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 im 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. Lets 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 ?