Why does acetic acid lose its buffering ability at very high pH values? Why does acetic acid lose its buffering ability at very low pH values? In other words, what is happening when the buffering range is exceeded? The pKa of formic acid is 3.75.

Because the optimal range of buffering for a formic acid potassium formate buffer is 2.74 ≤ pH ≤ 4.74.

Explanation:

Every buffer solution has an optimal effective range due to pH = pKa ± 1. Outside this range, there is not enough acid molecules or conjugate base molecules to sustain the pH without variation. There is a certain amount of both molecules that has to be in the solution to maintain a pH controlled.

Being for the formic acid the pKa 3.74, the optimal effective range is between 2.74 and 4.74. Upper or lower these range a formic acid/potassium formate buffer does not work.