Phosphate buffers and confusing results

[Calli]

I really just want to make my own phosphate buffers....i use:

 

Sodium phosphate mono and di (156 and 142g/mol respectively)

 

i use the general HH equation (pH = pka + log (salt/acid))

 

where the salt monobasic and acid is dibasic.

 

Ive worked out the ratio's (i.e for concentration acid + base = 1) and get answers that seem to be corroborated on the net:

 

i.e for ph 7:

 

7 = 7.2 + log (1-acid/acid)

acid = 0.613

salt = 1- 0.613 = 0.387

 

 

this works with others answers i.e.:

http://www.physicsforums.com/showthread.php?t=163100

(very old post i googled)

 

using this pka (7.21 ~7.2) you should expect 50:50 for a pH at 7.2 right? well all the table on the net give a 50:50 ratio of mono:di when the desired pH~6.9

 

which is what happened when i made a solution of ~50:50 i got a pH of 6.8 when i really wanted 7.2!

 

ANY HELP PLEASE AND THANKYOU! or if you would suggest a chemistry forum?

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note: the pKA for sodium phosphate is 7.21

 

but these tables make it look like the pkA is 6.9 or so. which i cannot back up on the net on pKA tables.

[CharonY]

First of all the Henderson-Hasselbalch equation does not give perfect results as it does not take effects like ionic strengths into account.

Second, it is possible that one of the pKa values is actually a thermodynamic (i.e. theoretical) one that may differ slightly from empirical values. In the lab the latter supersedes the former.

I do believe that at RT normally you will get pH of around 6.8 rather than 7.2.

[pippo]

I make PO4 buffer among hundreds of other reagents for a living. Although I cant address your question from that tech point of view (pKa's etc), If you wish simply to make your own, all you need to concern yourself with is the final pH. Check it on a calibrated meter, of course, and adjust with phos acid or NaOH 1N or whatever....it doesnt metter.

 

Of course, it has to be filtered through 0.2u filter.......