Question

Propose a buffer system using ascorbic acid. Explain how this solution can protect against acids and bases, specifically.

Answer 1

Propose a buffer system using ascorbic acid. Explain how this solution can protect against acids and bases, specifically

1)Buffer: Buffer is the solution containing a conjugate weak acid/weak base pair that resistant to a change in pH when a strong acid or strong base is added.

2)Ascorbic acid –sodium ascorbate buffer.

i)Preparation: For ascorbic acid dissociation constant is Ka = 7.9x10^-5

Let us calculate pKa for ascorbic acid.

pKa = -log (Ka))

pKa = -log(7.9x10^-5)

pKa = 4.10

If we prepare a buffer by mixing equal volumes of equimolar solutions of acid and conjugate base (i.e. salt) its pH will be numerically equal to pKa of the acid. This is evident from Henderson –Hasselbalch equation for acid buffer,

pH = pKa + log{[Salt]/[Acid]}

For, [Salt] / [Acid] = 1 we gat log{[salt]/[Acid]} = 0

Hence pH = pKa.

Hence we can prepare a ascorbic acid with pH = 4.10 by mixing equal volumes of equimolar solutions of ascorbic acid and sodium ascorbate.

3)

I)The capacity of a buffer to resist a change in pH (against acid and bases addition) is a function of the i) absolute concentration of the weak acid and weak base and also ii) their relative concentrations i.e. [Conjugate base]/[Acid] this ratio.

II)More moles of weak acid and weak base that buffer has, the more strong the base of acid (respectively) it can neutralize and maintain the buffer pH.

III)Relative concentrations i.e. [Conjugate base]/[Acid] this ratio determines the magnitude of pH change. A buffer with pH = pKa of acid i.e. [Conjugate base]/[Acid] this ratio =1 i.e. equimolar concentrations of weak and base resist pH change effectively. Greater amount of strong acid or base needed tobe added to have pH change of 1 unit.

4)Ionization of ascorbic acid show here.

Buffer system works on the basis of Le’ Chatelier’s principle states that,

A system in equilibrium resist the change by nullifying the constraint imposed on it working against the change.

On addition of acid i.e. H⁺ ions the stable conjugate base (formed by sodium ascorbate salt mainly) get protonated back to unconjugated ascorbic acid and added H+ ions get consumed.

And on addition of base i.e. OH^- ions ascorbic acid ionizes H⁺ ions to more extent and these protons neutralizes added HOHH^- ions by forming H2O.

And hence the mechanism of buffer in maintaining pH.