Question

The pKa of Ampicillin is 2.5. What is the between the conjugate base and acid of ampicillin when the stomach acid has a PH of 1.4? What is the ratio when in the small intestine (the small intestine has a PH of 6?). From your answers, explain if more ampicillin is absorbed into the bloodsteam through the stomach or small intestine?

Answer 1

This should be claculated with buffer equation, since it is easier to calculate:

pH = pKa + log(conjugate/acid)

the ratio is = conjugate/acid

so

pH = pKa + log(ratio)

pKa = 2.5 so

pH = 2.5 + log(ratio)

for the stomach acid, pH = 1.4

substitute and solve

1.4 = 2.5 + log(ratio)

ratio = 10^-(1.4-2.5) = 12.5892

ratio = 12.5892

Q2

for intestine, pH = 6

so

pH = pKa + log(ratio)

6 = 2.5 + log(ratio)

ratio = 10^(6-2.5) = 3162.27

Clearly, this is absorbed in its ionic state, so it must be absorbed in the ph = 6, i.e. small intestine