A Java applet that calculates the amount of citric acid and sodium citrate necessary to achieve a buffer at a given pH and strength.
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. Change the molecular weights if the values for your salts differ from those given.
Press the calculate button, and the approximate masses of anhydrous citric acid and sodium citrate dihydrate for one liter of solution will be displayed. For example, a pH of about 4.0 with a buffer strength of 10 mM is obtained using 1.3 g citric acid monohydrate and 1.1g sodium citrate dihydrate. The molar ratios of citric acid and sodium citrate are given in the Java control panel.
How the calculation works
Using the three pK's of citric acid and the pH, the ratio of each citrate pair 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 citric acid (anhydrous: 192.1, monohydrate: 210.1) and sodium citrate dihydrate (294.1) 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.
Version 1.1, November 19, 2000. Scott Calabrese Barton
Version 1.0, September 8, 1997. Jeffrey Clymer
Index of other pages by Jeffrey Clymer