Register to reply

Henderson-Hasselbalch & phosphate buffers

Share this thread:
chops369
#1
Sep11-11, 12:27 PM
P: 57
This is an example calculation about the phosphate buffer system from my Biochemistry textbook.

1. The problem statement, all variables and given/known data
If the total cellular concentration of phosphate is 20 mM (millimolar) and the pH is 7.4, the distribution of the major phosphate species is given by

pH = pKa + log10 [HPO42-] / [H2PO4-]

7.4 = 7.20 + log10 [HPO42-] / [H2PO4-]

[HPO42-] / [H2PO4-] = 1.58

Thus, if [HPO42-] + [H2PO4-] = 20 mM, then

[HPO42-] = 12.25 mM and [H2PO4-] = 7.75 mM


2. Relevant equations
pH = pKa + log10 [A-] / [HA]

pH = -log10 [H+]


3. The attempt at a solution
I understand everything up until they provide the concentrations of each phosphate species. Since their ratio as shown in the equation is 1.58, one can clearly assume that [HPO42-] > [H2PO4-]. But the fact that no explanation is provided for arriving at their specific concentrations is driving me insane.

The Henderson-Hasselbalch equation shows that, when [HPO42-] / [H2PO4-] = 1, pH = pKa. But since we are at pH = 7.4, they obviously can't be equal. I think the solution must involve taking the 0.2 difference into account somehow.
Phys.Org News Partner Science news on Phys.org
Bees able to spot which flowers offer best rewards before landing
Classic Lewis Carroll character inspires new ecological model
When cooperation counts: Researchers find sperm benefit from grouping together in mice
Borek
#2
Sep11-11, 12:52 PM
Admin
Borek's Avatar
P: 23,396
Quote Quote by chops369 View Post
[HPO42-] / [H2PO4-] = 1.58

Thus, if [HPO42-] + [H2PO4-] = 20 mM, then
These are two simultaneous equations in two unknowns - just solve.
chops369
#3
Sep11-11, 02:15 PM
P: 57
Quote Quote by Borek View Post
These are two simultaneous equations in two unknowns - just solve.
Wow. I stared at that problem for 2 hours...I can't believe the answer was right there. Thanks for clearing that up.


Register to reply

Related Discussions
Henderson Hasselbalch equation question Biology, Chemistry & Other Homework 0
Henderson-Hasselbalch equation Biology, Chemistry & Other Homework 3
Henderson-Hasselbalch equation Biology, Chemistry & Other Homework 6
Protein pH titration (Henderson-Hasselbalch) Biology, Chemistry & Other Homework 5
Henderson Hasselbalch equation! Where did I go wrong? Biology, Chemistry & Other Homework 3