POSTLAB

De Leon, Jo-Anna Maria N.
3ChEA Group No. 2

Experiment No. 5
Spectrophotometric Determination of pKa

I. Introduction The equilibrium constant of the dissociation of an indicator in water will be determined experimentally using spectroscopic method.This experiment aims to demonstrate the principle of equilibrium and the interaction of matter with light.

II. Methodology

Figure . UV-VIS Spectrophotometer 10 solutions covering the entire pH range from 1 to 13 were prepared using the quantities shown in the table below. Solutions 2 to 9 were prepared by diluting the mixture of 0.1 M NaH2PO4 and 0.1 M Na2HPO4 to 25 ml.
Table 1. Composition of the mixtures
Solution No.
Vol. of Indicator (ml)
Vol of NaH2PO4
(ml)
Vol of Na2HPO4 (ml) pH 2
1
5
0
2
3
1
5
1
3.375
4
1
5
5
4.75
5
1
10
5
6.125
6
1
5
10
7.5
7
1
1
5
8.875
8
1
1
10
10.25
9
1
0
5
11.625

Solution 1 was prepared by diluting the mixture of methyl red indicator, distilled water and HCl to 25 ml while Solution 10 was prepared by mixing the methyl red indicator and 0.1 M NaOH. The pH of solutions 2 to 9 was first adjusted using the potentiometer and by adding 0.1 M HCl to basic solutions and 0.1 M NaOH to acidic solutions. It is adjusted in a way that pH 2 to 12 was equally distributed among the solutions. The absorbance of solutions 1 and 10 was measured using the spectrophotometer. The solutions were scanned from 350 nm to 800 nm at 5nm intervals. After that, the absorbance and wavelength of the 2 solutions was plotted in the same x-y plane using Microsoft Excel. The difference between the absorbencies of solutions 1 and 10 was also computed and included in the graph. The plot showed a minima and maxima of the curve and the wavelengths of these points were also recorded.
After that, the absorbance of the other 8 solutions was scanned using the photometric mode and the two chosen wavelengths.
III. Results and Discussion

Figure . Plot of Absorbance vs Wavelength of solution 1 and 10 using the spectrophotometer
Table 2. Absorbencies at the two chosen wavelengths and pH of Solutions 1 and 10
Solution
pH
Abs (410.0)
Abs (524.0)
1
1
0.081399968
3
2
4.39
0.176863181
2.649729858
3
5.065
0.273637375
1.302518671
4
6.82
0.799340537
1.257945336
5
6.365
0.125723026
0.26076593
6
6.67
1.191562053
0.079850628
7
8.33
1.191442873
0.091172732
8
10.76
1.172135706
0.096655014
9
12.455
1.135189893
0.111910059
10
13
1.218020022
0.111433339

Figure . Plot of log[In]/[HIn] vs pH of solutions1 to 10

IV. Questions and Answers

1. You will observe that the two colors of the indicator at the extreme pH values give different scanning curves. Comment on the relationship between the color of the substance and the λmax
Color illustrates the approximate complementary relationship between the wavelengths of light absorbed and the wavelengths transmitted or reflected.

2. What is the significance of pKa?
The pKa is the negative logarithm of the equilibrium constant (Ka) of the acid-base reaction of a compound. It is important for the quantitative evaluation of systems involving acid-base equilibria in solution. The pKa of a compound tells how likely it will be ionized at a given pH. It is also essential for working with buffers; the design of these solutions depends on a knowledge of the pKa values of their components.[1]

3. Which is the favored species of the indicator (HIn or In-) in a neutral environment?

4. What are the other regions of spectroscopy aside from the visible region?
Different regions of the spectrum probe different types of energy levels of an atomic or molecular system. Radio waves have the least energy.

V. Conclusions

References https://www.boundless.com/chemistry/acids-and-bases/strength-of-acids/the-acid-dissociation-constant/ Appendix
[HIn]
[In-] log [In-] / [HIn] pKa 0.10
0.00
-
1.000
0.08800
0.01075
-0.913
5.303
0.04269
0.02440
-0.243
5.308
0.03959
0.07833
0.296
6.524
0.00833
0.01215
0.164
6.201
0.00097
0.05649
1.763
4.907
0.00060
0.08181
2.138
6.192
0.00035
0.09604
2.434
8.326
0.00027
0.11589
2.634
9.821
0.00
0.05168
-
13.000