DETERMINATION OF IRON BY ATOMIC ABSORPTION SPECTROPHOTOMETRY

OBJECTIVES:

1. The student will analyze a sample of iron ore using atomic absorption spectrophotometry.
2. The student will use the calibration curve method of analysis.
3. The student will use the method of standard addition for this analysis.
4. This method also depends on the fact that the entire sample is dissolved.
5. A known and unknown are treated alike.
6. Learn how to operate the Perkin-Elmer 2280 atomic absorption (AA) spectrophotometer.

INTRODUCTION:

In atomic absorption spectroscopy, metal atoms are vaporized into a flame, and the metal vapor will absorb radiation from the specific hollow cathode lamp in proportion to the number of atoms present. Beer's Law is followed in the part-per-million range (remember that ppm means mg of metal/liter of solution).

Each student will need to prepare a series of solutions of the metal that you wish to determine. The concentration of metal should be between 1 and 10 ppm of metal. You will need to prepare a series of standards in the desired concentration range.

Each student will need to be checked out on the AA this term. You will need to run your standard solutions and your unknown solutions. The instrument will read directly in ppm if you do the proper standardization, but it is recommended that you use the absorbance values and Beer's Law for the calculations.

PROCEDURE: Unknown and known should be run on the same day. Dilutions do not keep!

1. Solutions that each student should prepare:

FOR BOTH THE KNOWN AND THE UNKNOWN IRON SOLUTION, dissolve the metal sample in a 150-ml beaker on a hot plate under the hood. Cover the beaker with a watch glass. Quantitatively transfer the solution to a volumetric flask.

a (Fe). Standard iron stock solution (Fe(III) 1000 ppm): dissolve 0.500 g of iron wire in 20 ml of 1:1 HCl and 5 ml of concentrated HNO3 to oxidize all of the iron. Crumple wire and cover with acid, heat may be needed. Place a watch glass over the beaker. Boil until dense yellow brown fumes of NO2 cease. Transfer carefully to a 500-ml. vol. flask. Remember to rinse the watch glass. Dilute to 500 ml. This solution contains a known ppm of iron. Calculate the value using your weight.

b (Fe). Solution of unknown iron ore. Take 0.1 gram of the iron ore sample. Add 10 ml. of conc. HNO3 and boil for 10 minutes (or until dissolved). Add 10 ml. 1:1 HCl. Heat to boiling -- check to see if the sample is dissolved (no red particles). Dilute to 500 ml. in a volumetric flask after all of the sample is dissolved.

c. 1% (V/V) HCl for dilutions. 1 ml concentrated HCl per 99 ml H2O. Or plan to make your solutions in 100-mL volumetric flasks and then add 1 mL of concentrated HCl just before diluting with deionized water.

3. In 100 ml volumetric flasks prepare a series of standard solutions containing 1.00, 2.00, 4.00, 5.00, 6.00, 7.00 and 8.00 mL of your standard iron solution. (Use either a buret or a EPPENDORF pipet to measure the standard solution). Dilute to the mark with 1% HCl SOLUTION. After determining the absorbance, make a plot of absorbance vs. concentration USING EXCEL. The best results are obtained by using the regression equation of the data. Discard obviously bad points before doing the regression.

4. In 100 ml. volumetric flasks prepare a series of unknown solutions containing 1.00, 2.00, 2.50, 3.00, 4.00, and 5.00 mL of your unknown iron solution. (Use either a buret or a EPPENDORF pipet to measure the standard solution). Dilute to the mark with 1% HCl SOLUTION. Determine the absorbance of each solution. Using the data from the standard determine the concentration of each unknown solution. Also calculate the % of metal in the unknown.

5. Before making the first spectral measurement, you should receive instructions from Dr. Sherren on the use of the spectrophotometer. Set the wavelength for Fe [use 248.3 nm]. Sets slit width at 0.2 nm and maximize the wavelength. Read the section on the AA operation in the appendix.

6. Calculate the %metal in the unknown sample. Best results are obtained by calculating the best straight line on the computer. Use an Excel spread sheet and either make a chart and insert the trend line or do a linear regression of the known values. Then use the regression equation to calculate the value of each unknown solution.

CAUTION: Most solutions for the AA are prepared in dilute acid. Each ion will have a given recipe. Use 1% HCl for all dilutions.

CAUTION: Solutions of this dilute nature do not keep. You should prepare and do the determination the same day. All of you cannot use the instrument at once. It does go rapidly, but until you get the hang of it, it will take some time and some individual instruction.

