Chemistry: Ionic Compounds: Buffer Solutions and Buffer Action (For CBSE, ICSE, IAS, NET, NRA 2022)

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Buffer Solutions

  • The mixture of a weak base and a salt of common ion both behave in similar way.
  • The aqueous solutions containing weak acids/bases and a salt of common ion act as buffer solutions.
  • The Buffer solutions resist a change in its pH on adding a small amount of an acid or a base.
  • The buffer solutions are used in laboratory reactions, in industrial processes and in the bodies of plants and animals.
Buffer Solutions
  • It is often necessary to keep the pH near constant despite the addition of acids and bases.
  • The oxygen carrying capacity of hemoglobin and activity of the enzymes in our cells depends on the pH of our body fluids.
  • The pH of the blood is 7.4
  • The pH of saliva is 6.8.
  • The animals and plants are protected against the changes in pH by the presence of buffers.
  • There are two kinds of commonly used buffer-solutions they are:
    • A weak acid and a soluble ionic salt of the weak acid e. g. acetic acid and sodium acetate
    • A weak base and a soluble ionic salt of the weak base e. g. ammonium hydroxide and ammonium chloride.
  • The buffers with pH less than 7 are called acidic buffers
  • The buffers with pH above 7 are called basic buffers.
  • Acetic acid - sodium acetate buffer is an example of acidic buffer while ammonium hydroxide - ammonium chloride is a basic buffer.

Buffer Action

  • The buffer system contains a conjugate acid- base pair and the concentrations of these two pairs are high than hydronium ions. These are called as the acid reserve and the base reserve, respectively.
  • The additional acid or base reacts with these reserves and gets consumed without changing pH.
  • Let us consider a buffer solution containing acetic acid, CH3COOH and sodium acetate CH3COONa to understand the buffer action.
  • In acetic acid - sodium acetate buffer CH3COOH is the acid reserve while CH3COONa is the base reserve.
  • The weak acid dissociates partially while the salt undergoes complete dissociation.

CH3COOH (aq) + H2O (l) H3O + (aq) + CH3COO- (aq)

CH3COONa (aq) Na + (aq) + CH3COO- (aq)

  • When a strong acid such as HCI is added to this solution, it produces H3O + .
  • These H3O + (acid) react with an equivalent amount of the base reserve [CH3COO ] to generate undissociated acetic acid. H3O (aq) + CH3COO (aq) CH3COOH (aq) + H2O (l)
  • The net effect of this reaction is the increase in the concentration of the acid reserve and an equivalent decrease in the concentration of the base reserve. HCl (aq) + CH3COONa (aq) CH3COOH (aq) + NaCl (aq)
  • Similarly, when small amounts of a strong base like NaOH is added , it generates OH ions.
  • These additional OH neutralize some of the H3O + ions present in the solution, H3O (aq) + OH (aq) H3O (aq) + CH3COO- (aq)
  • The net effect of this reaction is the increase in the concentration of the base reserve and an equivalent decrease in the concentration of the acid reserve.
  • It is noted that when acid or the base is added only minor changes in the concentrations of the weak acid and the salt are caused.
  • The concentration of the hydronium ions and thereby the pH does not change significantly.

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