The pH of blood circulating in a human body is maintained around. `7.4` by the action of the buffer system

Options

A. `CH_(3)COOH//CH_(3)COONa`
B. `NH_(4)CI//NH_(3)`
C. `H_(2)PO_(4)^(-)//HPO_(4)^(2-)`
D. `CO_(2)//HCO_(3)^(-)`

Answer

D

Explanation

Blood, as a buffer solution, contains `H_(2)CO_(3)` as well as other conjugate acid-base pairs. Carbon dioxide might not appear to be an acid, since it has no proton of it froms carbonic acid `(H_(2)CO_(3))`:
`CO_(2)(aq.)+H_(2)hArrH_(2)CO_(3)(aq.)`
The conjugate base of carbonic acid is the bicarbonate ion, `HCO_(3)^(-)`:
`H_(2)CO_(3)(aq.)hArrHCO_(3)^(-)(aq.)+H^(+)(aq.)`
The overall reaction relating carbon dioxide and bicarbonate ion is the sum of these two reactions:
`CO_(2)(aq.)+H_(2)OhArrHCO_(3)^(-)(aq.)+H^(+)(aq.)`
The `pK_(a)` of the equilibrium for this buffer system is `6.37`. A second class of physiological buffer is based on the dihydrogen phosphate anion `(H_(2)PO_(4)^(-))` which is a weak acid and hydrogen phosphate `(HPO_(4)^(2-))`, its conjugate base: `H_(2)PO_(4)^(-)(aq.)hArrHPO_(4)^(2-)(aq.)+H^(+)(aq.)` Again the `pK_(a)(7.21)` is close to the physiological pH.