Chemical Indicators

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.


Kreshkov, A. P. Osnovy analiticheskoi khimii, 3rd ed., vol. 2. Moscow, 1971.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
"We examined a piece of melt that was 10 microns (0.01mm) in diameter, and analysed its chemical indicators such as H2O content, chlorine and deuterium/hydrogen ratio, and found that Earth's recycling process started about 600 million years earlier than originally thought," says Wilson.
The samples of seawater taken from the inspected beaches will be tested by physical and chemical indicators.
The charity has teamed up with the University of Manchester to learn how dogs like cocker spaniel Asher detect the chemical indicators of Parkinson's.
HIGHPOWER strives for continuous improvement while performing studies that involve verification and/or validation of biological indicators, chemical indicators, packaging materials, material compatibility, sterilization efficacy and device cleaning.
BAG Health Care's hygiene monitoring business markets, distributes and services biological, cleaning and chemical indicators, as well as a range of infection prevention products sold into the same central sterile supply customers.
When someone has taken cocaine, they excrete traces of benzoylecgonine and methylecgonine as they metabolise the drug, and these chemical indicators are present in fingerprint residue.
This division manufactures a broad suite of high-quality biological and chemical indicators which are used to accurately monitor the effectiveness of sterilization processes.
It should be noticed that chemical indicators include purity percentage and chemical analyzes of elements and zinter point.
The Sample Analysis at Mars (SAM) instrument, largest of the 10 science instruments for NASA's Mars Science Laboratory mission, will examine samples of Martian rocks, soil and atmosphere for information about chemicals that are important to life and other chemical indicators about past and present environments.
The most used chemical indicators in the optical fibres technique (Stanciu & Stanciu, 2003) are they that used the absorption and fluorescence phenomena's, the last variant being preferred because it presents a bigger sensibility.
STERIS also offers a complete line of sterility assurance products including barrier and packaging products, biological indicators and chemical indicators. In fact, just last month, STERIS was awarded a sole-source Amerinet Choice contract for a full range of their sterility assurance products.

Full browser ?