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Buffer solution
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A solution whose pH almost remains constant even when a few drops of an acid or base are added to it is called a buffer solution. The property of a solution to resist any change in pH is called Buffer action.
(1) Types of buffer solutions
(i) Salts of weak acid and weak base, e.g. CH3COONH4
(ii) Acidic buffers i.e. a weak acid + salt of this weak acid with a strong base (e.g. CH3COOH + CH3COONa).
(iii) Basic buffers i.e. a weak base + salt of this weak base with a strong acid (e.g. NH4OH + NH4Cl)
(2) Calculation of pH of a buffer mixture
(i) For acidic buffer mixture (Henderson equation) : If the weak acid is HA and its salt is BA, then
HA + H2O « H3 O+ + A-
BA ® B+ + A+
\ Ka = ([H3O+][A+])/([HA]) or [H3O+] = Ka ([HA])/([A-])
but [A-] = [BA], therefore, [H3 O+] = Ka ([HA])/([BA]) = Kb ([Acid])/([Salt])
or
pH = pKa + log ([Salt])/([Acid])
(ii) For basic buffer mixture like BOH + BA, in a similar manner,
pOH = pKb + log ([Salt])/(Base) ; As pH + pOH = 14, pH can be calculated.
(3) Buffer capacity : It is defined as the number of moles of an acid or a base required to be added to one litre of the buffer solution so as to change its pH by one unit. Thus,
It may be noted that buffer capacity is maximum when the concentration of the weak acid or the base and the salt are equal or when pH = pKa.
(4) Application of Buffer solution
(i) For the removal of phosphate ion in the qualitative inorganic analysis after second group using CH3 COOH / CH3 COONa buffer
(ii) For precipitation of hydroxides of third group of qualitative analysis, a buffer
NH4Cl / NH4 OH is used.
(iii) Blood pH is maintained with the help of H2 CO3 / HCO-3buffer inspite of many acidic foods we take.
Note : q pH of buffer depends on concentration ratio of the base to salt.
q Buffer solutions have reserve acidity and reserve alkalinity.
q Proteins and amino acid form buffer solution.
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