CHE 226 EXPERIMENT 8

Flame Photometric Determination of Sodium

Reference: D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed. Chapter 23, pp. 594­631.

NOTE: A CLEAN, LABELED 100-mL VOLUMETRIC FLASK FOR YOUR UNKNOWN MUST BE TURNED IN AT LEAST ONE PERIOD PRIOR TO THE DAY YOU ARE SCHEDULED TO DO THIS EXPERIMENT. EACH STUDENT WILL HAVE HIS/HER OWN UNKNOWN, EVEN IF WORKING IN PAIRS.

Background

Flame photometry, more properly called flame atomic emission spectrometry, is a fast, simple, and sensitive analytical method for the determination of trace metal ions in solution. Because of the very narrow and characteristic emission lines from the gas-phase atoms in the flame plasma, the method is relatively free of interferences from other elements. Typical precision and accuracy for analysis of dilute aqueous solutions are about ±1-5% relative.

The method is suitable for many metallic elements, especially for those metals which are easily excited to higher energy levels at flame temperature -- Na, K, Ca, Rb, Cs, Cu, Ba. Non-metals generally do not produce isolated neutral atoms in a flame, thus they are not suitable for determination by flame emission spectroscopy.

Flame photometry is an empirical method of analysis -- that is, you must calibrate the method carefully. Many different experimental variables affect the intensity of light emitted from the flame. Therefore, careful and frequent calibration is necessary for good results.

Apparatus

Coleman Flame Photometer, Model 21, with a total-consumption burner using natural gas and oxygen. Wavelength isolation is by use of a simple interference filter. Light from the flame is focused onto the end of a fiber optic cable (a "light pipe") which transmits the light onto a photodiode in the small electronics box by the flame photometer. The electronics therein convert the diode's output into a digital display.

CAUTION ­ Although the flame is quite small, it has so high a temperature that contact of the flesh with even the outer edge of the flame will instantly produce a third­degree burn. Except for lighting the flame, the hands should be kept completely out of the housing whenever the flame is burning, even if it has been turned very low between analyses.

Equipment: One 1000-mL volumetric flask, from your locker

Five 100-mL volumetric flasks, checked out from the TAs

10-mL and 25-mL volumetric pipets; or use a 50-mL burette

Clean all equipment and rinse thoroughly with deionized water before and after use. Use deionized water for all solutions.

Reagents

Standard Sodium Stock Solution: 0.100 g Na⁺ per L: dissolve 0.2542 g NaCl in 1.000 liter of deionized water. (0.100 g/L = 100 mg/L = 100 mg/mL = 100 ppm). If you cannot weigh out exactly this amount, get it as close as you can, record the exact weight, and correct your concentrations accordingly.

Procedure

1. Pipet 10.00, 20.00, 30.00, 40.00, and 50.00 mL of the standard 100-ppm sodium solution into the first, second, third, fourth, and fifth flasks, respectively. Dilute to the mark with water.

2. Use deionized water for the "blank".

3. Obtain the unknown from the instructor and dilute to the 100-mL mark with deionized water. Mix thoroughly.

4. Follow the instructions with the instrument and determine the emission intensity for each standard, the blank (deionized water), and the unknown.

Use of the Flame Photometer

1. When ready to take emission readings, call the TA to light the flame, stabilize the flame photometer, and instruct you in its proper and safe use.

2. Aspirate deionized water for at least 2 minutes to clean out and stabilize the unit. While aspirating deionized water, set the ZERO knob on the read-out unit to 0.0 units. There will be some bounce in the meter reading.

3. Aspirate the most concentrated standard solution, and use the GAIN knob to set the reading to 100.0 units. Recheck the ZERO setting with deionized water and the 100 setting again with the most concentrated standard.

4. Now aspirate all the standards and unknowns in turn, aspirating deionized water in between each one to clean the unit out. The burner shows a "memory effect".

5. Repeat steps 2-4 a second time to get a good duplicate set of measurements. Repeat the whole sequence a third time if necessary to get a third set of values.

6. When done, aspirate deionized water for at least 2 minutes.

7. Call the TA to shut down the photometer.

8. Clean your work areas up.

Waste Disposal

All Na solutions can be safely disposed of by, simply pouring down the drain and running some cold tap water to flush. Rinse out all glassware with deionized water.

Treatment of Data

Prepare a calibration curve by plotting the emission intensities as a function of Na concentration. Determine the concentration of sodium in the unknown sample by reading the concentration of the sample which corresponds to its emission intensity from the calibration curve. Report the concentration of sodium in ppm.

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