The most commonly used procedure is the titration of organic bases with perchloric acid in anhydrous acetic acid. These assays sometimes take some perfecting in terms of being able to judge the endpoint precisely.
The effect of this is that the inflection in the titration curves for very weak acids and very weak bases is small, because they approach the pH limits in water of 14 or 0 respectively , thus making endpoint detection relatively more difficult.
A general rule is that bases with pKa < 7 or acids with pKa > 7 cannot be determined accurately in aqueous solution.
Substances which are either too weakly basic or too weakly acidic to give sharp endpoints in aqueous solution can often be titrated in nonaqueous solvents. The reactions which occur during many nonaqueous titrations can be explained by means of the concepts of the Brønsted-Lowry theory. According to this theory an acid is a proton donor, i.e. a substance which tends to dissociate to yield a proton, and a base is proton acceptor, i.e. a substance which tends to combine with a proton. When an acid HB dissociates it yields a proton together with the conjugate base B of the acid:
Alternatively, the base B will combine with a proton to yield the conjugate acid HB of the base B, for every base has its conjugate acid and, every acid has its conjugate base.
It follows from these definitions that an acid may be either:
A base may be either:
Substances which are potentially acidic can function as acids only in the presence of a base to which they can donate a proton. Conversely basic properties do not become apparent unless an acid also is present.
Since dissociation is not an essential preliminary to neutralization, aprotic solvents are often added to 'ionizing' solvents to depress solvolysis (which is comparable to hydrolysis) of the neutralization product and so sharpen the endpoint.
Protophilic solvents are basic in character and react with acids to form solvated protons.
Amphiprotic solvents have both protophilic and protogenic properties. Examples are water, acetic acid and the alcohols. They are dissociated to a slight extent. The dissociation of acetic acid, which is frequently used as a solvent for titration of basic substances, is shown in the equation below:
Here the acetic acid is functioning as an acid. If a very strong acid such as perchloric acid is dissolved in acetic acid, the latter can function as a base and combine with protons donated by the perchloric acid to form protonated acetic acid, an onium ion:
Since the CH3COOH2+ ion readily donates its proton to a base, a solution of perchloric acid in glacial acetic acid functions as a strongly acidic solution.
When a weak base, such as pyridine, is dissolved in acetic acid, the acetic acid exerts its levelling effect and enhances the basic properties of the pyridine. It is possible, therefore, to titrate a solution of a weak base in acetic acid with perchloric acid in acetic acid, and obtain a sharp endpoint when attempts to carry out the titration in aqueous solution are unsuccessful.
|Indicator||Color change||Color change||Color change|
|Crystal Violet (0.5 per cent in glacial acetic acid)||violet||blue-green||yellowish-green|
|α-Naphtholbenzein (0.2 per cent in glacial acetic acid)||blue or blue-green||orange||dark-green|
|Oracet Blue B (0.5 per cent in glacial acetic acid)||blue||purple||pink|
|Quinaldine Red (0.1 per cent in methanol)||magenta||almost colorless|
The actual potential of the reference electrode need not be known accurately for most purposes and usually any electrode may be used provided its potential remains constant throughout the titration. The indicator electrode must be suitable for the particular type of titration (i.e. a glass electrode for acid-base reactions and a platinum electrode for redox titrations), and should reach equilibrium rapidly.
The electrodes are immersed in the solution to be titrated and the potential difference between the electrodes is measured. Measured volumes of titrant are added, with thorough (magnetic) stirring, and the corresponding values of emf (electromotive force) or pH recorded. Small increments in volume should be added near the equivalence point which is found graphically by noting the burette reading corresponding to the maximum change of emf or pH per unit change of volume. When the slope of the curve is more gradual it is not always easy to locate the equivalent point by this method. However, if small increments (0.1 cm³ or less) of titrant are added near the end point of the titration and a curve of change of emf or pH per unit volume against volume of titrant is plotted, a differential curve is obtained in which the equivalence point is indicated by a peak.