I am preparing a phosphate and a bicarbonate buffer at pH 7.4. I am not using the tradition of making a "rough" buffer and then adjusting the pH to the desired value as I want to target a certain value of osmolarity in both buffers. As such I just want to do all calculations beforehand to determine the exact amounts of the compounds I would need in the solution. I think I should be able to do that, except I want to accurately determine that ratio of the acid and conjugate base, accounting for the effect of temperature on pka. But the only values for the pka I could find were the generally quoted values which I believe are at 20 Celsius. Is this known as the thermodynamic pka? How do I account for the effect of temperature on the pka into my calculations? Or is not required to do so? And what is the difference between thermodynamic ka and apparent ka anyways? Greatly appreciate any help!
bicarbonate buffer: H2CO3 <--> HCO3- + H+
Phosphate buffer: H2P04- <--> HPO42- + H+
Pka = ([H+][A-])/([HA])
The Attempt at a Solution
I found a buffer calculator (http://www.biomol.net/en/tools/buffercalculator.htm) that allows you to input the temperature. When the recipe is produced, it also gives an apparent pka value. So is safe to simply use this apparent value in my calculations? Or are there any other considerations i must make such as ionic strength? But I don't fully understand what ionic strength is and how it affects the buffer.