Phosphate Buffer Calculator
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 monosodium phosphate, monohydrate and disodium phosphate, heptahydrate will be displayed. For example, a pH of about 7.4 with a buffer strength of 50 mM is obtained using 0.1558% monosodium phosphate monohydrate and 1.0374% disodium phosphate, heptahydrate. The buffer may be made by adding 0.16 g monosodium phosphate monohydrate and 1.04 g disodium phosphate, heptahydrate to 100 ml water.
How the calculation works
Using the three pK's of phosphoric acid and the pH, the ratio's of each of the phosphoric pairs is calculated. For example, the first ionization is given by:
H3PO4 --> H+ + H2PO4-
K1 = [H+][H2PO4-] / [H3PO4], or
[H2PO4-] / [H3PO4] = K1 / [H+]
Normalizing the amount of each of the four moieties, their relative amounts are calculated. The molar amounts of monosodium phosphate and disodium phosphate are derived from the following two equations.
Monosodium Phosphate = [H3PO4] + [H2PO4-]
Disodium Phosphate = [PO43-] + [HPO42-]
Using the molecular weights for monosodium phosphate, monohydrate and disodium phosphate, heptahydrate (the typical lab forms) together with the buffer strength, the actual percentages are calculated.
The pK's used for phosphoric acid are 2.148, 6.865 and 12.319. It's best to buffer at a pH close to one of the pK's, so use this calculation only for phosphate buffers in the pH range 4.8 to 8.8.
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.
A more general buffer page by Dave Robinson
Version No. 4,