Flame photometry is Based on principle that the colour of the flame gives up the concentration of metal ions in the solution as by beer-lambert’s law intensity depends upon the concentration and path length.
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Now ,What is Atomization ?
Reduction of metal ion in the solvent to metal atom by the flame.
Metals of group 1 and group 2 ,easily dissociate due to thermal energy provided by flame.Due to this thermal excitation ,some of the atoms are excited to a higher energy level where they are not stable and return to their ground state by emitting radiation characteristics to that particular element.
Emission process: since higher energy state is unstable the atoms jumps back to the stable low energy state with emission of energy in the form of radiation of characteristic wavelength, which is measured by photo detector.
Parts of Flame Photometer
Burner: Source of flame , that provide flame at constant rate.
Nebulizer :- it aspirate substance to be analyzed to flame as fine spray particles at a steady rate.
Photo detector:- it detect the emitted light & measure the intensity of radiation emitted by the flame.
Compressor:- it maintain air pressure to have steady Flame.
Note:- Absorbance due to electron excitation can be measured by using direct absorption techniques while the emitting radiation intensity is measured using the emission techniques.
Comparison of Emission intensities of unknown samples to either that of standard solutions by plotting calibration curve or to those of an internal standard (standard addion method) helps in quantitative analysis of the analyte metal in the sample solution.
Let’s understand how things works by considering an experiment
Aim: to determine the Li+, Na+,K+ by flame photometry.
•Firstly we prepare standard stock solution for each case by dissolving 254mg-NaCl , 190.7mg-KCl , 532.5mg-LiCO3 in three different 100ml measuring flasks and make up volume with water.(equivalent to 100ppm)
•Take 5 different 100ml measuring flasks in each case and add 5,10,15,20,25ml of stock solution & dilute with distilled water to obtain solution of different concentrations.(5,10,15,20,25ppm)
•Now Switch on the instrument & adjust senstivity control to minimum value then wait for some time(10min).
•Turn on Gas supply and ignite burner with help of lighter or autoignition.
•Adjust air supply with help of compressor until pressure indicate the value 10lbs per square inch.
• Now charged sample beaker with distilled water and place it in position so that atomizer aspirate ,now you’ll see that water is drawn up in the inlet tube by strain of air and atomized to fine mist.
•Regulate gas supply to obtain blue flame with cones to each burner hole. Place appropriate filter in position .(Na=589,K=766,Li=671)
• Now ,Spray standard solution containing ions to be determined then speay distilled water & adjust galvanometer to read zero by zero control.
•Spray standard solution again and readjust sensitivity control to get a deflection of 100 divisions on the meter for highest concentration.(100ppm)
• Then replace above solution by lower concentration solution and note the readings (must be less than 100 corresponding to their concentrations).
• Now plot a graph between meter Readings and concentration (it must be straight line passing through origin.
• Straight line obtained is known as calibration courve and then concentration of unkown sample can be determined by spraying it.
•Flame photometer has both quantitative and qualitative applications.flame photometer with monochromator emits radiations of characteristic wavelength which helps to detect the presence of a particular metal in a sample.
• Easy ,Quick and convenient
Selective and sensitive to even parts per million (ppm) to part per billion (ppb).
•It is difficult to obtai. Accurate results of ions witb higher concentration.
•Information about the molecular structure of the compound present in the sample cannot be determined.
•Elements such as carbon ,hydrogen ,halides cannot be detected due to their non-radiative nature.
•Only limited number of elements can be analyzed.