Which acid or base (along with its corresponding salt) should be used to generate a buffer solution with pH around 3.5

Formic acid

Sodium formiate

Explanation:

To determine acid or base that can generate a buffer solution with pH around 3.5, we have to think in the acid whose pKa = pH.

Although we have to also think in buffer capacity, a measure which can cause a change of 1 pH unit in 1 L of solution.

Buffer capacity does not only depend on the concentration of its components but also of the relationship between that concentrations.

When pH = pKa, buffer capacity is maximum which means that the concentration of conjugated species is the same and the ability to oppose pH changes is maximum.

One example with pH = pKa or nearly if:

COOH⁻  + Na⁺ →  NaCOOH

HCOOH  +  H₂O  →  COOH⁻  + H₃O⁺     Ka: 1.8×10⁻⁴

pKa = 3.74

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