Indicators, Chemical

substances introduced into the solution being analyzed to determine the end of a chemical reaction or the hydrogen ion concentration from their readily observable properties. Chemical indicators are used mostly in titrimetric analysis for determining the equivalence point (end point of titration). Changes in color, the formation or disappearance of a turbidity, luminescence, and other phenomena may be observed at this point (or in its vicinity) in the presence of chemical indicators as a result of a change in a certain characteristic of the solution under investigation. Chemical indicators are usually divided into five groups: acid-base, oxidation-reduction, complexometric, adsorption, and chemiluminescent.

The acid-base indicators are substances that change color as the pH value of the medium changes. These chemical indicators, regarded (according to the theory of W. Ostwald) as weak organic acids or bases, are used in neutralization methods as well as in pH determinations by colorimetric methods. The interval of pH values (pH range) in which the change in color is observed, is related to the dissociation constant of the indicator (pK) by the relation pH = pK ± 1. In performing the analysis, the indicator is selected such that the pH range of the color should include the pH value that the solution must have at the equivalence point. Analytical determinations by the neutralization method are most frequently performed using methyl orange (pH range, 3.1-4.4), methyl red (4.2—6.3), and phenolphthalein (8.0-9.8). The so-called mixed indicators, consisting of two chemical indicators, are used in addition to the individual indicators. An example of a mixed indicator is thymol blue + phenolphthalein. These mixtures are characterized by clear color changes within a narrow transition interval (up to 0.2 pH units). The acid-base indicators also include fluorescent (for example, α-naphthylamine, acridine) indicators, in the presence of which turbid or highly colored solutions on illumination with ultraviolet light either show fluorescence or have it disappear or exhibit color changes in it, and universal indicators, which consist of a mixture of several chemical indicators with different transition intervals and which undergo multiple color changes at various pH values and are used for the approximate determination of pH.

The reduction-oxidation indicators are substances capable of being oxidized or reduced within certain ranges of the redox potential and undergoing a color change at the same time. These chemical indicators (for example, methylene blue, diphenyla-mine, and starch) are used in the titrimetric analysis methods based on reduction-oxidation reactions.

Complexometric indicators are substances that form colored complex compounds with the ions being determined. Such chemical indicators, also called metallochromic, or metal indicators, are used in titrimetric analysis methods based on the application of aminopolycarboxylic acids. Water-soluble organic dyes, such as Eriochrome Black T, xylidine orange, and acid chrome blue C, are primarily used as complexometric chemical indicators.

Adsorption indicators are substances in the presence of which at the equivalence point, determined by precipitation (for example, during the titration of chloride ions with a standard silver nitrate solution), the color of the precipitate changes. Indicators of this type are mostly dyes (eosin and fluorescein, for example).

Chemiluminescent indicators are substances capable of generating visible luminescence at the equivalence point. They are used in the titration of highly colored solutions. These indicators include such substances as luminol and siloxene.