Titration is used to determine the concentration of an unknown acid or base. As with other types of titrations, acid/base titrations can be used to determine the concentration of hydronium or hydroxide ions and thus the pH of a solution. Starting with a solution of a unknown concentration of an acid or base, small volumes of a solution of known concentration are added until the neutralization point or equivalence point is reached. At this point, any hydroxide or hydronium ions from the original solution have been neutralized. Titrations are usually performed in the presence of an indicator, a substance that visibly changes when a certain pH is reached. The equivalence point of a titration does not always occur when the pH level reaches 7. If a weak base is titrated with a strong acid, for example, when all the base has been neutralized, the solution will still be slightly acidic. The indicator must therefore be chosen such that its change occurs in the approximate region of the equivalence point.
Titrations are generally plotted on Cartesian graphs. The x-axis shows the volume of the solution of known pH that has been added. The y-axis shows the pH of the solution. The titration curve generally has an almost horizontal slope except in the region of the equivalence point, where the slope is almost vertical. In other words, the area of the graph where the pH changes very quickly with the addition of only a small volume of titrant (the substance being added) is the region of the equivalence point. The sections of the graph with approximately horizontal slope imply that in these areas the
