[eleftheria]

I want to make citrate buffer 0.5M and pH4.5. Can someone help me with the recipe?
Thanks 

[Arkcon]

Well, I would start with the Henderson_haselbach equation, and look up the pKa for citric acid.  Then you can try to plug in the values yourself.  Once you're good at it, you'll be able to make any buffer you need.  Can you start your problem, along those lines, and see if it starts to work for you?

http://en.wikipedia.org/wiki/Henderson%E2%80%93Hasselbalch_equation

http://en.wikipedia.org/wiki/Citric_acid (since you're at the low end of the pH scale, you should use pKa1)

[Borek]

Yes. Buffer Maker.

You can also learn how to calculate buffers.

[eleftheria]

some recipes use citric acid and sodium citrate and some others only citric acid. if i use citric acid with sodium citrate, which analogy must i use?
thank you again!
I am a biologist and i don't know that things well....

[Borek]

Citric acid is not enough. Buffer solution must contain a pair of conjugate acid and base, so you need to either mix correct acid with correct conjugate base, or - starting with acid only - neutralize it with strong base so that the final ratio of acid and conjugate base is that granting requested pH.

Hopefullly you at least know what acin and conjugate base are 

[Borek]

since you're at the low end of the pH scale, you should use pKa1

No, pKa2.

pKa1 buffers with have too low pH.