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Offline sanderson

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Buffer
« on: July 22, 2008, 04:14:44 PM »
Hi,

I have questions regarding buffer preparation.  Can I make phosphate buffer by using phosphoric acid at desirable amount + water ~ 80% and adjusted the pH to.. say...3.5 with NaOH?  I understand that the best way to make buffer is to mix the acid form and the base form of the buffer at the same concentration to get the desirable pH.  But is there anything wrong with just mixing acid with NaOH?   Will the buffer made from different ways be the same?  Will the amount of Na+ in these buffers be the same if they are made differently? Thank you for all your help.

Offline enahs

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Re: Buffer
« Reply #1 on: July 22, 2008, 04:26:36 PM »
What is the definition of a buffer?

Yes you could, but if you go back to the definition of the buffer, it will have such little buffering capacity it is probably not worth it. And it is not good to add extra ions that server no purpose, changes in ionic strength can have profound effects on things sometimes.



Offline Borek

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Re: Buffer
« Reply #2 on: July 22, 2008, 04:34:56 PM »
In general it doesn't matter how you prepare your buffer. In some specific circumstances buffer solutions will slightly different when it comes to spectator ions. In some very specific situations you may need to account for the water being product of neutralization. But I doubt you will ever see such cvase in normal lab work.

Please download my Buffer Maker (see buffer calculator link in the signature) and play with the program, it will allow you to make buffers from anything you want.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Borek

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Re: Buffer
« Reply #3 on: July 22, 2008, 04:38:02 PM »
Yes you could, but if you go back to the definition of the buffer, it will have such little buffering capacity it is probably not worth it. And it is not good to add extra ions that server no purpose, changes in ionic strength can have profound effects on things sometimes.

There is almost no difference between solutions made by mixing 10 mmol amounts of NaH2PO4/Na2HPO4 and solution made by mixing 20 mmol H3PO4 with 30 mmol NaOH. The only difference is that the latter contains 30 mmol of water more. So buffering capacity is not a problem. Extra ions can be.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline sanderson

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Re: Buffer
« Reply #4 on: July 22, 2008, 04:46:29 PM »
First of all, thanks for all the comments.

My concern is whether Na+ concentration will be the same from different ways of preparation. 
And when I make the buffer by mixing phosphoric acid and NaOH I also adjust the final volume after the pH is at 3.5. 


Offline Borek

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Re: Buffer
« Reply #5 on: July 22, 2008, 04:50:42 PM »
Na+ concentration will be the same, as long as after mixing both reagents used you dilute your solution to the same volume. If not, this additional water can slightly change the result. Note, that as long as you don't prepare highly concentrated buffers, amount of water produced can be safely neglected.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline enahs

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Re: Buffer
« Reply #6 on: July 22, 2008, 05:04:12 PM »
Yes you could, but if you go back to the definition of the buffer, it will have such little buffering capacity it is probably not worth it. And it is not good to add extra ions that server no purpose, changes in ionic strength can have profound effects on things sometimes.

There is almost no difference between solutions made by mixing 10 mmol amounts of NaH2PO4/Na2HPO4 and solution made by mixing 20 mmol H3PO4 with 30 mmol NaOH. The only difference is that the latter contains 30 mmol of water more. So buffering capacity is not a problem. Extra ions can be.


I swear I read the post as saying taking NaOH and adding phosphoric acid to it!
Edit! It was an edit! Yeah, that's it!  :P

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