Phosphate Buffer Calculator
A Javascript that calculates the amount of monosodium phosphate and disodium phosphate necessary to achieve a buffer at a given pH and buffer 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. 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, 6.795 and 12.319. Different values for the pKa's of phosphoric acid appear in the literature. For pure water at 38 C, RL Jungas (Analyt Biochem 349; 1-15) concludes that the best value for the second dissocation constant of phosphoric is 7.18. However, under physiological conditions of ionic strength and pH, the best estimate is 6.795. 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. 5,
Index of other pages by the author. Please send comments to Jeffrey Clymer