A buffer solution is prepared by using a weak acid having $\mathrm{p}{K}_{\mathrm{a}}=4$ The approximate range it can be used effectively IS

The approximate buffer range is in the range $\mathrm{p}{K}_{\mathrm{a}}^{\circ }-1\text{ to }\mathrm{p}{K}_{\mathrm{a}}^{\circ }+1$.

The effective pH range of a buffer is one pH unit on either side of the weak acid's $\mathrm{p}{K}_{\mathrm{a}}^{ }$ value. A buffer's ability to withstand significant pH variations will be lost if its pH falls outside of this range.