Question

Many buffers are polyprotic, such as carbonic acid. Bicarbonate (HCO3) has a pKa of 10.33; while Carbonic acid (HCO3) has a pKa of 6.35. Use the Henderson-Hasselbalch equation to mathematically determine which form of this buffer predominates at pH 7.4 (homeostasis pH of blood). Besides your mathematical answer, state the form that predominates too.

Answer 1

Carbonic acid/bicarbonate; bicarbonate

Step-by-step explanation:

H₂CO₃ + H₂O ⇌ H₃O⁺ + HCO₃⁻; pKₐ₁ = 6.35

HCO₃⁻ + H₂O ⇌ H₃O⁺ + CO₃²⁻; pKₐ₂ = 10.33

`\text{pH}& = &\text{pK}_{\text{a}} + \log\frac{[\text{A}^(-)]}{\text{[HA]}}\\\\7.4 & = &\text{pK}_{\text{a}} + \log\frac{[\text{A}^(-)]}{\text{[HA]}}`

The best buffer is one for which pKₐ ≈ pH.

6.35 is closer to 7.4, so the carbonic acid/bicarbonate form of the buffer predominates

The pH of the blood is higher (more basic) than the pKₐ of carbonic acid, so its basic form (bicarbonate, HCO₃⁻) predominates.