How to determine the pH of citrate buffer system

I used this table (<http://microscopy.berkeley.edu/Resources/instruction/buffers.html>) to make a citrate buffer system. The Pka value is 6.4, and I multiplied the solution by ten to make a larger batch. I used 137ml of citric acid and 363ml sodium citrate in distilled water to make a buffer system with pH of 5.6. Now I'm trying to work backwards to prove this was correct, and using the equation pH = pKa + log(A-/HA) I am getting 6.4 + log 0.0137/0.0363 = 5.97, when it should be 5.6. Anyone know what I'm doing wrong? Thanks!


