The ionization constant of formic acid is 1.8 x 10-4. Around what pH will its mixture with sodium formate give buffer solution of higher capacity? Calculate the ratio of sodium formate and formic acid in a buffer of pH 4.25.

Buffer capacity is defined as the rate of change of pH with number of moles of acid or base added.

Buffer capacity = dpH / dn _{acid or base}

dpH is the change in pH and dn acid or base is the number of moles of acid or base added. It has been found that the buffer capacity is maximum near pK_a of the acid provided the total number of moles of salt and acid remain constant.
Therefore,
pK_a = – log (1.8 x 10^-4)
= – log 1.8 +4 = -0.26 + 4 = 3.74
It is the pH of high capacity buffer.
For buffer solution,
pH = pK_a + log ([salt]/ [base])
4.25 = 3.74 + log ([salt]/ [base])
Log ([salt]/ [base]) = 4.25 – 3.74 = 0.51
Therefore, ([salt]/ [base] = antilog (0.51) = 3.24