What is the set of chemical equations that describe the buffering action of phosphate buffered saline (PBS)? Calculate theoretically the pH of phosphate buffered saline.
1 Answer
Here's what these equations are.
Explanation:
A phosphate buffered saline (PBS) buffer usually contains the following species
- Sodium chloride,
NaClNaCl ; - Potassium chloride,
KClKCl ; - Disodium phosphate,
Na_2HPO_4Na2HPO4 ; - Monopotassium phosphate,
KH_2PO_4KH2PO4 .
The two species thata give PBS its buffer capacity are the hydrogen phosphate,
An equilibrium reaction is established between these two ions in solution, with dihydrogen phosphate acting as an acid, i.e. donating a proton, and hydrogen phosphate acting as a base, i.e. accepting a proton.
When a strong acid is added to the buffer, the excess hydronium ions will be consumed by the hydrogen phosphate ion
The strong acid will thus be converted to a weak acid. Likewise, when a strong base is added, the excess hydroxide ions will be consumed by the dihydrogen phosphate ion.
The strongbase will thus be converted to a weak base.
In relation to equation
- Excess hydronium ions will shift the equilibrium to the left;
- Excess hydroxide ions will shift the equilibrium to the right.
In order to calculate the pH of a PBS buffer, you can use the Hendeson-Hasselbalch equation
In your case, you have
To get the
By definition,
The H-H equation becomes
If the buffer contains equal concentrations of hydrogen phosphate and dihydrogen phosphate, then the pH of the solution will be equal to the
Usually, 1X PBS buffers have a pH of about 7.4. A bigger concentration of hydrogen phosphate is used, which will determine the pH to be bigger than
A common way to prepare 1X PBS buffers is to use
This will give you
You'd then use hydrochloric acid to adjust the pH to 7.4.
Read more on that here:
