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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: jmarjorie on April 12, 2005, 11:05:30 AM

Title: Prep Buffer Ph 7.4
Post by: jmarjorie on April 12, 2005, 11:05:30 AM

Dear All,

I would like to know how to prepare a 1 Litre buffer of pH 7.4 and strength 100mM, using NaHCO3 and Na2H2P2O7 or other acidic sodium phosphates.
It is important that the ingredients are all solids, but soluble in water at 25 deg C and the pH of 7.4 should be maintained even when the 1 Litre buffer is diluted to 100 Litres.
Thanks in advance,

jmarjorie

Title: Re:Prep Buffer Ph 7.4
Post by: Borek on April 12, 2005, 07:08:17 PM

Forget about carbonic buffer - it will require a lot of dissolved CO2.

pKa2 for phosphoric acid is 7.2 so if you prepare solution containing both H2PO4(-) and HPO4(2-) you should be able to achieve what you want.

Title: Re:Prep Buffer Ph 7.4
Post by: AWK on April 13, 2005, 01:46:04 AM

Phosphoric acid here is a bad choose because diluting phosphate buffer 100 times will change pH significantly. Borek can calculate this with his program.

Title: Re:Prep Buffer Ph 7.4
Post by: Borek on April 13, 2005, 08:38:08 PM

If you can avoid changes in the ionic strength of the solution 100mM buffer has exactly the same pH as 1mM. If you can't avoid changes in the ionic strength it is not possible to prepare buffer that will not change in such range of concentrations. It doesn't matter what acid you will use.

OP mentioned diluting buffer, but you may always add inert salt to buffer ionic strength - this way pH buffer IS diluted, and the pH doesn't change due to changes in ionic strength.