Chemistry Reactions Essay

1105 Words May 11th, 2013 5 Pages
Buffer solutions and common ion effect

A buffer solution resists (or buffers) a change in its pH. That is, we can add a small amount of an acid or base to a buffer solution and the pH will change very little.

How to calculate pH of buffer solution containing both acid and conjugate base? Dissociation constant definition 1.1 can be rearranged into
[pic]
or
[pic]
(note that due to sign change [A-] was moved to nominator).
This is so called Henderson-Hasselbalch equation (or buffer equation). It can be used for pH calculation of solution containing pair of acid and conjugate base - like HA/A-, HA-/A2- or B+/BOH. For solutions of weak bases sometimes it s more convenient to use equation in the form
[pic]15.3

Two common types of
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The pH of the buffer is governed by the equilibrium

CH3COOH --- CH3COO– + H+

The buffer solution has a large excess of CH3COO– ions produced by complete ionisation of sodium acetate,

CH3COONa ---CH3COO- + Na+

(1) Addition of HCl.

Upon the addition of HCl, the increase of H+ ions is counteracted by association with the excess of acetate ions to form unionised CH3COOH. Thus the added H+ ions are neutralized and the pH of the buffer solution remains virtually unchanged. However owing to the increased concentration of CH3COOH, the equilibrium shifts slightly to the right to increase H+ ions. This explains the marginal increase of pH of the buffer solution on addition of HCl.

(2) Addition of NaOH.

When NaOH is added to the buffer solution, the additional OH– ions combine with H+ ions of the buffer to form water molecules. As a result the equilibrium (1) shifts to the right to produce more and more H+ ions till practically all the excess OH– ions are neutralised and the original buffer pH restored. However, a new equilibrium system is set up in which [CH3COOH] is lower than it was in the original buffer. Consequently [H+] is also slightly less and pH slightly higher than the buffer pH values Operation of a Basic buffer as NH4OH/NH4Cl can also be explained on the same lines as of an acid buffer upon addition of HCl the H+ ions combine with

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