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Levelling effect is observed in the case of solvation of acids or bases in a solvent. Thus levelling effect is a phenomenon exhibited by solvents when an acid or base is dissolved in them.

Water forms ${\mathbf{H}}_{\mathbf{3}}\mathbf{O}\mathbf{\text{'}}$ion which is known as a hydronium ion and it is formed when a lone pair electron on the Oxygen atom is lost to bond with the proton. Hydronium ion also behaves as acid as it is capable of losing the proton during the course of the reaction.

Any acid that is said to behave stronger acid than a hydronium ion, when dissolved in water, forms a hydronium ion with water that the strength of the acid is 'levelled' to that of the water.

Similarly,${\mathbf{OH}}^{\mathbf{鈥�}}$ion is the strongest base in water. Any other base stronger than an ${\mathbf{OH}}^{\mathbf{鈥�}}$ion when dissolved in water its strength is levelled to that of${\mathbf{OH}}^{\mathbf{鈥�}}$ion.

Acetic acid is a weak acid and it doesn't dissociate completely into ions in solution. When we are titrating a very weak base, we would choose to titrate it against strong acid which is${\mathrm{HCIO}}_4$. The strong acid is not levelled to the strength of acetic acid so the presence of acetic acid as a solvent wouldn't interfere with the titration reaction. Hence acetic acid is the best choice as a solvent for the weak base.

Choose a strong base to titrate a very weak acid and in this case, it is tetrabutylammonium hydroxide (TBAH). Hence the suitable solvent must not interfere with the base. If acetic acid is used as the solvent, more of the hydroxide ion of TBAH is consumed by acid and the desired titration reaction will not occur due to the unavailability of the hydroxide ion. Instead, if we choose pyridine as a solvent it will not react and so this is the suitable solvent to titrate a very weak acid with TBAH.