Introduction Flame photometry, Experiment 10: Determination of Sodium by Flame photometry flame photometry” is a relatively old instrumental analysis method. Its origins date back to Bunsen's flame-color tests for the qualitative identification of select metallic elements. As an analytical method, atomic emission is a fast, simple, and sensitive now more properly called flame atomic emission spectrometry or method for the determination of trace metal ions in solution. Because of the very narrow ca. 0.01 nm) 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 with no major interferences present are about E1-5% relative. Detection limits can be quite low." Good elements typically have detection limits between about 1 ng/ml and 1 ug/ml. The method is suitable for many metallic elements, especially for those metals that are easily excited to higher energy levels at the relatively cool temperatures of some flames -Li, Na, K, Rb, Cs, Ca Cu, Sr, and Ba. Metalloids and nonmetals generally do not produce isolated neutral atoms in a flame, but mostly as polyatomic radial and ions. Therefore, nonmetallic elements are not suitable for determination by flame emission spectroscopy, except for a very few and under very specialized conditions. Flame photometry is a highly empirical, rather than an absolute, method of analysis such as gravimetry. That is, you must calibrate the method carefully and frequently. Many different experimental variables affect the intensity of light emitted from the flame and that finding its way to the detector. Therefore, careful and frequent calibration is required for good results. 19
Standard Sodium Stock Solution, 100.0 ppm. 1. Accurately (to 0.1mg) weight out by difference 0.1271 g of reagent grade NaCl into a small plastic weighing boat. It is very difficult and time consuming to weigh out exactly this amount. Get it as close as you reasonably can, record the exact mass, and correct your Preparation of solution: concentrations accordingly. Remember: NEVER transfer chemicals inside an analytical balance. 2. Carefully transfer the salt quantitatively into a 500-ml volumetric flask. Use a few squirts of deionized water from your wash bottle on the weighing boat and the sides of the flask to wash all of it down into the flask. (0.100 g Na/L=100 mg /L=100 ug/ml =100 ppm Na). 3. Add about 100 ml of deionized water to the flask, swirl several times, and dissolve all of the salt before diluting to volume with deionized water. Sodium Standard Calibration Solutions 1. Use deionized water for the "blank”. Pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of the standard 100-ppm sodium solution into the first, second third, fourth, and fifth 100-ml volumetric flasks, respectively. 3. Dilute carefully to the mark with deionized water and mix thoroughly. Unknown Solution Obtain the unknown from the instructor and carefully dilute to the 100-ml mark with deionized water. Mix thoroughly.
Flame Photometer is a law-temperature (air/natural gas) flame atomic emission lorometer designed for the routine determination of sodium and potassium in aqueous solutions. Additional filters are available for this instrument for lithium and calcium. The Higure below shows the main parts of the used flame photometer. lastrumentation: Flame Photo detector Filter Readout 0-0-1 4 10.0 Amplifier Gas inlet Nebulizer Constant bead drain Mixing chamber Waste Air 'U' tube Schematic of a flame photometer Equipment needed: • Wash bottles rinsed and then filled with deionized water. • One 500-ml volumetric flask. • Assorted volumetric and/or graduated transfer pipets. • Five 100-ml volumetric flasks for the standards. 30
Carefully follow the instructions provided you for use of the instrument and measure the emission intensity for the blank (deionized water), each standard, and the Procedure: unknown(s). 1. When you you are approaching the time to begin taking emission reading, call the Teaching Assistant to light the flam, stabilize the flame photometer, and instruct in its proper and safe use. The instrument should have been turned on and the flame lit for 15 minutes (aspirating deionized water) to ensure stability. 2. Thoroughly rinse all the equipment you will use in this experiment, first with lots of distilled water, secondly with deionized water from a rinse bottle. 