Ken Yokoyama
CHE 331
February 26, 2003
Dr. Rahni
EXPERIMENT 4: Determination of Calcium, Iron, and Copper in Food. (9-3)
Objectives
This experiment is designed to illustrate how trace amounts of several nutritionally
important elements can be determined by atomic absorption.
Theory
Atomic absorption spectroscopy (AAS) is a widely used technique for determining a large
number of metals. In the most common implementation of AAS, an aqueous sample containing
the metal analyte is aspirated into an air-acetylene flame, causing evaporation of the solvent
and vaporization of the free metal atoms. This process is called atomization. A line source
(hollow cathode lamp) operating in the UV-visible spectral region is used to cause electronic
excitation of the metal atoms, and the absorbance is measured with a conventional UV-visible
dispersive spectrometer with photomultiplier detector.
In this experiment, AAS will be used to determine copper in an aqueous sample. For the
measurements of Cu2+, the flame is used to produce a population of free atoms. The atomic
absorption slot burner head provides a path length of 10 cm. The copper resonance transition is
a doublet, the upper energy level is split into two states at 30784 cm and 30535 cm , and either
-1
-1
of the resulting wavelengths (324.75 nm or 327.40 nm, respectively) can be used. The 324.7
nm line is actually about twice as strong and will therefore give a larger fraction of light
absorbed. It will also be the stronger of the two emitted from the hollow cathode lamp.
Procedure
A. Calibration Curve
1.
After setting the instrument parameters for copper, make appropriate dilutions to
give calibration solutions of 1, 2.5, 5, and 10 ppm of Cu2+ from a 100 ppm
2.
3.
4.
Cu(SO4) stock solution.
Zero the instrument using deionized water in the furnace
Place each standard in the furnace, aspirate, and record the absorbance; repeat
three times.
Plot a calibration curve for the copper solutions (absorbance vs. concentration).
B. Effects of Solvents
1.
Repeat the same steps as part A, except diluting the stock Cu2+ solution with
20% EtOH and 50% EtOH at the concentrations of 1, 2.5, and 5 ppm.
C. Salt Effect
1.
Measure 5 ppm of CuNO3 and 5 ppm of CuCL2 and compare with the 5 ppm of
Cu(SO4), then make a histogram.
D. Unknown Determination
1.
Obtain three unknowns.
2.
With identical instrument settings as for preparing the calibration in Part A,
place the unknown in the furnace and record the absorbance; repeat three times.
3.
Note that serial dilutions may be required to obtain an absorbance reading
within the calibration range.
Treatment of Data
Instrument Parameters:
λ=324.7 nm
Slit = 0.5 nm
Vertical Setting = 5
Airflow = 45
Acetylene flow = 9
Data:
Height
5
11
17
12
Cu (ppm)
1
2.5
5
10
15
Abs.
0.62
0.66
0.686
0.61
-Used 5ppm Cu solution to
standardize the height
Diluted with Water
Abs 1
Abs 2
0.131
0.125
0.340
0.341
0.649
0.648
1.15
1.14
1.50
<-- over
Abs 3
0.130
0.339
0.650
1.15
Diluted with 20% EtOH
Abs 1
Abs 2
Abs 3
0.155
0.149
0.149
0.365
0.370
0.367
0.616
0.616
0.620
-The concentrations 15 ppm and over had absorption over 1.5.
Effects of other salts
Cu (5 ppm) Abs 1
Abs 2
Abs 3
0.352
0.349
0.348
CuNO3
0.177
0.179
0.175
CuCl2
-DI H2O was used as the solvent for these salts.
Unknowns
Cu (? ppm) Abs 1
1.3
Unk. 1
1.491
Unk. 2
0.429
Unk. 3
Abs 2
1.307
1.495
0.431
Abs 3
1.315
1.483
0.422
Diluted with 50% EtOH
Abs 1
Abs 2
Abs 3
0.116
0.115
0.115
0.322
0.325
0.324
0.657
0.651
0.651
Results:
Standard Curve
Cu (ppm)
1
2.5
5
10
15
Abs 1
0.131
0.34
0.649
1.15
1.50
Abs 2
0.125
0.341
0.648
1.14
<-- over
Abs 3
0.13
0.339
0.65
1.15
Standard Curve of
AVG.
