Calculate the concentration of ascorbate in blood plasma at

Calculate the concentration of ascorbate in blood plasma at pH 7.40 if [ascorbic acid] = 1.00 mu M. The pK_a of ascorbic acid is 4.10. Concentration of ascorbate = _________ M

Solution

Ascorbic acid is a weak acid, and ascorbate ion is the conjugate base of ascorbic acid. Ascorbate is found in the blood plasma in combined state.

The given problem can be solved using the Henderson-Hasselbach equation:

pH = pK_a + log ( [A^-] / [HA] )

Given,

pH = 7.4

[ascorbic acid], i.e. [HA] or concentration of weak acid = 1.00 mu molar = 1x 10^-6 M

pK_a of ascorbic acid = 4.10

Therefore, concentration of ascorbate ion or [A^-] has to be found out.

Replacing the values in the equation,

7.40 = 4.10 + log ([A^-] / 1x 10^-6)

or, 3.30 = log [A^-] - log (1x 10^-6)

or, log [A^-] = 3.30 - 6 = -2.7

or, [A^-] = 0.0019 M = 0.002 M (correct to 3 decimal place)

Therefore, concentration of ascorbate = 0.002 M