Flame Emission Spectroscopy : FES

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Introduction

1. Optimization of fuel-air ratio for sodium determination by Flame emission spectroscopy

2. Investigation of some factors affecting accuracy of sodium determination

3. Analysis of sodium in sample

Atomic emission spectroscopy (AES) employing flames, also called flame emission spectroscopy (FES) or flame photometry has found widespread application in elemental analysis. Its most important uses have been in the determination of sodium, potassium, lithium and calcium, particularly in biological fluids and tissues. For reasons of convenience, speed, and relative freedom from interferences, flame emission spectroscopy has become the method of choice for these otherwise difficult to determine elements. The method has also been applied, with varying degree of success, to determine of perhaps half the elements in the periodic table.

Theory

Flame photometry is an analytical technique based on the spectrum of an element when a solution containing it is aspirated into a flame (e.g. propane/air, acetylene/air) which is hot enough to cause the element to emit its characteristic radiation. The spectrum is normally relatively simple, consisting of only a few lines, and measurement of the intensity of one or more of these provides a highly sensitive measure of the concentration in the sample. When several elements are present, the required sprectral line is isolated either by passing the light from the flame into the entrance slit of a monochromator by means of a lens or mirror or by a system of optical filters. The intensity of the isolated radiation is measured by a photosensitive detector coupled to an amplifier and recorder.
In the analysis of sodium and potassium in the presence of calcium, some interference by the latter due to spectral overlap occurs. The addition of a sufficient quantity of aluminium ions to the analyte solution tends to reduce the emission due to calcium and hence minimize the interference.
Potassium determination by flame emission is affected mainly by ionization of potassium at the high temperatures associated with air/acetylene or hotter flames, especially at low concentrations of the elements. This effect is however, neglible for the air/propane flame as used by the flame analyser in this experiment. Therefore, addition of radiation buffers is not required.

Procedure

A. Optimization of the fuel/air flow rate for determination of sodium

(1) Prepare a calibration series containing 1.25, 2.5, 5.0, and 10 ppm sodium in deionized distilled water (50 mls each) from the 50 ppm standard stock sodium solution provided.

(3) Set the sodium filter in position before the photocell and set the air pressure to the burner as recommended (~20 psi)

(4) Depress ignition switch to light the flame and slowly and slowly increase the fuel flow rate. Once the flame is lit, observe the flame, and carefully adjust the fuel flow rate until a non-luminous flame is obtained. Allow the temperature to equilibrate for about 3-5 minutes.

(5) Aspirate the 2.5 ppm sodium standard and adjust the sensitivity knob to obtain about an emission reading of about 0.3. Re-zero the instrument while aspirating deionized distilled water

(6) Aspirate again the 2.5 ppm solution, and carefully change the fuel flow rate until a maximum signal is obtained, but avoid using a luminous flame.

(7) Re-zero with deionized distilled water again. The instrument is now ready for use.

B. The effect of aspiration rate on sensitivity of detection of sodium

(1) Into each of four 25 ml volumetric flasks, pipette the required volume of standard 100 ppm sodium standard to give a final concentration 2.5 ppm. Add to the flasks 2.5, 5.0, 7.5, and 10.0 mls respectively of methanol and make up to the mark with deionized distilled water.

(2) Aspirate the 2.5 ppm sodium made up in distilled water only and note the emission reading.

(3) Using deionised distilled water, zero the instrument and then determine the emission of the distilled water standards.

(4) Aspirate the methanol samples in order of increasing methanol content, and note the corresponding readings.

(5) Plot emission vs methanol (0-40%) content of the solution.

What precautions should be taken when determining sodium in alcoholic beverages?

C. Interference of sodium emission by calcium and the countering effect of aluminium

(1) Into five 25 ml volumetric flasks, add the required volume of the 100 ppm sodium standard stock solution that will give respective final concentration values of 1.25, 2.5, 5.0, and 10 ppm. To each flask add 10 ml of the 1000 ppm calcium standard stock solution provided and make up to the mark with deionized distilled water.

(2) Prepare a blank solution containing 10 ml of the 1000 ppm calcium stock solution in 25 ml volume solution.

(3) Into five other 25 ml volumetric flasks, add similar quantities of sodium and calcium solutions. Then add to each 5 ml of the aluminium solution provided and make uo to the mark with distilled water.

(4) Prepare a blank containing 10 ml 1000 ppm calcium standard stock solution and 5 ml of the aluminium solution in 25 ml volume solution.

(5) Aspirate the standard solutions in distilled water, with distilled water as blank, then the sodium-calcium solutions, and then the sodiun-calcium-aluminium solutions, and note the emission/sodium concentration readings.

(6) Note and comment on the change of color of the flame.

(7) Aspirate the respective blanks and subtract their emission values from the sample solution values to provide blank-corrected emission values.

(8) Plot corrected emission values vs sodium concentration of each set of solutions on the same sheet of graph paper, or using a plotter.

Comment on coincidence of the calibration curves, and the effectiveness of the Al⁺³ in suppressing Ca⁺² interference.

D. Analysis of sodium in sample

(1) Place triplicate, accurately weighed or measured quantities of the sample provided into separate boiling tubes

(2) To each tube add 5 ml pure nitric acid and allow to predigest for at least one hour in a fume hood.

(3) Prepare an acid blank simultaneously

(4) Reflux the material gently on dry heating block for one hour in a fume hood, ensuring that fumes are properly vented.

(5) Cool and dilute with 10 ml distilled water

(6) Filter through a Whatman fifter paper No. 1 into 100 ml volumetric flasks and rinse the boiling tube and filter paper with 25 ml distilled water.

(7) To each flask add 10 ml of Aluminium stock solution and make up to the mark with distilled water.

(8) Aspirate your sample solutions along with the solutions of section D (5) and determine the mean sodium content of the samples.

References

1. Flame Emission and Atomic Absorption Spectroscopy, Vols 1, 11, 111.
   Dean, J.A., Rains,T.C. Eds. Marcel Decker, NY. 1969-75
2. Analyst 77 430-436 (1952)
3. Analyst 82 200 (1957)



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