Question

THERMODYNAMICS

You have a phosphate buffer at pH 7.8. What ionic constituents are present in the buffer?

What are the fractions of each ionic constituent present at pH 7.8?

Answer 1

A buffer solution is combination of a weak acid and its conjucate base or a weak base and its conjucate acid

phosphate buffer is combination of Weak acid H2PO4^- and its conjucate base HPO4^2-

H2PO4^- <------> HPO4^2- + H⁺

Henderson - Hasselbalch equation is used to calculate the fracrion of weak acid and conjucate base

pH = pKa + log([A^-]/[HA])

where, A^- and HA are conjucate base and weak acid respectively

pKa of H2PO4^- = 7.20

required pH = 7.8

7.8 = 7.20 + log([HPO4^2-]/[H2PO4^-])

log([HPO4^2-]/[H2PO4^-]) = 0.60

[HPO4^2-]/[H2PO4^-] = 3.981

[HPO4^2-] = 3.981×[HPO4^-]

if total buffer concentration = 1M

[H2PO4^- ] + 3.981[H2PO4^-] = 1M

4.981[H2PO4^- ] = 1M

[H2PO4^-] = 0.2008M

[HPO4^2-] = 0.1 - 0.2008M = 0.7992

Therefore,

Fraction of H2PO4^- = 0.2008

Fraction of HPO4^2- = 0.7992