Question

3. Write out the chemical reactions that take place in a buffer when acid/base is added.

a). A phosphate buffer system is the major intracellular buffering system. Write out the three acid dissociation equilibrium equations for phosphoric acid. Identify which equation is the buffering system at physiological pH 7.4.

b). When hydrochloric acid is added to this buffer, write out the net chemical reaction equation that takes place to neutralize the acid. When acid is added, what species in the buffer is formed?

c). When sodium hydroxide is added to this buffer, write out the net chemical reaction equation that takes place to neutralize the base. When base is added, what species in the buffer is formed?

Answer 1

a) The formula of Phosphoric acid is H₃PO₄ . Now , it is a tribasic acid , which means that on addition of base , three protons come out of the moiety to neutralize the base.Now , due to the presence of four oxygen atoms , the negative charge created by the departure of proton is ne utralized by the resonance of one double bonded oxygen atom.So , the pKa₁ of the acid is very low ( around pH =3 ). Now , the conjugate base produced by the second proton departure is not so stabilized as thye first one due to increase in electron density and the degree of stabilization with resonance is lesser than first one.Hence , the second proton gets released at pH = 7.4.

When we titrate phosphoric acid with strong base, we see total three crests can be seen. Now , at the middle point of the second crest , we see the pH is almost equal to 7.4 , which denotes the departure of the second proton.

b) The aim of a buffer system is to maintain the pH of the system.Now , adding base or acid will make certain changes into the system.According to Hendersen - Hasselbalch equation ,

pH = pKa + log ( [A^-] / [HA]) ........... (x)

Now , adding hydrochloric acid means increasing the concentration of proton concentration , which in return , increases the amount of unreacted acid concentration.Here , the buffer system is

1) PO₄^3- + H⁺ HPO₄^2-

2) HPO₄^2- + H⁺ H₂PO₄^-

3) H₂PO₄^- + H⁺ H₃PO₄

So , adding HCl will increase the concentration of H⁺, which induces the formation of H₃PO₄ unreacted ( follows La - Scateliar principle).

So , from equation (x) , adding H⁺ , increases the concentration of HA which , decreases pH of the system. So, the buffer will become acidic buffer.

c) Adding strong base will abstract the proton from system, driving the equation to the formation of conjugate bases. So , from Hendersen - Hasselbalch equation , the amount of conjugated base is increased which , in terms increases the pH.

HA + OH^-H⁺ + A^-

increasing the amount of base , increases pH and hence , forms basic buffer.

1) H₃PO₄   H⁺ + H₂PO₄^-

2) H₂PO₄^- H⁺+ HPO₄^2-

3 ) HPO₄^2- H⁺ + PO₄^3-