Solving the HH equation for a Phosphate Buffer

[peak99]

I have a relatively simple problem. I'm used to making Tris buffer and always use the HH equation to figure it out initially for my ratio of acid/salt in percentage for my given pH.

This is not working for phosphate buffers and really getting me frustrated right now. I know i could just use a table but i like to know what I am doing wrong:

I have Sodium phosphate monobasic and dibasic (acid/salt respectively)

the pKa is 7.21

knowing: pH = pK + log (salt/acid)

I want a pH of 7.2. Let's say pKa is 7.2 for simplicity:

[acid] + [salt] = 100% (1)
salt = 1-acid

7.2 = 7.2 + log (1-acid/acid)
10^ 0 = (1-acid)/acid
2*acid = 1
acid =0.5
salt= 0.5

i know its reallllly .506 and .494 for pka is really 7.21

BUT using this ratio does not give anything close to 7.2: more like 6.8 ( i made a 0.5L of 200mM using 49.4mmol Na2HPO4 an 50.6mmol NaH2PO4 = 100mmol/0.5L = 200mM) the pH was 6.8

So:
I Double checked phosphate buffer charts and low and behold the 50:50 ratio in fact gives a pH of ~6.8 or 6.9.

Why is this if the pKA is 7.21 ?

 

[Borek]

ionic-strength-activity-coefficients.

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[peak99]

Thanks,

I considered this, but i had thought that ionic strength was only a problem in higher concentrations this complicates life...

Aside,
How come HH works for Tris buffers even with the addition of NaCl, but not phosphate?

 

[Borek]

It works for both cases. Could be TRIS pKa value that you use is already corrected for ionic strength.

Check out Buffer Maker, it does the ionic strength calculations automatically.

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[epenguin]

I think if you work out what you have got in the solution, if each is at pH equal to their pK, the phosphate buffer has 5X the ionic strength as the same concentration of tris, so ionic strength effects would be expected greater. Just try diluting the two buffers and tell us what you observe.