Flame tests and flame emission spectroscopy guide
Chemical analysis • Identification of ions
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Flame test procedure and cause of colours
A small sample of a metal compound is introduced into a hot flame using a clean nichrome wire or a dampened loop. Heat excites outer electrons to higher energy levels; electrons return to lower levels and emit photons at characteristic wavelengths, producing visible colours. Flame colour depends on the energy differences between electronic levels in the metal atoms. Impurities and the flame type change the apparent colour; therefore, controlled conditions and clean equipment are necessary for reliable identification.
Common flame colours and species identification
Distinct metal ions produce characteristic flame colours: sodium yields a strong yellow, potassium yields lilac (often masked by sodium), lithium yields crimson red, calcium yields orange-red, strontium yields red, barium yields apple-green, and copper yields blue-green. Identification uses observed colour matched to known standards. Limiting factors include mixed-metal samples (colour mixing), low concentration (colour too weak), and sodium contamination (dominant yellow emission). Observations require note of intensity and possible masking to avoid incorrect identification.
Principle and data from flame emission spectroscopy (FES)
FES atomises and excites sample atoms in a flame or plasma, disperses emitted light by wavelength, and records emission lines and intensities with a detector. Each metal produces emission lines at characteristic wavelengths; peak position identifies the element and peak intensity relates to concentration. Interpretation of FES data uses wavelength (or line) matching for identification and calibration curves for quantification. Instrumental parameters, such as resolution and background correction, affect detection limits and accuracy.
Interpreting charts and tables from FES
FES output appears as spectra with peaks at specific wavelengths or as tabulated peak wavelengths and intensities. A peak at a known wavelength identifies the corresponding metal ion; proportional changes in peak intensity indicate relative concentration changes given a valid calibration. Quantitative interpretation requires a calibration curve prepared from standards. Signal intensity may saturate at high concentrations and fall below detection limits at low concentrations; therefore, sample dilution or concentration adjustments ensure readings lie within the calibrated range.
Advantages of instrumental methods over chemical tests
Instrumental methods provide higher sensitivity, enabling detection of ions at much lower concentrations than visible tests. Instruments offer greater selectivity because they separate emissions by wavelength, preventing colour masking and allowing multiple elements to be distinguished in mixtures. Additional advantages include quantitative results, faster processing of multiple samples, reproducibility through automated procedures, and reduced sample size. Limitations include instrument cost, need for calibration, and potential spectral interferences that require correction.
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