Henderson-Hasselbalch & phosphate buffers

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In summary, the major phosphate species in a solution with a pH of 7.4 and a total cellular concentration of 20 mM is [HPO42-] = 12.25 mM and [H2PO4-] = 7.75 mM, based on the Henderson-Hasselbalch equation and simultaneous equations.
  • #1
chops369
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This is an example calculation about the phosphate buffer system from my Biochemistry textbook.

Homework Statement


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


Homework Equations


pH = pKa + log10 [A-] / [HA]

pH = -log10 [H+]


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.
 
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  • #2
chops369 said:
[HPO42-] / [H2PO4-] = 1.58

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

These are two simultaneous equations in two unknowns - just solve.
 
  • #3
Borek said:
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.
 

Related to Henderson-Hasselbalch & phosphate buffers

1. What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation is a mathematical expression used to calculate the pH of a buffer solution. It takes into account the concentration of the weak acid and its conjugate base, as well as the acid dissociation constant (pKa) of the weak acid.

2. How is the Henderson-Hasselbalch equation used in phosphate buffers?

The Henderson-Hasselbalch equation is used to determine the pH of a phosphate buffer solution, which is a common type of buffer used in biological and biochemical experiments. The equation takes into account the pKa values of the two phosphate species (H2PO4- and HPO42-) present in the buffer solution, as well as their respective concentrations.

3. What is the significance of using phosphate buffers in biological experiments?

Phosphate buffers are important in biological experiments because they can maintain a relatively constant pH even when small amounts of acid or base are added. This is because the two phosphate species present in the buffer can act as both an acid and a base, helping to neutralize any changes in pH.

4. How do phosphate buffers help maintain pH in living organisms?

In living organisms, phosphate buffers play a crucial role in maintaining the pH of bodily fluids and tissues. They can neutralize any excess acid or base produced in metabolic processes, which helps to keep the body's pH within a narrow range that is necessary for proper cellular function.

5. Are there any limitations to using phosphate buffers?

Phosphate buffers have limitations in terms of their effectiveness at extreme pH values (very acidic or basic). They also have a limited buffering capacity, meaning they can only maintain a stable pH within a certain range of acid or base concentrations. In addition, phosphate buffers may not be suitable for all types of experiments, as they can interact with certain molecules and alter their behavior.

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