What are buffer solutions? Explain types of buffer solutions with examples.

(N/A) Definition: The solutions which resist change in $pH$ on dilution or with the addition of small amounts of acid or alkali are called buffer solutions. Buffer solutions are of two types: acidic and basic.
$(A)$ Acidic buffer solutions: The $pH$ value of these solutions is less than $7.0$. The examples of acidic buffers are as under:

Acid $+$ Salt of acid	Approx $pH$
$CH_{3}COOH + CH_{3}COONa$	$4.75$
$C_{6}H_{5}COOH + C_{6}H_{5}COONa$	$3.7$
$C_{6}H_{4}(COOH)_{2} + C_{6}H_{4}(COOH)(COOK)$	$2.9$
$HCOOH + HCOONa$	$3.7$

$pH$ of an acidic buffer is calculated as follows:
$pH = pK_{a} + \log \frac{[Salt]}{[Acid]}$
$(B)$ Basic buffer solutions: The $pH$ value of these solutions is greater than $7.0$. These consist of a weak base and its salt with a strong acid. Example: $NH_{4}OH + NH_{4}Cl$ $(pH \approx 9.25)$.
$pOH = pK_{b} + \log \frac{[Salt]}{[Base]}$