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ACID–BASE INDICATORS 277

high a value of pH. However, the graph in Figure 6.3 shows how the pH of the
titration solution changes dramatically near the end point: in fact, only a tiny incre-
mental addition of alkali solution is needed to substantially increase the solution pH
by several pH units. In other words, a fraction of a drop of alkali solution is the only
difference between pH 7 (at the true volume at neutralization) and pH 8 when the
phenolphthalein changes from colourless to puce pink.
Table 6.7 lists the pH changes for a series of common pH indicators. The colour
changes occur over a wide range of pHs, the exact value depending on the indicator
chosen. Methyl violet changes from yellow to blue as the pH increases between 0 and
1.6. At the opposite extreme, phenolphthalein responds to pH changes in the range
8.2 to 10.