Mlambo MM : 21507060
DETERMINATION OF IRON
IN PRONUTRO BY ATOMIC
SPECROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY 2
AIM
The aim of this experiment was the determination of iron, magnesium and calcium in
a dry breakfast cereal then minimise the effect of potentially interfering substances in
the cereal by the use of the standard addition technique.
Introduction
Atomic spectroscopy is one of the most widely used methods for quantitative
elemental analysis. There are a number of situations where elemental composition is
important for example how much iron in an ore sample, how much lead in your
drinking water, calcium in intracellular fluids. Trace amounts of certain metals like
iron, calcium and magnesium, nutritional value is somewhat well known in nutrition.
Minerals are an essential element of the human diet, aiding in cellular body
processes from rebuilding tissue to maintaining ion gradients. Often times, minerals
such as iron are found in highest content in foods like spinach and red meats. It is
inconvenient for humans to rely solely on only a few foods to provide the necessary
nutrients in their diets, so it is necessary for cereal companies to add these minerals
as supplements and to allow diets to be more varied and balanced.
In this experiment calcium, iron and magnesium were determined using the method
of standard addition by using an atomic absorption spectroscopy. From the results
obtained calibration graphs of absorbance versus volume and concentration were
plotted. The concentrations of each metal were determined.
METHOD
Three 100ppm stock solutions of calcium, iron and magnesium were prepared from
their salts in 250ml volumetric flasks. A 2g duplicate of cereal was weighed after
being crushed in a mortar. The two masses were then placed in 2 250ml beakers
which were then placed in on a hot plate in a fume hood. 25ml of nitric acid and
perchloric acid were cautiously added to the beakers. They were warmed up until the
sample was colourless. They were transferred to 100ml volumetric flasks and diluted
to the mark with distilled water. 10ml 0f the sample solution was added to thirteen
50ml volumetric flasks. One of them was for a blank, in the rest 2ml, 4ml, 6ml and
8ml standards were prepared from the stock solutions then filled to the mark using
distilled water.
Table of weighed masses
Compound
Mass (g)
CaCl2
0.1820
MgCl2
0.1865
Sample
2.0065
Table of instrument setup
Element
Wavelength
Slit width
Slit height
Calcium
422.67
1.8
0.60
Iron
248.33
1.8
0.60
Magnesium
285.21
2.7
1.05
RESULTS
ANALYSIS OF CALCIUM
Standard
Volume
Concentration Absorbance
1
2
3
4
5
0
2
4
6
8
0
4
8
12
16
0.0005
0.185
0.262
0.342
0.457
absorbance vs volume of calcium
0,5
y = 0,0535x + 0,0353
R² = 0,9708
0,45
0,4
0,35
Abs
0,3
0,25
0,2
0,15
0,1
0,05
0
0
2
4
6
8
volume in ml
CALCULATIONS
Calculations From Graph
2. Volume from graph = -0.65ml
3. CX = - (VS)0x Cs)/VX
= - (-0.67x100)/50
CX = (1.34x50)10
= 6.70ppm
= 1.34ppm
Calculations From Equation
4. Y = 0.0535x+0.0353
0 = 0.0535x + 0.0353
x = -0.666
VS = -0.666ml
5. CX = - (VS)0 x Cs)/VX
= - (-0.666 x 100)/50
= 1.332ppm
CX = (1.332 x 50)/10
= 6.66ppm
10
absorbance vs concentration of calcium
0,5
y = 0,0268x + 0,0353
R² = 0,9708
0,45
0,4
0,35
Abs
0,3
0,25
0,2
0,15
0,1
0,05
0
0
2
4
6
8
10
12
14
conc in ppm
CALCULATIONS
y = 0.0268x + 0.0353
CX = (1.317 x 50)/10
0 = 0.0268x + 0.0353
= 6.59ppm
x = 1.317ppm
CX = 1.317ppm
ANALYSIS OF IRON
Standard
Volume
Concentration
Absorbance
1
2
3
4
5
0
2
4
6
8
0
4
8
12
16
0.023
0.120
0.222
0.310
0.384
16
18
absorbance vs volume of Fe
0,45
y = 0,0456x + 0,0294
R² = 0,9964
0,4
0,35
Abs
0,3
0,25
0,2
0,15
0,1
0,05
0
0
1
2
3
4
5
6
volume in ml
CALCULATIONS
Calculations From Graph
. Volume from graph = -0.7600ml
