Spectrophotometric Determination of Acid Dissociation Constant of Methyl Red

SPECTROPHOTOMETRIC DETERMINATION OF THE ACID DISSOCIATION CONSTANT OF METHYL RED VIII. Data Sheet Table 1. Wavelength readings for samples in maximum absorption of HMR and MR Sample

Absorbance for wavelength 520.4 (HMR) 0.180 0.125 0.060 0.007 0.008 0.005 0.173 0.238 0.371 0.538

1 2 3 4 5 6 7 8 9 10

Absorbance for Wavelength Wavelength 418.0 (MR ) 0.008 0.006 0.003 0.076 0.056 0.030 0.299 0.278 0.232 0.188

Table 2. Various pH readings of samples and the basic and acidic forms of Methyl Red Solution Stock Standard MR HMR Samples

pH

Molarity 3.713330 x  M 1.856665 x  M 1.856665 x  M 1.856665 x  M 1.392499 x  M 9.283327 x  M 4.641664 x  M 1.392499 x  M 9.283327 x  M 4.641664 x  M

10.69 1.66 1.63 1.64 1.62 9.53 9.33 7.86 5.62 5.37 5.01 4.64

1 2 3 4 5 6 7 8 9 10

Table 3. Computed values for sample data Sample

pH

1

1.63

-

Log ([MR ]/[HMR])

Path length (b)

-2.139262081 1.00 cm

[HMR]

[MR-]

1.39E-05

1.01E-07

pKa

3.7693 3.7693

2

1.64

-1.781126252 1.00 cm

9.67E-06

1.60E-07 3.421126

3

1.62

-1.661905075 1.00 cm

4.64E-06

1.01E-07 3.281905

4

9.53

1.727843206

1.00 cm

2.86E-07

1.53E-05

7.802157

5

9.33

1.416507314

1.00 cm

4.31E-07

1.13E-05

7.913493

6

7.86

1.322808781

1.00 cm

2.86E-07

6.02E-06

6.537191

7

5.62

0.677365145

1.00 cm

1.24E-05

5.90E-05

4.942635

8

5.37

0.490742751

1.00 cm

1.75E-05

5.42E-05

4.879257

9

5.01

0.194552238

1.00 cm

2.80E-05

4.38E-05

4.815448

10

4.64

-0.088704143 1.00 cm

4.11E-05

3.35E-05 4.728704

0.2 y = 12926x + 0.0017 R² = 0.9977

0.18 0.16 0.14 0.12    H    p

0.1 y = 4955.1x + 0.008 R² = 0.9944

0.08 0.06

y = 538.6x + 0.0007 R² = 0.9868 y = 215.44x + 0.0047 R² = 0.4286

0.04 0.02

0 0.00E+00 2.00E-06 4.00E-06 6.00E-06 8.00E-06 1.00E-05 1.20E-05 1.40E-05 1.60E-05

Concentration

Figure 1. Plot of pH vs Concentration of Samples 1-6

pH vs Log ([MR-]/[HMR]) 12 10 y = 2.0647x + 5.1922 R² = 0.9394

8    H    p

6 4 2 0

-2.5

-2

-1.5

-1

-0.5

0

Log ([MR-]/[HMR])

0.5

1

1.5

2

Figure 2. Plot of pH vs Log ([MR-]/[HMR]) of Samples 1-10

Experimental pKa Literature pKa Mean Standard Deviation Range Relative Standard Deviation (ppt) Confidence Limits (95%) Possible Sources of Error  –  Effect on Parameters

5.1922 4.95 5.20912 1.67833 0.213930712 32.22% 4.17 to 6.25 or 5.20912 ±1.04 A) On solution preparation -Going beyond the mark on the volumetric flask in adding H 2O; lower concentration; no effect on pKa -incomplete dissolution of methyl red; indeterminate effect on concentration B) On reading absorbance -Presence of stray radiation; lower absorbance; lower pKa -Faulty resolution of spectrophotometer; indeterminate effect on absorbance -Temperature is not constant; pKa shifts depending on temperature change