Buffer solution





It is practically observed that when a drop of decinormal HCI or NaOH is added to a litre of pure water, larger increase in the concentration of H^+ions or OH^- ions takes place respectively and thus the pH value is changed. But when a drop of decinormal HCI is added to a mixture of weak acid and its strong salt (e. g. , CH_3COOH + CH_3COONa) there is a very slight change of H^+ ion concentration.

Similarly, when a decinormal solution of N aOH is added in a mixture of weak base and its strong salt (e. g., NH_4OH + NH_ 4Cl) then a slight change in OH ion concentration takes place. It means the pH value in these media remains approximately the same and these solutions resist any change of acidity or alkalinity.

'Such solutions which reserve acidity or alkalinity are known as buffer solutions.'

Types of Buffers:

1.Simple buffers:

Such buffers are:

(i) Salts of weak acid and weak base e. g. ,

CH_3COONH_4, (NH_4)_2 CO_3, NH_4CN etc.

(ii) Proteins and amino acids.

(iii) A mixture of an acid salt and normal salt of a poly basic acid.

E.g.

Na_2HPO_4 + Na_3PO_4; NaH_2PO_4 + Na_3PO_4, NAHCO_3 + Na_2CO_3 etc.

 

2. Mixed buffers:

Such buffers are of following two types:

(i) Acidic buffer mixtures: A weak acid with its salt of strong base.

E. g.

CH _3COOH +CH _3 COONa; H_3BO_3 + NA_3BO_3; H_2CO_3 + Na_2CO_3; H_3PO_4 + NaH_2PO_4

 

(ii) Basic buffer mixtures: A weak base with its salt of strong acid.

E. g. NH_4OH, NH_4Cl, glycine + glycine hydrochloride etc.

 

The characteristics of such solutions are:

(i) They possess a definite pH value.

(ii) The pH value remains steady on storing or on dilution.

(iii) The pH value is not changed on the addition of a little amount of strong acid in acidic buffer (e. g., CH_3COOH + CH_3COONa) and strong base in basic buffer (e.g. , NH_4 OH + NH_4Cl).

 

Preparation: Buffer solutions are usually made of a mixture of an acid and its salt of the alkali.

The list of some important buffer solutions are given below:

  1. Acetic acid + sodium acetate.
  2. Boric acid + borax.
  3. Citric acid + sodium citrate.
  4. Hydrocyanic acid + potassium cyanide.
  5. Sodium carbonate + sodium bi-carbonate
  6. NaH_2PO_4 + Na_2HPO_4


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