0.129
0.340
0.649
1.15
STDEV
0.00321
0.00100
0.00100
0.00569
y = 0.1118x + 0.0492
R2 = 0.9944
Cu
1.2
Absorbance
1
0.8
0.6
0.4
0.2
0
0
2.5
5
7.5
Concentration (ppm)
10
-The correlation of determination is 0.99 for the standard curve of concentration vs.
absorbance.
Salt Effect
5 ppm
Cu(SO4)
CuNO3
CuCl2
Abs 1
0.649
0.352
0.177
Abs 2
0.648
0.349
0.179
Abs 3
0.650
0.348
0.175
AVG.
0.649
0.350
0.177
STDEV
0.00100
0.00208
0.00200
% Abs
100.00%
53.9%
27.3%
Salt Effect
100.00%
-CuSO4 has the least
interference of the salts.
80.00%
60.00%
40.00%
20.00%
0.00%
Cu(SO4)
CuNO3
CuCl2
Effects of Solvent
Solvent
Water
Cu (ppm)
1
2.5
5
1
20% EtOH
2.5
5
1
50% EtOH
2.5
5
Abs 1
0.131
0.340
0.649
0.155
0.365
0.616
0.116
0.322
0.657
Trials
Abs 2
0.125
0.341
0.648
0.149
0.370
0.616
0.115
0.325
0.651
Abs 3
0.13
0.339
0.65
0.149
0.367
0.62
0.115
0.324
0.651
AVG.
0.129
0.340
0.649
0.151
0.367
0.617
0.115
0.324
0.653
STDEV
0.00321
0.00100
0.00100
0.00346
0.00252
0.00231
0.000577
0.00153
0.00346
Effects Of EtOH on Cu
0.700
0.600
Absorbance
0.500
Water
20% EtOH
50% EtOH
0.400
0.300
0.200
0.100
0.000
1
2.5
5
Cu (ppm)
- 20% EtOH seems to have the best sensitivity for copper.
Unknowns:
Unk. 1
Unk. 2
Unk. 3
Abs 1
1.30
1.49
0.429
Abs 2
1.31
1.50
0.431
Abs 3
1.32
1.48
0.422
% Error
Unk. 1
Unk. 2
Unk. 3
Actual
Expected
Conc.(ppm) Conc.(ppm) % Error
11.2
10
12.2%
12.8
10
28.4%
3.39
2.5
35.6%
AVG.
1.31
1.49
0.427
STDEV Conc. (ppm)
0.00751
11.2
0.00611
12.8
0.00473
3.39
Conclusion:
The experiment went well. The precision was good too, with the range of the
standard deviations between 0.000577-0.00346. The SD decreased as the concentration
increased for the DI H2O and 20% EtOH solvents, but it increased for the 50% EtOH.
The standard curve came out well also, with a correlation coefficient of 0.997 and the
samples showed a 99.4% of shared variance. The salt effects showed that 5 ppm of
Cu(SO4) had the most absorption compared to 5 ppm of CuNO3 and 5 ppm of CuCl2.
CuNO3 had a 50% absorption than Cu(SO4) and CuCl2 had a quarter absorption than
Cu(SO4). This showed that Cu(SO4) had the least interference with the absorption of Cu2+
ion.
The 20% EtOH solution seems to show the best effect; it has a best absorption for
1 ppm and 2.5 ppm, but the least for 5 ppm. The 5 ppm 20% EtOH may have been
switched with the 5 ppm 50% EtOH looking at the trends. The 20% EtOH may be the
best solvent for absorption, because it may vaporize better in the nebulizer allowing the
Cu2+ to absorb better. 50% EtOH may interfere with the absorption, because the Cu(SO4)
may not dissociate as well as the DI H2O and the 20% EtOH. The 20% EtOH may be the
right combination for dissociating the Cu and allowing the solvent to vaporize enough to
get a better absorption than the DI H2O and the 50% EtOH.
The unknowns had absorption measurements with standard deviations below
0.008 for three measurements for each sample. Unknown #1 had a concentration of 11.2
ppm and unknown #2 had a concentration of 12.8 ppm. If the concentrations for each of
two unknowns were supposed to be 10 ppm, then there was a 12.2% and a 28.4% error
respectively in accuracy. Unknown #3 had concentration of 3.38 ppm and if the
concentration was supposed to be 2.5 ppm, then there was a 35.6% error.