Definition of buffer solution:- A solution which resists change in pH when a small amount of a strong acid or a strong base is added to it is called buffer solution.
Buffer action:- The resistance offered by a buffer solution to change in pH on the addition of acid or base is called buffer action.
Explanation of buffer solution
Pure water has pH value equal to 7.0 but even the purest form of water cannot acidic this value for very long time. The acorn is that carbon dioxide in the air dissolve in water and give its a slight acidic character. Thus buffer solution resist the change in pH. For example- Blood is a buffer since it maintains a ph. of 7.4 under wide range of physiological conditions.
Preparation buffer solution
A buffer solution is usually prepared from (i) Weak acids and its salts (ii) Weak base and its salts.
Acidic buffer solution
A weak acid and its salts with a strong base like CH3COOH/CH3COONa make a solution called acidic buffer. Acidic buffer have pH less than 7.0. Consider an acidic buffer solution consisting of acetic acid and sodium acetate when a strong base NaOH is added the added OH ion are removed by the reaction with acetic acid molecules.
CH3COOH + OH ⟶ CH3COO + H2O
When strong acid is added H ion of acid reacts with the acetate ion of buffer.
H ion + CH3COO ion ⟶ CH3COOH
Both of these reactions go to completion hence the added OH ion and H ion are removed and pH of buffer solution remains constant.
Basic Buffer Solution
A weak base and its salt with a strong acid like NH4OH/NH4Cl make a solution called basic buffer. Basic buffer solution ph more than 7.0. Consider as basic buffer solution consisting of ammonium hydroxide and ammonium chloride on adding a strong acid such as HCl the added hydrogen ions are removed by reaction with NH4OH.
H ion + NH4OH ⟶ NH4 ion + H2O
Where as the OH ions of the base added are removed by reaction with the ammonium ions of the buffer solution.
NH ion + OH ion ⟶ NH4OH
pH of buffer solution
pH of buffer solution is determine by two way. (1) The value of Ka of weak acid or Kb of the weak base.
(2) The ratio of the concentration of the weak acid and its conjugate base. The equation for the ionization of a weak acid is:
HA ⟶ H ion + A¯ ion
Ionization constant Ka of a weak acid is: Ka = [H+] [A¯] / [HA]
On rearranging [HA]Ka = [H+] [A¯]
[H+] = Ka [HA] / [A¯]
Taking -log on both side
-log [H+] = -log ( Ka [HA] / [A¯]
-log [H+] = -log Ka - log [HA] / [A¯]
pH = PKa - log [HA] / [A¯]
pH = PKa + log [A¯] / [HA]
This equation is known as Henderson-Hasselbalch equation.
Application of buffer solution and Uses of buffer solution
1. The use of buffer solution is important in some industrial processes, where process would be interrupted by large change in ph. Such as manufacture of leather, photographic materials and dyes.
2. The bacteriological research culture media are generally buffered to maintain a constant pH required for the growth of bacteria being studied.
3. Buffer are important in biological systems because the biochemical reaction in both plants and animals are often very sensitive to pH change Human blood buffered to a ph of 7.4 by means of bicarbonate, phosphate and complex protein system.
4. Many shampoos used a citric acid to maintain a slightly acidic pH balance
5. In bewaring industry, buffer solution are added before fermentation begins.
6. Protein studies must be performed in buffered media because the magnitude and kind of electrical charge.
>Methods for determination of pH of water
