Chapter 15, Problem 87AE

Phosphate buffers are important in regulating the pH of intracellular fluids at pH values generally between 7.1 and 7.2.

a. What is the concentration ratio of H₂PO₄⁻ to HPO₄²⁻ in intracellular fluid at pH = 7.15?

${\text{H}}_2{\text{PO}}_4{}^{-}(aq)\rightleftharpoons {\text{HPO}}_4{}^{2-}(aq)+{\text{H}}^{+}(aq)\begin{array}{cc}& K_{\text{a}}=6.2\times {10}^{-8}\end{array}$

b. Why is a buffer composed of H₃O₄ and H₂PO₄⁻ ineffective in buffering the pH of intracellular fluid?

${\text{H}}_3{\text{PO}}_4(aq)\rightleftharpoons {\text{H}}_2{\text{PO}}_4{}^{-}(aq)+{\text{H}}^{+}(aq)\begin{array}{cc}& K_{\text{a}}=7.5\times {10}^{-3}\end{array}$