CAUTION: Between each run, determination, or standardization; you will need to aspirate 1% HCl for a minimum of one (1) minute to clean out the burner chamber. After finishing, you will also need to aspirate 1% HCl into the flame to clean out the mixing chamber. If in doubt, come get Dr. Sherren.

CAUTION: Often it is necessary to take a solution in the AA and use the technique of standard addition. If your unknown values are much lower that the Absorbance values for the standards, check with Dr. Sherren on the directions for standard addition.

EXPERIMENT:

This experiment is to be done during regular laboratory hours. Dr. Sherren needs to check you out on the instrument before you begin the first and second times.

SAFETY AND DISPOSAL INFORMATION: Dispose of all test solutions and in the "AA EXPERIMENT WASTE" container. CAUTION: Remember to treat all chemicals with respect.

REFERENCES

Skoog, West, Holler: Fundamentals of Analytical Chemistry (1996) pp. 614-638.

Sawyer, Heineman, & Beebe: Chemistry Experiments for Instrumental Methods (1984) pp. 242-253.

"General Information" section of manufacturer's Analytical Methods book.

Return to the Table of Contents

INTRODUCTION TO THE OPERATION OF THE INSTRUMENT
The Perkin-Elmer 2280 is a microprocessor controlled, single beam atomic absorption spectrophotometer.
                1. Make sure the GAIN and LAMP 1 dials are turned fully counterclockwise,
                then turn on the instrument and the air compressor. Find the log book and signin.
                2. Turn on the AA. (It takes 30 minutes for the lamp to warm up for max. use.)
                Set the Lamp current (see instrument manual): The proper lamp current can be
                found, circled in ink, on the base of the lamp.
                3. Turn on the acetylene tank. It must read over 90 on the tank gage.

4. Set the wavelength
       for Fe -- use 248.3 nm. Set slit width at 0.2 nm and maximum nm.
       for Cu -- use 324.8 nm. Set slit width at 0.7 nm and maximum nm.
       for Ni -- use 232 nm. Set slit width at 0.2 nm and maximum nm.
5. Check the fuel gage. It should read 17 or 18.
6. Check the oxidant gage. It should read 33 or 34
7. Light the flame.
8. Aspirate a 1% HCl blank for about 1 minute.
9. Set AZ (autozero). Set the average = 3 and t = 2.
10. Aspirate the standard for 1 minute. Read the absorbance.
11. Aspirate 1% (v/v) HCl for one minute (or 90 seconds). Does it read 0.00? If way off (0.4 or more), repeat steps 7 to 11.
12. To read your solution, aspirate for one minute, and then take at least 3 readings.
13. Average the readings for each solution.
14. Calculate the % metal for each of your solutions.
15. Reject bad data and calculate the mean and relative standard deviation for your unknown.
INTRODUCTION TO THE OPERATION OF THE INSTRUMENT
The Perkin-Elmer 2280 is a microprocessor controlled, single beam atomic absorption spectrophotometer. Among its principle components are a hollow cathode lamp (in our case it also happens to be a multielement lamp), a premix burner, and a grating monochromater.
The parts of the instrument manual which will be of most use to you are the following:
topic section in manual
- instrument controls and indicators 7A
- normal operating procedure 9C,1
- controls for the digital readout Appendix 2, sec. 3
    burner control
- operation of the digital readout Appendix 2, sec. 4
burner control
- averaging and statistical operations 10I
The burner height and fuel/air ratio must be optimized. This may be done for you or you will be shown how to do it.

INSTRUMENT SHUTDOWN
1. Before shutting off the flame, aspirate deionized water for at least 5 minutes to prevent possible explosions (see section 12B,4). Then shut off the flame by carrying out the following steps in sequence:
a) close the main valve of the acetylene tank,
b) press the FLAME OFF button or let the flame die out by itself,
c) turn off the air compressor,
d) press CHK FUEL long enough to depressurize the acetylene lines.
2. Turn the GAIN and LAMP 1 dials fully counterclockwise. Turn off the instrument power and sign out of the log book.

REFERENCES
Skoog, West, Holler: Fundamentals of Analytical Chemistry (1996) pp. 611-636.
Sawyer, Heineman, & Beebe: Chemistry Experiments for Instrumental Methods (1984) pp. 242-253.
"General Information" section of manufacturer's Analytical Methods book.

CAUTION:The iron ore should be dried on the day it is weighed.

This may mean re-drying the sample.
DO NOT OVER DRY THE SAMPLE!