The Availability of Catechol Sulphonphthalein as a Polychrom Indicator

When used in a chemical sense, the word "indicator" brings to mind a substance which marks the completion, in solution, of a chemical reaction by showing, on the addition of an excess of a reagent, a change in color. As generally used, however, the term is more limited. In a number of oxidation-reduction reactions solutions of potassium permanganate are used, and, since solutions of this compound are intensely colored themselves, while solutions of its reduction products are practically color less, this compound acts automatically as its own indicator. That is, no substance not involved in the reaction under observation need be added. The same is true of oxidation-reduction reactions involving the use of solutions of iodine. In starch, we have a substance which serves as a very delicate indicator for free iodine, and more often than not recourse is had to it.