Buffers are defined as substances in solution that offer resistance to change in pH as acids or bases are added to or formed within the solution. Buffer solutions usually contain mixtures of weak acids and their salts (conjugate base) or weak bases and their salts (conjugate acids).
Weak acid & Weak base:-
1) CH3COOH CH3COO- + H+
(Acetate Ion)
[CH3COO] [H] / [CH3COOH] = ka = 1.8 X 10-5
log [CH3COO]/ [CH3COOH] + log [H] = log 1.8 X 10 -5
- log [CH3COO]/ [CH3COOH] - log [H] = - log 1.8 X 10 -5
- log [CH3COO]/ [CH3COOH] + pH = - log 1.8 X 10 -5
pH = -log ka + log [CH3COO]/ [CH3COOH]
pH = -log ka + log [base] / [Acid]
in buffer solution acid = base; log 1 = 0
pH = Pka = 1.8 X 10-5
2) [H2CO3] = [H+] +[HCO3-]
[H] [HCO3]/ [H2CO3] = ka1
- log [H] [HCO3]/ [H2CO3] = -log ka1
-log [H] – log [HCO3]/ [H2CO3] = - log ka1
pH = Pka1 + log [HCO3]/ [H2CO3]
pH = Pka1 + log [Conjugate base]/ [acid]
When, base = acid.
pH = pk
Buffering capacity of any substance is higher at pH = pk
CH3COOH CH3COO- + H+
(Acetate ion)
[CH3COOH] [H]/ [CH3COOH] = k = 1.8 X 10-5
pH = Pk = - log 1.8 X 10-5 + log [CH3COO-]/ [CH3COOH]
pH = 4.74 + log [CH3COO-]/ [CH3COOH]
* Prepare a solution of Acetic acid with pH = 5
CH3COONa CH3COO- + Na+
pH = 4.74 + log [CH3COO-]/ [CH3COONa]
5.0- 4.74 = log [CH3COO-]/ 1 mole
A| 0.26 = [CH3COOH]
Conjugate Base: -
Alkali: - It accepts H+.
Acid: - It releases H+.
CO2 + H2O H2CO3
H2CO3 H+ +HCO3-
(Acid) (Conjugate base)
HCO3 H+ + CO3-
(Acid) (Conjugate base)
Carbonate buffer system:-
CO2 + H2O H2CO3 Carbonic acid
H2CO3 H+ + HCO3- bicarbonate ion
HCO3 H+ +CO3- carbonate ion