SPECTROPHOTOMETRIC DETERMINATION DISSOCIATION CONSTANT OF METHYL RED
OF
THE
ACID
C.F. B ATIFORRA DEPARTMENT OF CHEMICAL ENGINEERING , COLLEGE OF ENGINEERING
, DILIMANQUEZON CITY , PHILIPPINES UNIVERSITY OF THE PHILIPPINES DATE PERFORMED: MAY
2, 2015
INSTRUCTOR S NAME: MARIEL CLORES ’
ABSTRACT The experiment aimed to apply spectrophotometric concepts and beer's law in the determination of the acid dissociation constant of o f methyl red. Methyl red is a commonly used indicator because of it's property to change color within a range of pH. A spectrophotometer was used to determine the absorptivity constants and subsequently, the concentrations c oncentrations of HMR and MR- in a twocomponent system. The dissociation constant was calculated by obtaining pH and concentration values of the solutions. The calculated value for the pKa was 4.854 which had a 2.92% deviation from the literature value of 5.00. We conclude that this method is effective in determining the concentrations of unknown solutions containing two systems.
INTRODUCTION Methyl red is a common indicator that has an acidic form (HMR) and a basic form (MR-). At pH 6.2 and above it appears as a yellow solution and at pH 4.4 and below it appears as a red solution.
The samples to be measured will have two components meaning that the absorbance of one species has an effect on the absorbance of another species so we will modify beer's law into the following equations:
Our objective in this experiment is to determine the dissociation constant of methyl red by using and applying spectrophotometric concepts. This will be done by using the henderson-hasselbach equation and by graphical analysis. By measuring the pH and calculating the respective concentrations of methyl red components, we can calculate for the dissociation constant, denoted by pK a.
Ahmr = ɛ hmr, ʎ hmrbchmr + (1) Amr- = ɛ hmr, ʎ mr-bchmr +
We must first obtain the absorptivities of both species so two sets of solutions containing only one species was prepared. The absorbance of these solutions were measured.
METHODOLOGY
ɛ hmr-, ʎ hmrbcmrɛ mr-, ʎ mr-bcmr-
(2)
The equations above can be used to simtultaneously compute for the concentrations of both species through systems of equations. Once the concentrations of each species are obtained, the dissociation can now be computed.
Preparation of Solutions
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50mL of methyl red stock solutions was prepared by dissolving 0.0572g of methyl red into a 150mL beaker containing 30mL of 95% ethyl alcohol. The solution was tranferred to a 50mL volumetric flask where it was diluted to the mark. 50mL of methyl red standard solution was prepared by adding 25mL of 95% ethyl alcohol to 2.50mL of the methyl red stock solution in a 50mL volumetric flask. Dilute to mark.
*All volumes in mL
100mL of 0.040M sodium acetate trihydrate was prepared by dissolving 0.544g of sodium acetate in distilled water. 12.5mL of this solution was diluted to prepare 50mL of 0.010M.
The absorbance of the sample solutions was then measured in the spectrophotometer. The pH of solutions 7-10 was then measured using a pH meter.
Determination of max wavelengths The spectra of the HMR solution and MR- solution was obtained by using a UV Vis Spectrophotometer with water in the reference cell. From this we can obtain the wavelength of maximum absorption of both solutions.
RESULTS AND DISCUSSION 50mL of working red methyl red standard solutions were prepared each for acidic and basic forms of methyl red. For the acidic solution, 5.00mL methyl red standard solution was pipeted into a 50mL volumetric flask containing 5.00mL of 0.1M HCl solution. Dilute to mark. The basic solution was prepared by pipeting 5.00mL methyl red standard solution into a 50mL volumetric flask containing 12.50mL of 0.040M NaOAc solution. Dilute to mark. 10 Sample solutions were prepared. Solutions 1-3 only contained HMR, solutions 4-6 only contained MR- and solutions 7-10 contained both.
Table 2.1 Conc vs. Absorbance Solution
HMR
1 2 3 4 5 6
MR
0.408 0.265 0.138 0.019 0.008 0.011
0.056 0.036 0.021 0.152 0.077 0.055
Conc (M) 0.0001597 0.0001062 0.0000526 0.0001597 0.0001062 0.0000526
The figure below graphically shows the concentrations of the solutions vs their absorbances.
0.45
Table 1.1 Sample Solutions*
0.4 0.35
Solution
HCl
HMR
1 2 3
0.408 0.265 0.138
0.056 0.036 0.021
Solution
NaOAc
MR-
4 5 6
4.96 10.0 15.04
15.04 10.0 4.96
Solution
MR
HOAc
NaOAc
7 8 9 10
6.00 6.00 6.00 6.00
1.20 2.40 4.80 7.20
12.80 11.60 9.20 6.80
e c n a b r o s b A
0.3 0.25 0.2 0.15 0.1 0.05 0 0
0.0001
0.0002
Concentration E HMR, HMR E MR, MR
E HMR, MR E MR, HMR
2
Figure 1.1 Molarity vs Absorbance plot
7
We can calculate for the molar absorptivities of the two species in two wavelengths by getting the best fit line and then getting their slopes.
Wavelength
Absorptivity
ʎ hmr
25221 3269 747.1 9061
HMR
ʎ mr- ʎ hmr
MR-
ʎ mr-
pH
7 8 9 10
3.96 x 10-6 7.78 x 10-6 1.81 x 10-5 2.84 x 10-5
6.44 x 10-5 5.58 x 10-5 5.17 x 10-5 3.80 x 10-5
6.09 5.73 5.29 5.00
After calculating for concentrations of [HMR] and [MR-], we now solve for the value of pKa using Henderson-Hasselbach equation and plotting the log[MR-] / [HMR] vs pH.
y = 1.016x + 4.854
3
0
0.5
1
1.5
log (MR- / HMR)
Figure 2.1 log [MR-]/ [HMR] vs pH The pKa value of the system is graphically shown as the y-intercept. Our calculated value for pKa is 4.854 and our Ka value is 1.4 x 10-5. This results in a 2.92% deviation from the literature value of the pKa which is 5.00. A possible error that may have an effect on the calculated values is the improper handling of the UV-Vis spectrophotometer. Improper handling can cause an increase or decrease in the measured absorbance.
Table 3.1 Conc. of Unknown Samples [MR-]
4
0
A hmr = 25221bchmr + 747.1bcmrA mr- = 3269 bchmr + 9061bcmr-
[HMR]
H p
1
After getting the molar absorptivities, we can now use them to calculate the unknown concentrations of HMR and MR - in solutions 7-10. We can calculate them by using two equations of beer's law.
No.
5
2
Table 2.2 Calculated Absorptivities Solution
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CONCLUSION AND RECOMMENDATION
the can the by
The calculated pKa had a 2.92% erorr deviation from the literature value. We can conclude that the experiment was successful in obtaining the dissociation constant of methyl red. REFERENCES [1] Purcell, K. F., Kotz, J. C. Organic Chemistry. W.B. Saunders Company, Philadelphia. 1977. [2] Cotton, F.A., Wilkinson, G., Murillo, C.A., Bochmann, M. Advanced Inorganic Chemistry 6th ed.John Wiley and Sons Inc., Toronto. 1999. [3] Stafford, F.E., J. Chem Educ.1962, 39, 626.
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APPENDIX Ahmr = ɛ hmr, ʎ hmrbchmr + ɛ hmr-, ʎ hmrbcmrAmr- = ɛ hmr, ʎ mr-bchmr + ɛ mr-, ʎ mr-bcmr-
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