Acid–base titration


An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor the progress of the acid–base reaction. If the acid dissociation constant of the acid or base dissociation constant of base in the analyte solution is known, its solution concentration can be determined. Alternately, the pKa can be determined if the analyte solution has a known solution concentration by constructing a titration curve.

Alkalimetry and acidimetry

Alkalimetry and acidimetry are a kind of volumetric analysis in which the fundamental reaction is a neutralization reaction. Alkalimetry is the specialized analytic use of acid-base titration to determine the concentration of a basic substance. Acidimetry, sometimes spelled acidometry, is the same concept of specialized analytic acid-base titration, but for an acidic substance.

Indicator choice

A suitable pH indicator must be chosen in order to detect the end point of the titration. The color change or other effect should occur close to the equivalence point of the reaction so that the experimenter can accurately determine when that point is reached. The pH of the equivalence point can be estimated using the following rules:
When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour.
The point at which the indicator changes colour is called the end point. A suitable indicator should be chosen, preferably one that will experience a change in colour close to the equivalence point of the reaction.

Mathematical analysis: titration of weak acid

The pH of a weak acid solution being titrated with a strong base solution can be found at different points along the way. These points fall into one of four categories:
  1. initial pH
  2. pH before the equivalence point
  3. pH at the equivalence point
  4. pH after the equivalence point
for more rigorous calculation, using a RICE chart is required. In fact the equations below are a simplification of the RICE chart.

Single formula

More accurately, a single formula that describes the titration of a weak acid with a strong base from start to finish is given below:
where
" φ = fraction of completion of the titration

Gallery

Graphical methods

The titration process creates solutions with compositions ranging from pure acid to pure base. Identifying the pH associated with any stage in the titration process is relatively simple for monoprotic acids and bases. The presence of more than one acid or base group complicates these computations. Graphical methods, such as the equiligraph, have long been used to account for the interaction of coupled equilibria. These graphical solution methods are simple to implement, however they are infrequently used.