Summary

In solutions where the concentration of the base of the conjugate pair is higher than that of the acid

[H₃O⁺] must be lower than K_a for the system to be at equilibrium.

K_a = [H₃O⁺] ×	[conjugate base]
	[acid]

Thus pH is higher than pK_a.

Provided the concentration of the base is no more than ten times the concentration of the conjugate base, the pH will be within 1 unit of pK_a.

K_a = [H₃O⁺] ×	10
	1

K_a = [H₃O⁺] × 10

Taking the negative logarithm of all terms
pK_a = pH – 1
pH = pK_a+1

This is shown in the graph of pH change on addition of NaOH to CH₃CO₂H (pK_a 4.76). At 22.5

the ratio [conjugate base] to [acid] is 10. The pH is 5.76.

titration curve ethanoic acid with sodium hydroxide	CH₃CO₂^–
	CH₃CO₂H/ CH₃CO₂^–
	CH₃CO₂H

Buffers where the ratio [conjugate base] to [acid] is between 1 and 10 have pH between pK_a and pK_a +1.