Buffers: -A buffer solution is that solution that allows solutions to resist large changes in pH upon the addition of limited amounts of acid Or base. Chemically buffer solutions are made up of a mixture of two substances, a conjugate base and a conjugate acid. Buffers can be divided according to their chemical nature into two Types : 1- Acidic buffers which contains a weak acid and its salt of A strong base. 2- Basic buffers which contains a weak base and its salt of a strong acid. How do buffers resist changes in pH ? Together the two species,conjugate base and conjugate acid ( the buffer components) will resist large changes in pH by partially Absorbing the added H + ions and OH - ions as seen in the following Buffer example HA /A - When an acid is added the added H + will react with the conjugate

Base, H + + A - → HA, When a base is added the OH - ions will react with the conjugate Acid, OH - + HA → H 2 O + A - (producing water and the Conjugate base ) So in both cases as seen the limited amount of H ions or OH - ions Added (which could change significantly the pH of the solution) Reacted with the two basic components of the buffer (the Conjugate base and conjugate acid) producing products that do Not have a significant effect on the pH. Note :1- Buffered solutions do show a change in pH upon the Additon Of acid or base, but the change is insignificant compared to the Change if no buffer was present. 2- The amount of change recorded depends on the strength of the Buffer and the [A - ] / [HA] ratio. Example: a) Describe the components of an acetate buffer. b) show the mechanism by which it resists changes in pH.

Solution: The acetate buffer is composed of a mixture of CH 3 COOH Representing the conjugate acid, and CH 3 COO - representing the Conjugate base, so the acetate buffer is a solution of the following Composition CH 3 COOH/ CH 3 COO -. b)When an acid is added it will react with the conjugate base H + + CH 3 COO - → CH 3 COOH When a base is added it will react with the conjugate acid Component of the buffer, OH - + CH 3 COOH→ H 2 O + CH 3 COO Buffer Capacity : The quantitative measure of the buffers resistance to changes in PH upon the addition of acid or base. Buffer capacity can be Defined as the number of moles of H + or OH ions required To cause a one unit change in pH in 1liter of buffer. -

Preparation of buffers Example ; Describe the preparation of 2 liters of a 0.25M formate Buffer, pH 4.5, starting from 1M formic acid and solid sodium Formate HCOONa. Pk a : of HCOOH is Solution; First calculate the proportions of the two formate species Present, From Henderson – Hasselbalch equation, PH = pK a + log [A - ] [HA] The 0.25M formate buffer represents the sum concentration of the Two components of the buffer, ( 0.25M of formate buffer = [HCOOH] + [HCOONa]) Let y = [A - ], so [HA] = 0.25 – y, (A - is the conjugate base HCOONa, while HA represents the conjugate acid HCOOH ) Thus pH = log y 0.25 – y