INDICATOR

The Uses of INDICATOR

Analytical chemistry.The pH ranges of several typical indicators are as follows: alizarin yellow R 10.1–12.0 yellow to red methyl orange 3.1–4.4 red to yellow phenolphthalein 8.3–10.0 colorless to red phenol red 6.8–8.4 yellow to red litmus 4.4–8.3 red to

Definition

An organic substance (usually a dye or intermediate) that indicates by a change in its color the presence, absence, or concentration of some other substance, or the degree of reaction between two or more other substances. The most common example is the us

Definition

A compound that reversibly changes color depending on the pH of the solution in which it is dissolved. The visual observation of this change is therefore a guide to the pH of the solution and it follows that careful choice of indicators permits a wide range of end points to be detected in acid–base titrations.
Redox titrations require either specific indicators, which detect one of the components of the reaction (e.g. starch for iodine, potassium thiocyanate for Fe3+) or true redox indicators in which the transition potential of the indicator between oxidized and reduced forms is important. The transition potential of a redox indicator is analogous to the transition pH in acid–base systems.
Complexometric titrations require indicators that complex with metal ions and change color between the free state and the complex state. See also absorption indicator.

Definition

indicator: A substance used to showthe presence of a chemical substanceor ion by its colour. Acid–base indicatorsare compounds, such as phenolphthaleinand methyl orange,that change colour reversibly, dependingon whether the solution isacidic or basic. They are usually weakacids in which the un-ionized formHA has a different colour from thenegative ion A–. In solution the indicatordissociates slightlyHA?H⁺ + A^-In acid solution the concentration ofH⁺ is high, and the indicator islargely undissociated HA; in alkalinesolutions the equilibrium is displacedto the right and A– is formed. Usefulacid–base indicators show a sharpcolour change over a range of about 2 pH units. In titration, the point atwhich the reaction is complete is theequivalence point (i.e. the point atwhich equivalent quantities of acidand base are added). The end point isthe point at which the indicator justchanges colour. For accuracy, the twomust be the same. During a titrationthe pH changes sharply close to theequivalence point, and the indicatorused must change colour over thesame range.
Other types of indicator can beused for other reactions. Starch, forexample, is used in iodine titrationsbecause of the deep blue complex itforms. Oxidation–reduction indicatorsare substances that show a reversiblecolour change betweenoxidized and reduced forms.

Agricultural Uses

Indicator is an organic substance (usually a dye or intermediate) which indicates, by a change in its color, the presence or absence or concentration of some other substance, or the degree of reaction between two or more other substances.T he most common example is the use of acid-base indicators such as litmus, phenolphthalein and methyl orange to indicate the presence or absence of acids and bases, or the approximate concentration of hydrogen ion in a solution. Their chief use is in analytical chemistry.