This paper is a lab report on the determination of the absorptivity value of food dye. It starts off with listing the objectives for the experiment and then it goes on to describe some bit of theory on the experiment itself. Beer’s law which states that Absorbance of light of a substance is directly proportional to the product of its concentration and absorptivity value and the path length of the vial. The paper looks at the process of setting up and performing the experiment with all the apparatus needed. The major apparatus was the colorimeter which determined the intensity of the light. After the methods and the materials used, the paper looks at the results and their analysis. It is at this point that we realize how to plot a calibration curve that helped in the establishing of a relationship between the Absorbance and Concentration and the slope of the graph which was then used to calculate the absorptivity value of the food dye. It then moves to the conclusion where we realize that Beer’s law holds for any substance that can absorb light.
Keywords: absorbance, intensity, concentration,
An Experiment to Determine Absorptivity Value of Food Dye Using Beer’s Law
Objective
This experiment was aimed at determining the absorptivity value of food dye using Beer’s law. The experiment was also aimed at realizing how to utilize the linear equation in Beer’s law to calculate the absorptivity value of the food dye.
Introduction
There are various methods of determining the concentration of a solution and thus it’s absorptivity value. The experiment was aimed at investigating and verifying Beer’s law. The law relates the concentration of a solution to its light absorbance. Beer’s law is given by the following equation:
A=ELC where,
A: Absorbance E: absorptivity value L: path length of the vial
C: concentration of the solution
From the above equation, A
is directly proportional to E, L, and C. L is a constant since the vial does not change width during the experiment. C changes as the number of drops of the dye to the solution increases. E is the factor under investigation.
Materials/Methods
Apparatus
Colorimeter
Dropper
Test tube
Procedure
The experiment incorporated the use of a colorimeter. Its principles of operation are fairly simple. A white LED shines through the vial and on the other side, a photoresistor (light sensor) detects the amount of light passing through the solution at any given moment. Addition of more solute particles (food dye) to the solvent (water) increased the amount of light that was being absorbed by the solution resulting in less light reaching the photoresistor. The initial intensity, I0, is the amount of light emitted by the LED. The intensity, I, is the amount of light that is detected by the photoresistor. After each reading of a value of I, the value is saved by pressing the left button and reset by pressing the right button.
At the start of the experiment, it was crucial to know the mL in a single drop of solution and thus, 2 mL of the solution was collected in the dropper and then the dropper was emptied drop by drop. The number of drops taken to empty the dropper were counted and divide with the 2 mL to obtain the value of a single drop.
A suitable table of results was drawn.
Drops Added
I
The first value of I, Ii, was recorded when the concentration was zero. The vial was filled with water and the intensity value was measured and saved. Five more drops of the solution were added to the vial and the solution was mixed then left to settle as the intensity value stabilized. Upon stabilization, the number of drops added and the intensity value were recorded in the table of results.
The experiment was repeated twenty-nine other times for consistency of results and accumulation of enough test data.
Results
Upon the completion of the experiment and collection and recording of data, it was necessary to analyze the data. The concentration and the absorbance were calculated from the number of drops added and the intensity respectively.
C=(x*V*R)/(x*V*(1-R)) +20 where,
C: Concentration x:
# drops added V: mL of solution per drop of solution= 20mL
R: ratio of dye to water in solution = 1:10
A=log (Ii/I) where,
A: Absorbance Ii: Initial I: Intensity
Table of Results
Drops Added
Concentration, C
Intensity, I
Absorbance
0
0
884
0
5
0.001236094
886
-0.000981457
10
0.002444988
881
0.001476357
15
0.00362757
883
0.000491561
20
0.004784689
861
0.011449114
25
0.00591716
832
0.026328939
30
0.007025761
792
0.047727083
35
0.00811124
745
0.074295992
40
0.009174312
697
0.103219487
45
0.010215664
683
0.112031561
50
0.011235955
531
0.221357744
55
0.012235818
471
0.273431358
60
0.013215859
560
0.198264238
65
0.014176663
590
0.175600253
70
0.01511879
593
0.173397572
75
0.016042781
593
0.173397572
80
0.016949153
582
0.18152928
85
0.017838405
566
0.193635834
90
0.018711019
551
0.205300666
95
0.019567456
534
0.218911008
100
0.020408163
521
0.229614542
105
0.021233569
507
0.241444306
110
0.022044088
495
0.251847066
115
0.022840119
486
0.259815996
120
0.023622047
478
0.267024368
125
0.024390244
471
0.273431358
130
0.025145068
461
0.28275134
135
0.025886865
454
0.289396412
140
0.02661597
447
0.296144742
145
0.027332705
441
0.302013676
From the results above, two graphs were plotted, a graph of Absorbance against Concentration and a graph of Intensity against Concentration.
A Graph of Absorbance Against Concentration
Using the absorbance equation, A=ELC, analysis was done to know what EL represented since A and C were the only two variables in the equation. A/C
= EL. This implies that EL represents the slope of the graph. Closer inspection of the graph made sense since the slope is usually calculated from dividing the change in the y-axis variant with the change in the x-axis variant. The data showed some form of consistency between the start and end points of the experiment. The value makes perfect sense when you consider the relation between all the variables and we also consider Beer’s law. The value was the same (around eleven point something) on the points that were located on the line of best fit, the only difference was in the decimal points. The other points’ values were either higher or lower. This meant that the value of EL remains constant through the data points if and only if great accuracy and precision is maintained throughout the experiment.
The trend line for this graph is given by the following equation
y = 11.862x - 0.0073 R² = 0.8831
It was possible to determine E, the absorptivity value after finding the equation of the line of best fit.
From A=ELC, we know that EL was approximately 11.862
E=11.862/2.6= 4.562 mLmol-1cm-1
A Graph of Intensity Against Concentration
The XY scatter plotted graph on closer inspection revealed that the line formed after joining the scatter points would be a curve and not a line of best fit as the other graph. This made sense because there was no clear linear relationship or equation between the intensity and the concentration. This was quite hard to comprehend at first until I realized intensity is connected to the absorbance and that is connected to the concentration using Beer’s law and they form a curve instead of a straight line.
Conclusion
It can be concluded that the experiment was a success and all the data collected ensured that it was possible to calculate the absorptivity value E. The experiment also enabled the realization of the Beer’s law equation which was only realized after the drawing and analysis of the two graphs. The analysis of the data managed to prove without a doubt that Beer’s law holds for any substance. In the experiment, a calibration curve was plotted in excel. The linear equation derived from the calibration curve was then manipulated and the absorptivity value of food dye was found to be E=11.862/2.6= 4.562 mLmol-1cm-1
Reference
Thomasson, K., Lofthus-Herschman, S., Humbert, M., " Kulevsky, N. (1998). Applying Statistics in the Undergraduate Chemistry Laboratory: Experiments with Food Dyes. Journal Of Chemical Education, 75(2), 231. doi: 10.1021/ed075p231
