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Phosphate buffer - (Sep/11/2005 )

I need to make a 0,15M phosphate buffer pH 5,0.
How can I make it?
Please, help me!!!!!!!!! unsure.gif


look here


yp theo a good reference laugh.gif
btw : the two solutions you'll need to prepare must be at 0.15M. Adjust by mixing the two solutions and monitoring pH.


Why not use KH2PO4 alone and just adjust pH with H3PO4 (if it needs to be more acidic) or KOH (to be more basic)?
Any reason you mix the two (KH2PO4 and K2HPO4) ?


well i don't see any contradiction to that. But mixing K2HPO4 and KH2PO4 is easier as adding H3PO4... Moreover, buffering capacity of the mix is better.


QUOTE (fred_33 @ Sep 21 2005, 02:55 AM)
Moreover, buffering capacity of the mix is better.

Really? ...
It it essentially the same buffering system, I really did not see the reason for better buffering capacity.
If you think about it, K2HPO4+ H3PO4 would produce KH2PO4, then why bother mixing the two.
And making one buffer followed by adjusting pH sounds much easiler than making two and calculating how to mix.


well, i was thinking of the K+ ion. But you're right, it's the buffering capacity of H2PO4 and HPO4...
For preparing the buffer, i don't pass a long time to calculate. I start with the acidic solution and just add the second one, monitoring the pH. and i think it's easier as the pH increase slower as allows more precision when you're close to the final pH...