Question

Can you help me solve the next to columns Log and calculated PKA

Answer 1

Since, absorbance corresponds to /or proportional to concentration of absorbing species.

And ,since system with acid and conjugated base behaves as buffer solution:

pH = pKa + log[ A- / HA ]

Or pH = pKa + log [A - Aint/ Atotal -A]

#1

pH = 6.56 = pKa + log [ 0.067 - 0.020 / 0.769 - 0.067]

6.56 = pKa + (-1.20)

pKa = 7.76

#2

pH = 6.88 = pKa + log [ 0.258 - 0.020 / 0.769 - 0.258]

6.56 = pKa + (-0.34)

pKa = 6.90

#3

pH = 7.11= pKa + log [ 0.235 - 0.020 / 0.769 - 0.235]

7.11= pKa + (-0.40)

pKa = 7.51

#4

pH = 7.57 = pKa + log [ 0.284 - 0.020 / 0.769 - 0.284]

7.57 = pKa + (-0.26)

pKa = 7.83

#5

pH = 7.77= pKa + log [ 0.331 - 0.020 / 0.769 - 0.331]

7.77 = pKa + (-0.15)

pKa = 7.92