Dilution of Buffer Solutions
Dilution Effects on pH changes – Buffer
For avoidance of doubts, in this post, we are referring to acidic buffer.
In Acid-base Equilibria, there is a portion of the syllabus on buffers. Normal questions will ask for an outline of how the buffer works. They might ask you what is the mass of strong acid or base to a weak acid/base to add to create a buffer at a certain pH.
A more advanced question will ask you what effects will dilution have on the pH. Generally, dilution has NO effect on the pH. Why is this so? If you look at the buffer formula, pH = pKa + lg [salt]/[acid], dilution does not affect the [salt]/[acid] ratio. (volume cancels out)
But the issue comes when the question further asks you will the pH change be larger if you dilute the buffer? We have to be careful here to watch out for the conditions. It is important to note whether the total volume of the buffer changed. Why is this important?
Buffer: 3 Scenarios
Firstly, if the volume changed drastically, i.e. large volumes of water was added to the buffer, the pH will tend to move towards 7. Which is the pH of water. This is because concentration of H+ ions tends to be closer to the amount from auto-ionization of water.
Second, if the total buffer volume remained constant, it means that dilution AND removal of some of the solution had happened. (since adding water increases the total volume) In this case, although the [salt]/[acid] ratio remains constant, adding a small amount of strong acid will cause a larger change in the ratio. Note that removing some of the solution decreases the number of moles of both salt and acid.
Example
Before buffer dilution: [salt]/[acid] ratio is 5/5. So when a small amount of strong acid is added, say 0.1 mol, the ratio becomes (5-0.1)/(5+0.1)
4.9/5.1
After dilution: ratio becomes 0.5/0.5. So when a small amount of acid is added, the ratio becomes 0.4/0.6
It should be apparent that the salt to acid ratio is significantly different after dilution. Thus the pH change will also be larger.
Third, if the total volume DID NOT remain constant, it means that total volume has increased. If so, then the number of moles of salt and acid remained the same after dilution. So when a small amount of strong acid is added, the ratio is 4.9/5.1, as if there was no dilution! Why?
e.g. for a buffer with 5 mol of both salt and acid in 1 dm3 of water, we dilute it with another 1 dm3 of water. The ratio will remain the same since the volume will cancel out.
Hence you see that what’s crucial is actually the number of moles of salt and acid, as these will determine how large is the pH change.
Read also: Daily application of buffer solutions
Sharing is caring
Has this explanation helped you? If it has, share it with your friends via the sharing buttons below!
Connect with me on other socials here
Feel free to browse through the blog for more posts on other interesting topics 🙂 eg check out this post on Energetics! Find out which orbital 3d or 4s has higher energy!
Back to home of Top Chemistry Tuition
Click here to learn about A level Chemistry syllabus
Read: How to choose the right JC Chem Tuition or O level Chem Tuition