Buffers A guide for the preparation and use of buffers in biological systems V2

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8Buffers, Buffer Capacity and Range Buffers are aqueous systems that resist changes in pH when small amounts of acid or base are added. Buffer solutions are composed of a weak acid (the proton donor) and its conjugate base (the proton acceptor). Buffering results from two reversible reaction equilibria in a solution wherein the concentration of proton donor and its conjugate proton acceptor are equal. For example, in a buffer system when the concentration of acetic acid and acetate ions are equal, addition of small amounts of acid or base do not have any detectable influence on the pH.This point is commonly known as the isoelectric point. At this point there is no net charge and pH at this point is equal to pKa.

 

[CH3COO ̄]pH = pKa+ log ________________[CH3COOH]At isoelectric point [CH3COO ̄] = [CH3COOH] hence, pH = pKa Buffer capacity is a term used to describe the ability of a given buffer to resist changes in pH on addition of acid or base. A buffer capacity of 1 is when 1 molof acid or alkali is added to 1 liter of buffer and pH changes by 1 unit. The buffer capacity of a mixed weak acid-base buffer is much greater when the individual pKa values are in close proximity with each other. It is important to note that the buffer capacity of a mixture of buffers is additive.Buffers have both intensive and extensive properties. The intensive property is a function of the pKa value of the buffer acid or base. Most simple buffers work effectively in the pH scale of pKa± 1.0. The extensive property of the buffers is also known as the buffer capacity. It is a measure of the protection a buffer offers against changes in pH. Buffer capacity generally depends on the concentration of buffer solution. Buffers with higher concentrations offer higher buffering capacity. On the other hand, pH is dependent not on the absolute concentrations of buffer components but on their ratio.Using the above equation we know that when pH = pKa the concentrations of acetic acid and acetate ion are equal. Using a hypothetical buffer system of HA(pKa= 7.0) and [A–], we can demonstrate how the hydrogen ion concentration,[H+], is relatively insensitive to external influence because of the buffering action.

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