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Investigation of some factors affecting accuracy of sodium determination
and their application in the determination of sodium by Flame Emission Spectroscopy

1. Introduction:

Atomic emission spectroscopy (AES) employing flames, also called flame emission spectroscopy (FES) or flame photometry has found widespread application in elemental analysis (1). 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.

Flame photometry is based on the emission spectrum of an element which is excited in a flame (e.g. propane/air, acetylene/air) which is hot enough to cause the element to allow emission of characteristic radiation. The spectrum may be relatively simple, consisting of only a few lines, or may be complex, broad bands.

Measurement of the intensity of a portion of a spectrum characteristic of an element can provides a measure of the concentration in the sample. The required spectral line or a portion of the spectrum is isolated either by a monochromator or by an optical filter. 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 occurs, due to spectral overlap. 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 (2, 3).

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. However this effect is negligible in the air/propane flame used by the flame analyser in this experiment. Therefore, addition of radiation buffers is not required for potassium analyses with this instrument.

2. Experimental procedures:

(A) Optimization of the fuel/air flow rates for determination of sodium:

(a) Prepare a calibration series containing 1.25, 2.5, 5.0 and 7.5ug/mL sodium respectively in distilled water (25mL each), using the 50ug/mL standard sodium solution provided.

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

(c) Depress ignition switch to light the flame 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 system to equilibrate for about 15min.

(d) Aspirate the 2.5ug/mL sodium standard and adjust the sensitivity knob to obtain an emission reading of about 30 units. Re-zero the instrument while aspirating distilled water.

(e) Aspirate again the 2.5ug/mL solution, and carefully change the fuel flow rate until a maximum signal is obtained. Avoid using a luminous flame, which creates a high background signal.

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

(B) Investigation of effect of aspiration rate on emission signals of sodium:

(a) Into each of four 25mL volumetric flasks, pipette the required volume of standard 100ug/mL sodium standard, to give a final concentration of 2.5ug/mL.

(b) Add to the flasks 2.5, 5.0, 7.5 and 10mL respectively of ethanol and make up to the mark with distilled water. These correspond to 10 to 40% by volume of ethanol in sodium solutions. Similarly prepare a series of 10-40% ethanol blank solutions in distilled water.

(c) Using distilled water, zero the instrument and then aspirate the 2.5ug/mL sodium standard in distilled water (0% EtOH), noting its emission value.

(d) Aspirate and measure the emission intensities of the distilled water sodium standards (1.25 to 7.5ug/mL).

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

(f) Plot emission vs ethanol (0-40% v/v) content of the solution, corrected for blanks.

(g) Exercise: What precautions should be taken when determining sodium in alcoholic beverages?

(C) Interference of sodium emission by calcium and the countering effect of aluminium:

(a) Into five 25mL volumetric flasks, add the required volume of the 100ug/mL sodium standard stock solution that will give respective final concentration values of 1.25, 2.5, 5.0, and 7.5ug/mL.

(b) To each flask, add 10mL of the 1000ug/mL calcium standard stock solution provided and make up to the mark with distilled water.

(c) Prepare a blank solution containing 10mL of the 1000 ppm calcium stock solution in 25mL solution.

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

(e) Prepare a blank containing 10mL of 1000ug/mL calcium standard stock solution and 5mL of the aluminium solution in 25 mL solution.

(f) Aspirate in the following order:

(i) The standard solutions in distilled water (1.25-7.5ug/mL), with distilled water as blank.

(ii) The sodium-calcium solutions (1.25-7.5ug/mL)and blank.

(iii) The sodium-calcium-aluminium solutions (1.25-7.5ug/mL) and blank.

(iv) Sample solutions and blank (preparation in later section).

(v) The sodium-ethanol solutions and blanks.

(vi) Tabulate the emission readings. Note the change of color of the flame in the three types of solutions.

3. Exercises:

(A) Plot blank-corrected emission values vs sodium concentration of each set of solutions (i), (ii) and (iii) on the same sheet of graph paper, or using a plotter.

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

(B) Plot the emission values for the sodium-ethanol solutions, corrected for ethanol- water blanks, vs EtOH concentration.

(a) Discuss the effect of viscosity and fuel/air ratio changes respectively, on the instrumental response to a fixed sodium concentration.
(b) How would you determine the concentration of sodium in an alcoholic beverage?

4. Analysis of sodium in sample:

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

(b) To each tube add 5mL conc. nitric acid and reflux at 135-140°C for 1h. in a fume hood.

(c) Prepare an acid blank simultaneously

(d) Cool and dilute with 10mL distilled water

(e) Filter into 100mL volumetric flasks and make to volume with distilled water rinses of the boiling tubes and filter papers.

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

5. 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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