Buffer regions in weak acid-strong base titrations are caused by the presence of conjugate acid-base pairs
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Chemical reference sources confirm that buffer regions during the titration of a weak acid with a strong base are formed and maintained by the presence of conjugate acid-base pairs.
dissociation in the context of acid–base reactions. The chemical species HA is an acid that dissociates into A−, called the conjugate base of the acid, and a
In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted
K
a
{\displaystyle K_{a}}
) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction
HA
At half-neutralization the ratio [A−]/[HA] = 1; since log(1) = 0, the pH at half-neutralization is numerically equal to pKa. Conversely, when pH = pKa, the concentration of HA is equal to the concentration of A−.
The buffer region extends over the approximate range pKa ± 2. Buffering is weak outside the range pKa ± 1. At pH ≤ pKa − 2 the substance is said to be fully protonated and at pH ≥ pKa + 2 it is fully dissociated (deprotonated).
If the pH is known, the ratio may be calculated. This ratio is independent of the analytical concentration of the acid.
In water, measurable pKa values range from about −2 for a strong acid to about 12 for a very weak acid (or strong base).
A buffer solution of a desired pH can be prepared as a mixture of a weak acid and its conjugate base. In practice, the mixture can be created by dissolving the acid in water, and adding the requisite amount of strong acid or base. When the pKa and analytical concentration of the acid are known, the extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated using an ICE table.
The experimental determination of pKa values is commonly performed by means of titrations, in a medium of high ionic strength and at constant temperature. A typical procedure would be as follows. A solution of the compound in the medium is acidified with a strong acid to the point where the compound is fully protonated. The solution is then titrated with a strong base until all the protons have been removed. At each point in the titration pH is measured using a glass electrode and a pH meter. The equilibrium constants are found by fitting calculated pH values to the observed values, using the method of least squares.
The total volume of added strong base should be small compared to the initial volume of titrand solution in order to keep the ionic strength nearly constant. This will ensure that pKa remains invariant during the titration.
A calculated titration curve for oxalic…
Figure is used with the permission of J.A. Freyre. The Titration Curve
The titration curve is a graph of the volume of titrant, or in our case the volume of strong base, plotted against the pH. There are several characteristics that are seen in all titration curves of a weak acid with a strong base. These characteristics are stated below. - The initial pH (before the addition of any strong base) is higher or less acidic than the titration of a strong acid
- There is a sharp increase in pH at the beginning of the titration. This is because the anion of the weak acid becomes a common ion that reduces the ionization of the acid. - After the sharp increase at the beginning of the titration the curve only changes gradually. This is because the solution is acting as a buffer. This will continue until the base overcomes the buffers capacity. - In the middle of this gradually curve the half-neutralization occurs. At this point the concentration of weak acid is equal to the concentration of its conjugate base. Therefore the pH=pKa. This point is called the half-neutralization because half of the acid has been neutralized.
buffered solution An aqueous solution consisting of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH
This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant
buffered solution
Also simply called a buffer.
An aqueous solution consisting of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH when strong acids or bases are added.
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