint for the element.

This is the method used by astronomers to determine which elements are in stars. By collecting the light from stars and passing them through spectroscopes then it is possible to identify the elements in the atmospheres of stars. Chemists can also use these emission spectrum to identify particular elements present. All that is needed is the emission spectrum for a large number of ions. Then compare the spectrum of your unknown ions with those in a data bank. There is the added advantage that the intensity of the line (whether it is bright or faded) will indicate the concentration of the particular ion present. This is simply a case of matching up the lines in the spectra, a very tedious and repetitive task that computers are well designed to carry out!

Flame emission spectroscopy is an instrumental method used to identify metal ions present in solutions. The sample is placed in a hot flame, this will excite the electrons in the metals to higher energy levels, once they fall back down to their original energy level inside the ion they will emit light at very particular wavelengths, this light is then passed through a spectrometer and a line spectrum produced. This technique is very sensitive and can detect trace amounts of any particular ion. It is also quickly and easy to do and will produce reliable and accurate results. The only problem is that the sample may contain lots of different metal ions, so the emission spectra will contain a mix of lots of emission lines for the ions present, luckily most ions have very characteristic parts to their line spectra that with a bit of practice makes it easy to identify particular ions e.g. in the sodium emission spectra you will see 2 very bright yellow lines at wavelength 588nm and 589nm, these lines are like a big glowing sign- they are very distinctive of sodium ions-you could also use a computer to do the analysis the sample for you!

Key Points