3. Fill the tall, 25-ml, capped polyethylene vials with the Blank (deionized water), the five standards (1, 2, 3, 4, and 5 ppm Na) and the unknown solutions in that order – and place them in the plastic holder designed for them. Because water droplets cling to the vials, their insides will need to be pre-rinsed with small amounts of their solutions first. Put a ml or two into a vial, cap it, shake it, then shake the contents into the sink. Do this at least 3 times for each vial. 4. Aspirate deionized water until the meter reading stabilizes, this may take 30-90 sec. Use the blank knob to set the meter reading to 0.00. then aspirate the highest standard (5ppm) until the meter reading has stabilized. 5. Repeat the two-step calibration procedure with deionized water and the 5ppm standard as many times as it takes to get them both stabilized at 0.00 and 5.00, respectively. 6. Aspirate the blank, the 5 standards, and the unknowns in that order. Take three replicate readings of each solution once the meter reading has stabilized. 32
2 For the second calibration run, place the unknown solutions between the two standards whose readinds bracket that of unknowns, so that the concentrations of the solutions aspirated now all increase monotonically. Atomic emission instruments work best when going from low to high 8. Repeat the whole process of calibration and taking triplicate reading as before at least 1 or 2 more times. The more data you have to review, the better you will be able to detect and eliminate determinate error - inaccuracy in the final reported concentrations. value. 9. When completely done with the experiment, aspirate deionized water to clean out the aspirator/burner, clean the work areas up thoroughly, and notify a TA that the instrument is ready to be shut down. 10. Thoroughly rins all the glass and plastic ware provided you for the experiment with deionized water. Drain the water vout and put the equipment back in the drawer. Data treatment: 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. Depending on the drift in the instrument and other factors, it may be better to average all three values for each solution and obtain one final value for the unknown, or to get three
separate values for the unknown, each using its "own" calibration curve, and average the three values. References: D.A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed., Chapter 23, pp. 594-631.
جامعة البترا Colantirritu is a measure of column capability 600- 500- 2.78 Peak 2.783 1 400- 4.60 Peak 4.600 2 300- 200- 100- 0 5 10 Compound N Reten. Time [min] 2.783 4.600 Total Result Table (Uncal - Calib\00000002 - ASM2. 110V) I!! Calibration curve could not be constructed for multiple compounds !!! Area Height Area Amount Amount% Response Peak Type (MAU.S) [MAU) [%] [UL] [%] 1667.015 343.399 56.6 1667.015 Error 1277.412 214.162 43.4 1277.412 Error 2944.426 557.561 100.0 1 2. Peak 2.783 Peak 4.600
ITAL.Or 044) 8. Calculate the separation factor (a) for all peaks (selectivity). Selectivity, a, is a measure of column capability to discriminate between different sample components, 9. Calculate the resolution between peaks 1&2, 3&4, and report all results in table 1. k 2 a- Calculations and Treatment of Results: k's where a must always be greater than one. Efficiency, N, is a numerical measure of the rate of zone spreading in the column. A large value of N mcans high efficiency and narrow peaks. Retention time 2 tr N-5.54 x 5.54 Resolution, Rs, is a measure of how well the column has separated two peaks. 2 Atr Rs WA+WB A retention time difference between peaks A and B. W = Peak width at base of beak. Both A tr and W must be measured in the same units. w 1/2 Width at half height or 2 tr 2 Retention time N- 16 x "Width of peak at base = 16 W! Prepare a table from the above parameters and comment on column and instrument performance. PEAK I PEAK 2 PEAK 3 PEAK 4 PEAK 5 where N at half-height is more reproducible to measure and can give a slightly larger value, especially for tailing peaks. Calculate plate height, h, for each peak. hLN where h is independent of column length. Capacity factor, k', is a measure of column ability to retain a sample component relative to an unretained component (or solvent front). Table 1 Absolute retention time Void time Adjusted retention time Baseline width Win Half-height width Xa Separation factor Capacity factor X RS Resolution N Theoretical plates h Plate height t's VR TVM tretm VM IM IM Vk (tr) - retention volume (time) for a sample component. VM (tm) - retention volume (time) for an unretained substance. where k' must always be greater than zero. 55 54