Question

Explain the buffering components of urine, the chemical equation, where the system is found and find the ka and pKa of the system.

Answer 1

The phosphate buffer is the major buffer present in urine. The buffering action of the phosphate buffer can be shown as

H₂PO₄ (aq) <======> H⁺ (aq) + HPO₄^2- (aq)

The buffering action of the phosphate buffer in urine can be nicely explained by noting that H₂PO₄^- behaves as an acid and HPO₄^2- is a base. When a strong acid (H⁺) is added, the monohydrogen phosphate, HPO₄^2- combines with H⁺ to form dihydrogen phosphate, H₂PO₄^-. In this way, the phosphate buffer neutralizes a strong acid.

HPO₄^2- (aq) + H⁺ (aq) --------> H₂PO₄^- (aq)

When a strong base (OH^-) is added to the urine buffer, H₂PO₄^- neutralizes OH^- by forming HPO₄^2-.

H₂PO4^- (aq) + OH^- (aq) ---------> H₂O (l) + HPO₄^2- (aq)

The pK_a of the phosphate buffer is 7.21.

It is known that

pK_a = -log (K_a)

where K_a is the acid ionization constant of the phosphate buffer.

Since pK_a = 7.21, hence,

K_a = antilog (-pK_a)

= antilog (-7.21)

= 6.16*10^-8 (ans).