The pH buffer range and dissociation constant of Tris and other buffers

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Biological buffers such as Tris, Bicine, etc., are very common auxiliary reagents used in biochemical experiments. Different buffers have different buffer ranges. The buffer range is usually calculated by adding or subtracting the corresponding dissociation constant pKa. This article introduces pH The relationship between buffer range and dissociation constant.


In fact, the pH buffer range used in most of the more demanding or more precise experiments is the dissociation constant pKa plus or minus 0.5. Here, two buffers, Tris and Bicine, are used as examples to illustrate the relationship between their dissociation constant and the pH buffer range.


Dissociation equilibrium in Tris buffer

Tris is slightly alkaline when dissolved in water. When dissolved in water, the lone electron pair on the amino nitrogen atom of Tris will combine with hydrogen ions in the solution to become HTris+ ions with a structure similar to ammonium salts. If the hydrogen in the water is looted, it will be out of the shop. Hydroxide is alkaline. The Tris buffer also needs to add some acid such as hydrochloric acid, the hydroxide radical is neutralized to leave the chloride ion. In the system, HTris+ ions and Tris molecules form an acid-base buffer pair, and the dissociation constant:


Ka = [H+][A-]/[HA] (monobasic acid) (for dibasic acid, replace [A-] with [HA-]), and pH = lg [H+], then pH=pKa+lg (A-/HA)


In the formula, A- is the negative ion of A, and H is the hydrogen ion-this is the Henderson-Hassel-Balch equation. In general, there is no direct relationship between pH and pKa. However, at the half-metering point, where the amount of salt and unionized acid or alkali is equal, the pH and pKa are equal.


Tris dissociation constant and buffer range

For Tris buffer solution, when the molecular weight of dissociated HTris+ is equal to that of undissociated Tris, the pH is equal to pKa; so pKa plus or minus 1 means that the degree of dissociation is 1/11 to 10/11; and pKa plus or minus 0.5 means The degree of dissociation is about 1/4 to 3/4. It can be seen that there is still a significant difference in the extreme degree of dissociation between the two cases.


The same Bicine is similar, except that it directly ionizes hydrogen ions. The acid-base ions of Bcine and Bicine molecules form an acid-base buffer pair. The pH is still the dissociation constant minus the negative logarithm of the ratio of dissociated/undissociated particles. Buffers, especially Tris, have a large degree of dissociation that is affected by temperature, and the pH buffer range can be calculated according to the formula. Desheng is a manufacturer of in vitro diagnostic reagents, which can provide various biological buffers.