How to use
Type the desired pH into the first cell, and type the intended buffer strength (in millimoles per liter) in the second cell. Press the calculate button, and the approximate percentages of anhydrous citric acid and trisodium citrate dihydrate will be displayed in %weight/volume. For example, a pH of about 4.0 with a buffer strength of 10 mM is obtained using 0.12% anhydrous citric acid and 0.11% trisodium citrate dihydrate. The buffer may be made by adding 0.12 g anhydrous citric acid and 0.11 g trisodium citrate dihydrate to 100 ml water.
How the calculation works
Using the three pK's of citric acid and the pH, the ratio's of each of the citrate pairs is calculated. For example, the first ionization is given by:
H3Cit --> H+ + H2Cit-
K1 = [H+][H2Cit-] / [H3Cit], or
[H2Cit-] / [H3Cit] = K1 / [H+]
Normalizing the amount of each of the four moieties, their relative amounts are calculated. The molar amounts of citric acid and sodium citrate are derived from the following two equations.
Citric Acid = [H3Cit] + (2/3)[H2Cit-] + (1/3)[HCit2-]
Sodium Citrate = [Cit3-] + (2/3)[HCit2-] + (1/3)[H2Cit-]
Using the molecular weights for anhydrous citric acid and sodium citrate dihydrate (the typical lab forms) together with the buffer strength, the actual percentages are calculated.
The pK's used for citric acid are 3.15, 4.50 and 5.75. It's best to buffer at a pH close to one of the pK's, so use citrate buffers only in the pH range 3-6.
What buffer strength to use? Too low will give a weak, drifting buffer, while too much may negatively affect other desired properties, such as taste. A 10 mM buffer is in general a good starting point.
Acetic Acid Buffer Calculator