[Juliaffe]

Hello guys! From citric acid  (0.1 M C6H8O7*H2O) and sodium phosphate (0.2 M Na2HPO4*2H2O), how can I adjust the volume of each stock solution to create the buffer aiming at different pH (4; 4.4; 4.8, and so on)? Mix X ml of citric acid to Y ml of phosphate solution and dilute the mixture to finally get 400 ml of buffer at the desired pH.
How can I figure out X and Y in the case that I want to make this buffer at pH 4?
Can you please explain this? Thanks a lot!!

[Borek]

Calculations are not easy. You can use a specialized software like the Buffer Maker, or google for McIlvaine buffer recipes.

[Babcock_Hall]

Table 10.45 in DD Perrin and B Dempsey's book "Buffers for pH and Metal Ion Control" may help.

[Juliaffe]

Thank you very much @Borek and @Babcock_Hall  I found the McIlvaine buffer recipes, however, I still cannot understand the way they calculated it. I am not specializing in Chemistry, so it becomes even harder for me..

[Borek]

As I said, calculations are not easy. See the introduction here: http://www.chembuddy.com/?left=pH-calculation&right=toc

[Babcock_Hall]

Citrate has three pK_a values.  Phosphate has two relevant pK_a values.  In both cases the pK_a values will be affected by ionic strength, making the problem even more difficult.  There was a thread on this buffer about six years ago:  http://www.chemicalforums.com/index.php?topic=45278.0