Which equation correctly represents the Henderson–Hasselbalch expression for pH in terms of bicarbonate and carbonic acid?

Which equation correctly represents the Henderson–Hasselbalch expression for pH in terms of bicarbonate and carbonic acid?

• A. pH = pKa + log [HCO3-]/[H2CO3]
• B. pH = pKa + log [CO2]/[HCO3-]
• C. pH = pKa - log [HCO3-]/[H2CO3]
• D. pH = 7.4 + log [Na+]/[Cl-]

Answer: (A)

In this buffering system, pH is set by the ratio of the buffer’s base form to its acid form. For the bicarbonate–carbonic acid couple, bicarbonate (HCO3–) is the conjugate base and carbonic acid (H2CO3) is the weak acid. The Henderson–Hasselbalch form is pH = pKa + log([HCO3–]/[H2CO3]). The pKa in this system is the dissociation constant of carbonic acid, about 6.1 at body temperature. Practically, H2CO3 is in equilibrium with dissolved CO2, so [H2CO3] tracks [CO2(aq)], and the ratio of [HCO3–] to [H2CO3] determines the pH: increasing bicarbonate raises pH, while increasing carbonic acid lowers it. The given expression uses the correct base-to-acid ratio and the correct sign. The other forms would invert the relationship or use unrelated species, so they don’t describe this buffer accurately.

In this buffering system, pH is set by the ratio of the buffer’s base form to its acid form. For the bicarbonate–carbonic acid couple, bicarbonate (HCO3–) is the conjugate base and carbonic acid (H2CO3) is the weak acid. The Henderson–Hasselbalch form is pH = pKa + log([HCO3–]/[H2CO3]). The pKa in this system is the dissociation constant of carbonic acid, about 6.1 at body temperature. Practically, H2CO3 is in equilibrium with dissolved CO2, so [H2CO3] tracks [CO2(aq)], and the ratio of [HCO3–] to [H2CO3] determines the pH: increasing bicarbonate raises pH, while increasing carbonic acid lowers it. The given expression uses the correct base-to-acid ratio and the correct sign. The other forms would invert the relationship or use unrelated species, so they don’t describe this buffer accurately.