copy of buffers.ppt

8/11/2019 Copy of buffers.ppt

1/4

Buffers:-A buffer solution is that solution that allows solutions to resistlarge changes in pH upon the addition of limited amounts of acid

Or base .Chemically buffer solutions are made up of a mixture of twosubstances , a conjugate base and a conjugate acid .Buffers can be divided according to their chemical nature into twoTypes : 1- Acidic buffers which contains a weak acid and its salt ofA 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 ( thebuffer components) will resist large changes in pH by partiallyAbsorbing the added H+ions and OH-ions as seen in the followingBuffer example HA /A-

When an acid is added the added H+

will react with the conjugate


2/4

Base , H+ + A- HA ,When a base is added the OH- ions will react with the conjugate

Acid , OH- + HA H2O + 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 doNot have a significant effect on the pH .

Note :1- Buffered solutions do show a change in pH upon theAdditon Of acid or base, but the change is insignificant comparedto the Change if no buffer was present.

2- The amount of change recorded depends on the strength of theBuffer 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.


3/4

Solution: The acetate buffer is composed of a mixture of CH3COOH

Representing the conjugate acid, and CH3COO- representing the

Conjugate base, so the acetate buffer is a solution of the following

Composition CH3COOH/ CH3COO-.b)When an acid is added it will react with the conjugate baseH+ + CH

3COO- CH

3COOH

When a base is added it will react with the conjugate acid

Component of the buffer , OH-+ CH3COOH H2O + CH3COOBuffer Capacity :

The quantitative measure of the buffers resistance to changes inPH upon the addition of acid or base. Buffer capacity can beDefined as the number of moles of H+ or OH ions requiredTo cause a one unit change in pH in 1liter of buffer.

-


4/4

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 sodiumFormate HCOONa . Pk

a: of HCOOH is 3.75.

Solution; First calculate the proportions of the two formate speciesPresent,

From HendersonHasselbalch equation ,PH = pK

a+ log [A-]

[HA]The 0.25M formate buffer represents the sum concentration of theTwo components of the buffer ,( 0.25M of formate buffer = [HCOOH] + [HCOONa])Let y = [A-] , so [HA] = 0.25y , (A-is the conjugate baseHCOONa , while HA represents the conjugate acid HCOOH )Thus pH = 3.75 + log y

0.25y

copy of buffers.ppt

Documents