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### Topic: Buffer soln - how much concentration is correct?  (Read 4963 times)

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#### charzhino

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##### Buffer soln - how much concentration is correct?
« on: January 10, 2012, 07:05:09 AM »
Hi, I want to make a buffer solution using citric acid using 1M sodium hydroxide. I will plan to make the citric acid solution at 0.1M. What is the general concentration this can be, maximum/minimum? I am using it between 3-6 pH and plan to make this particular solution till 5.5 pH. Will making the concentration more than 0.1M alter the buffer's properties?Thanks alot.

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##### Re: Buffer soln - how much concentration is correct?
« Reply #1 on: January 13, 2012, 12:27:24 AM »
The buffer capacity is depending on the concentrations. pH value only depend on the concentration ratio of citrarate/citric at constant temperature.

#### Borek

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##### Re: Buffer soln - how much concentration is correct?
« Reply #2 on: January 13, 2012, 03:22:58 AM »
pH value only depend on the concentration ratio of citrarate/citric at constant temperature.

This is true, but in a twisted way. Yes, for a given ratio pH is constant, but when you dilute the solution ratio changes and pH approaches 7. See discussion on the Henderson-Hasselbalch equation page.
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##### Re: Buffer soln - how much concentration is correct?
« Reply #3 on: January 16, 2012, 09:24:23 AM »
Borek, According to my knowledge buffer is a system which can keep its pH approximately constant upon adding a little amount of acid, base or water. So, dilution may not severely effect pH you explained. Specially concentration ration is not changed upon addition of water.

But, dilution changes the ionic strength and increase the activity coefficient, and decrease pH. Dilution is not effecting to the pH of the buffer in any other way upon a few times dilution.

But buffer capacity is decreased and  reaches to zero upon continued dilution.

#### Borek

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##### Re: Buffer soln - how much concentration is correct?
« Reply #4 on: January 16, 2012, 10:30:08 AM »
dilution may not severely effect pH you explained. Specially concentration ration is not changed upon addition of water.

Page I linked to contains result of exact equilibrium calculations. Please either read the page, or try to calculate pH on your own for different solutions of the acetic buffer (not using HH equation, as it will be misleading - but if you want to, please do, however, show then concentrations of all ions present in the solution; I will show you that these solutions are impossible).

Quote
But buffer capacity is decreased and  reaches to zero upon continued dilution.

Both things happen at the same time - buffer capacity gores down, pH drifts in the direction of 7. Both things are part of the same process.
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##### Re: Buffer soln - how much concentration is correct?
« Reply #5 on: January 17, 2012, 11:54:24 PM »
I had a look at the page you attached. According to the page you refer, See, the pH is remaining unchanged upon 10 times dilution. Little bit changed upon 100 times dilution. First decimal point of pH changed only upon 1000 times dilution.

Dilution is not effecting to the pH of the buffer in any other way upon a few times dilution.

Buffer is being effective until 1000 times dilution as your reference. As I told upo,n a few times dilution pH is not changed and your reference is also an evidence for that. I am not misleading.

#### Borek

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##### Re: Buffer soln - how much concentration is correct?
« Reply #6 on: January 18, 2012, 05:34:57 AM »
It is all a matter of how strong the statement "buffer don't change pH upon dilution" you want to be. In a reasonable dilution range pH doesn't change, unfortunately, way too often people treat this idea as if it was a given truth that holds always.

So far all your posts contained two statements - that buffer capacity tends to zero with the dilution and that pH of the buffer doesn't change with dilution. Each of these statements has its range of applicability. Listing them both together without any comment, you seem to be suggesting they work identically at the same time. They don't, which is why your posts can be confusing for people not understanding limitations of the buffer systems.
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