The indicators shown in this photograph have all been used to determine the end point in acid-alkali titrations. In the 17th century Robert Boyle investigated various substances that underwent a reversible colour change when exposed to acid or alkali. Two are shown in the picture. Red cabbage is greenish yellow in alkali and mauve in acid. Syrup of violets is greenish yellow in alkali and purple in acid. Litmus, an extract of lichen, is shown at the bottom of the picture, with the well-known books of litmus strips. It is sufficiently stable to keep and does not fade quickly in use. The burette shown on the right dates from 1876. A white tile is usually put under the solution being titrated so the end point can clearly be seen.
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