Experiment 10 Cavite 1

Theo Victor Cavite

Melanie Bucsit-Carpio

Chem 28.1

September 26, y

Spectrophotometric Determination of Acid Dissociation

Constant Methyl Red

Results and Discussion

In the same manner as the previous experiment, the concept of Beer’s Law would still be applied for this

experiment. The main difference between the two is that this experiment now deals with solutions having more

than one kind of absorbing species, namely MR- and HMR.

In a multi-component system, the total absorbance is equal to the summation of absorbances of individ-

ual species. Given n absorbing species and the absence of any interfering substance , the total absorbance is

given by

where εn is the molar absorptivity of the nth species, b as the path length in cm, cn as the molar concentration of

the nth absorbing species.

After solutions of HMR, MR-, HCl, HOAc and NaOAc were already prepared, six sample solutions were

then prepared out from varying volume composition of the mentioned solutions. The purpose of these solutions

were to determine the molar absorptivity at corresponding wavelengths. In solutions 1-3 only varying volumes of

HMR and HCl were added. Meanwhile, solutions 4-6 comprise only of mixture of MR- and NaOAc. The addition

of HCl and NaOAc was to ensure that the pH value at each solution is maintained at roughly equal to 2 for the

first three solutions and at pH 8 for solutions 4-6. This isolation is necessary to make sure that only one form of

methyl red is present at each solution.

The maximum absorption wavelengths of HMR (λHMR) and MR- (λMR-) were determined from their spectra

between 350 and 600 nm. The values obtained for λHMR and λMR- are 521.6 nm and 430.4 nm, respectively. After

which, the absorbance of the six sample solutions at these two wavelengths were determined. The plot of these

measured absorbances versus the concentration at each solution results to a slope equal to the molar absorptiv-

ity.

Experiment 10 Cavite 2

Figure 1. The Absorbance vs Concentration of Solutions 1-3. Both slopes of the line represent the molar absorptivity of HMR at differ-

ent λ maximum.

Figure 2. The Absorbance vs Concentration of Solutions 4-6. Both slopes of the line represent the molar absorptivity of MR- at different

λ maximum.

Table 1. Molar Absorptivity Constant Values in L/mol.cm

εHMR at λHMR εMR- at λHMR εHMR at λHMR εMR- at λHMR

48089 4808.9 5770.7 20198Recall that the slope represents the molar absorptivity. These values are tabulated in Table 1. Solutions 7-

10 were then prepared to finally determine the acid dissociation constant for methyl red, which is theoretically

equal to 10-5.4. To find the acid dissociation constant, it is empirical to measure the pH at each solution. This is

because this part of the experiment mainly rely with the Henderson-Hasselbalch Equation dictated by

Aside from the pH, the [MR-] and [HRM] need to be calculated first. HMR and MR- concentrations are easily

calculated through equation 1. Two equations would be generated with two unknowns for each solution - one at

λHMR and another one at λMR-. Once these concentrations are already known, the plot of pH vs log ([MR-]/[HMR])

yields a y-intercept equal to pKa, as suggested from equation 2.

Experiment 10 Cavite 3

Figure 3. pH vs log ([MR-]/[HMR]) of Solutions 7-10.

The linear relationship observed in Figure 3 implies that the values are consistent with the equation 2.

The y-intercept of the equation of the best-fit line is numerically equal to pKa of methyl red. Therefore, the exper-

imental value of the acid dissociation constant of methyl red generated from this experiment is equal to 10-5.4723.

If the pKa is manually solved at each solution instead of plotting, the relative standard deviation is equal to

10 ppt with a range of 0.168. These pKa values are tabulated in Table 2 and the data set’s rsd suggests a highly

precise data. In addition, the percent relative error of the measured pKa from the plot relative to the theoretical

value (5.54) only amounts to 1.3%. Thus, this implies that the data is not only precise but also highly accurate.

Table 2. Calculated values

Solution pH log ([MR-]/[HMR]) pKa

7 6.31 0.914776842 5.395223158

8 5.94 0.477798167 5.462201833

9 5.61 0.046896147 5.563103853

10 5.15 -0.261330421 5.411330421

In measuring the absorbances of all solutions, matched cells were used - with water in the reference

cell. These cells were used to avoid having errors or y-intercept in the absorbance vs. concentration plot. As

seen from Beer’s Law, ideally no y-intercept should be observed. As much as possible, it should be corrected.

The pKa value at 95 % confidence level lies inclusively between 5.3 and 5.7. Possible sources of error in-

clude those from miscalibration of the pH meter. Also, errors in the preparation of solution may result to pH at the

desired value. Hence, interfering substances may cause inaccuracy in the measured absorbance

Experiment 10 Cavite 4

References

[1] Skoog, Douglas A. et al. Fundamentals of Analytical Chemistry 8th ed. Thomson Higher Education, Califor-

nia, USA. 2004

[2] Harris, Daniel C. Quantitative Chemical Analysis 7 th edition . W.H. Freeman and Company, USA. 2007