Question

Assays were preformed by mixing AChE, THC, and 340 microM DTNB in 100 mM pH phosphate bufer, pH 8.0 containing 5% methanol. Ignoring the methanol, determine the concentrations of the conjugate base and the weak acid in the phosphate buffer. The total buffer concentration is 100 mM. Please indicate the source where you obtained the pKa value to use in the Henderson-Hasselbalch equation. You must determine the correct conjugate pair.

Answer 1

Phosphate-buffer buffer solution commonly used in biological research. It is a water-based salt solution containing sodium hydrogen phosphate and sodium dihydrogen phosphate.

The equation of the buffer is

H₂PO₄^- +    H⁺        H₃PO₄
weak base                              conjugate acid
dihydrogen phosphate           phosphoric acid

pKa of KH₂PO₄ is around 6.86

so from the Henderson-Hasselbalch equation.

pH = pK_a + log [OH^-]/[H]

8.0 = 6.86 + log(Na2HPO4/NaH2PO4)

Solving for this we get

Na2HPO4 = 15.37 g/L

NaH2PO4 = 0.9559 g/L