3. CX = - (VS )0 x CS )/VX
= - (-0.76x100)/50
CX = (1.52x50)/10
= 7.653ppm
= 1.52ppm
Calculations From Equation
4. Y = 0.045x + 0.0294
0 = 0.045x + 0.0294
x = -0.65ml
VS = -0.65ml
5. CX = - (VS )0 x Cs)/VX
= - (-0.65 x 100)/50
= 1.307ppm
CX = (1.307 x 50)/10
= 6.535ppm
7
8
9
absorbance vs concentration of Fe
0,45
y = 0,0228x + 0,0294
R² = 0,9964
0,4
0,35
Abs
0,3
0,25
0,2
0,15
0,1
0,05
0
0
2
4
6
8
10
12
14
conc in ppm
CALCULATIONS
Y = 0.0228x + 0.0294
CX = (1.289 x 50)/ 10
0 = 0.0228x +0.0294
= 6.447ppm
x = 1.289ppm
ANALYSIS OF MAGNESIUM
Standard
Volume
Concentration
Absorbance
1
2
3
4
5
0
2
4
6
8
0
4
8
12
16
0.0006
1.505
1.545
1.568
1.596
16
18
absrbance vs volume of Mg
2
y = 0,1627x + 0,5922
R² = 0,5475
1,8
1,6
1,4
Abs
1,2
1
0,8
0,6
0,4
0,2
0
0
1
2
3
4
5
6
volume in ml
CALCULATIONS
Calculations From Graph
. Volume from graph = -3.64ml
3. CX = - (VS x CS)/ VX
= - (-3.64x100) /50
CX = (7.28x50)10
= 36.4ppm
=7.28 ppm
Calculations From Equation
4. Y = 0.1627x +0.5922
0 = 0.1627x +0.5922
x = -3.6398ml
VS = -3.6398 ml
5. CX = - (VS x Cs)/ VX
= - (-3.6398 x 100)/50
= 7.280ppm
CX = (7.280 x 50)/10
= 36.4ppm
7
8
9
Abs
ABSORBANCE VS CONCENTRATION OF Mg
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
y = 0,0813x + 0,5922
R² = 0,5475
0
2
4
6
8
10
12
14
16
18
conc in ppm
CALCULATIONS
Y = 0.0813x + 0.5922
CX = (7.284 x 50)/10
0 = 0.0813x + 0.5922
= 36.42ppm
x = 7.284ppm
Discussion
The method of standard addition for the determination of calcium, magnesium and
iron was performed well and the graphs of absorbance versus volume and
concentrations of standards were plotted. The concentrations of the samples were
determined from extrapolations from the graphs as well as by using the equation
obtained from the graphs. The concentrations of iron, calcium and magnesium are
closed to each other thus showing good results with good precision. What happened
is that, the calculated concentrations of magnesium do not even fall within that of
standards. There could have been a few different major sources of error which would
have caused the concentration to be in that way.
Some errors might have arose on the part when mixing the standard solutions for the
standard addition portion of the experiment as each member of the group was
performing a different task, resulting in a slightly busy and confusing atmosphere
where measurements for the solutions could have been done incorrectly. There is
also the very great possibility that could have been erred when doing preparations,
which would have caused multiple errors in the rest of the data. There could also
have been random errors associated with measurements taken by the instrument.
Conclusion
In this experiment, we used two methods in order to quantify the concentrations of
calcium, magnesium and iron. The concentrations of calcium, iron and magnesium
from extrapolation in the absorbance versus volume graph were found to be
6.66.ppm, 6.535ppm and 36.40ppm and from the equation they were found to be
6.59ppm, 6.447ppm and 36.42ppm respectively.
Advantages of standard addition technique
1. It does not suffer from spectral interference,which occurs in emission
spectroscopy.
2. It is independent of flame temperature.
3. By atomic absorption technique, traces of one element can easily be
determined in presence of high concentration other elements.
Disadvantages of standard addition technique
1. In aqueous solution, the anion affects the signal to a noticeable degree.
2. This technique has not proved very successful for the estimation of elements like
V, Si, Mo ,Ti and Al because these elements give oxides in the flame.
REFERENCES
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www.csun.edu>hcchm003
sites.chem.colostate.edu>experiments
web.pdx.edu>teach>expt-atomicspec