Correlating the pH of a solution with pKa for a weak acid-strong base titration
| Ka = [H3O+] × | [conjugate base] |
| [acid] |
Reaction of the added titrant is fast and equilibrium is achieved on mixing. The relationship of [H3O+] in the flask to Ka of the acid being titrated depends on the ratio [conjugate base]/[acid].
At points in the titration when the concentration of the acid in the flask is higher than that of its conjugate base
Halfway to the equivalence point, the concentration of the acid in the flask equals the concentration of its conjugate base
At points in the titration when the concentration of the base of the conjugate pair is higher than that of the acid
| Ka = [H3O+] × | [conjugate base] |
| [acid] |
| Ka = [H3O+] × | [conjugate base] |
| [acid] |
As shown in the equation at high [acid], [H3O+] is higher than Ka.
Thus pH is lower (more acidic) than pKa.
Roll the mouse over the image to connect this statement with the titration curve.
Thus pH is lower (more acidic) than pKa.
Roll the mouse over the image to connect this statement with the titration curve.
Halfway to the equivalence point, the concentration of the acid in the flask equals the concentration of its conjugate base
[H3O+] equals Ka; therefore –log[H3O+] = –log Ka and pH = pKa.
At points in the titration when the concentration of the base of the conjugate pair is higher than that of the acid
[H3O+] is less than Ka.
Thus pH is higher than pKa (on the base side).
Thus pH is higher than pKa (on the base side).