A buffer solution is a solution that resists a change in pH upon the addition of small amounts of an acid or a base. It is generally a mixture of a weak acid and its salt with a strong base (acidic buffer) or a weak base and its salt with a strong acid (basic buffer).

• $HNO_2$ is a weak acid and $NaNO_2$ is its salt with a strong base (NaOH). This pair forms an acidic buffer.
• NaOH is a strong base, $HNO_3$ is a strong acid, and HCl is a strong acid. Solutions containing strong acids or strong bases along with their salts do not act as buffer solutions.