- #1
peak99
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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 ?
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 ?