chemical indicator
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What is a chemical indicator?
A chemical indicator is a substance that signals the presence or absence of a chemical species, such as an acid or an alkali in a solution, typically through a visible color change. Indicators are effective even at low concentrations and do not influence the conditions for which they are used.
These indicators are classified as acid-base, oxidation-reduction, or specific-substance indicators, each with a characteristic transition range. The common application of indicators is the detection of end points of titrations, where the color change signals the completion of a reaction. Besides color changes, the formation or disappearance of turbidity can also serve as an indicator.
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What are chemical indicators used for in science?
Chemical indicators are substances that signal the presence or absence of a chemical species, typically through a visible color change, when a threshold concentration is reached. They are commonly used to detect the end points of titrations.
Indicators are classified based on their function, such as acid-base, oxidation-reduction, or specific-substance indicators, each with a characteristic transition range. For example, methyl yellow is an acid-base indicator that turns from yellow to red when the hydrogen ion concentration exceeds a certain level. The pH of a solution can be determined using a chemical acid-base indicator, which is a dye that is either a weak acid or a weak base.
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How does a chemical indicator show changes in a substance?
A chemical indicator visually signals the presence or absence of a chemical species, typically through a color change. For instance, methyl yellow turns yellow in an alkaline solution, but when acid is added to neutralize the solution, it turns red.
Indicators change color when a solution's acidity, oxidizing strength, or specific chemical concentration reaches a critical range. These indicators are classified as acid-base, oxidation-reduction, or specific-substance indicators, each with a distinct transition range. Besides color changes, indicators can also cause the formation or disappearance of turbidity.
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What are some common examples of chemical indicators?
Chemical indicators signal the presence or absence of a chemical species, typically through a visible color change. They are used to detect the end points of titrations, with common applications including acid-base, oxidation-reduction, and specific-substance indicators.
Common examples:
* Methyl yellow Imparts a yellow color to an alkaline solution, changing to red when neutralized by an acid.
* Litmus Turns blue in alkaline solution and red in acid solution.
* Phenolphthalein Is colorless in acid solution and red in alkaline solution.
* Fluorescein Detects the completion of the reaction of silver ion with chloride ion, changing from yellow-green to red.
* Diphenylcarbazone An indicator for mercuric ion, changes from yellow to violet when the mercuric ion concentration is increased from 0.000001 to 0.00001 mole per litre.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do acid-base indicators like litmus paper work?
Acid-base indicators, like litmus, signal the presence or absence of a threshold concentration of a chemical species, typically through a color change. Litmus, derived from lichens, turns red in acidic solutions and blue in alkaline solutions, acting as a common indicator of acidity or alkalinity.
These indicators are visible even at low concentrations and don't influence the conditions they're used to test. The color change occurs when the acidity of the solution reaches a critical range. Other indicators include methyl yellow and ferrous 1,10-phenanthroline.
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Why are chemical indicators important in laboratory experiments?
Chemical indicators are substances that signal the presence or absence of a chemical species, such as an acid or an alkali in a solution, typically through a visible color change. They are essential in laboratory experiments, particularly in titrations, where they detect the endpoint of a reaction.
Indicators are effective even at low concentrations and do not influence the conditions of the experiment. They are classified based on their function, such as acid-base, oxidation-reduction, or specific-substance indicators, each with a characteristic transition range. The color change of an indicator occurs when the acidity, oxidizing strength, or concentration of a specific chemical species reaches a critical range. The use of indicators allows for sensitive and sharp detection of the completion of a reaction.
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chemical indicator, any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. An example is the substance called methyl yellow, which imparts a yellow colour to an alkaline solution. If acid is slowly added, the solution remains yellow until all the alkali has been neutralized, whereupon the colour suddenly changes to red.
Like most indicators, methyl yellow is visible even if its concentration is as low as a few parts per million parts of solution. Used at such low concentrations, indicators do not have any influence on the conditions for which they are recommended. The common application of indicators is the detection of end points of titrations.
The colour of an indicator alters when the acidity or the oxidizing strength of the solution, or the concentration of a certain chemical species, reaches a critical range of values. Indicators are therefore classified as acid-base, oxidation-reduction, or specific-substance indicators, every indicator in each class having a characteristic transition range. Methyl yellow, an acid-base indicator, is yellow if the hydrogen ion (acid) concentration of the solution is less than 0.0001 mole per litre and is red if the concentration exceeds 0.0001. Ferrous 1,10-phenanthroline, an oxidation-reduction indicator, changes from red to pale blue when the oxidation potential of the solution is increased from 1.04 to 1.08 volts; and diphenylcarbazone, an indicator for mercuric ion, changes from yellow to violet when the mercuric ion concentration is increased from 0.000001 to 0.00001 mole per litre. Each of these indicators thus has a relatively narrow transition range, and each is capable of giving a sensitive, sharp indication of the completion of a reaction, that is, the end point.
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Although the visible change of the indicator is usually a colour change, in some cases it is a formation or disappearance of a turbidity. If, for example, a soluble silver salt is added to a solution of cyanide that contains a trace of iodide, the solution remains clear until all the cyanide has reacted to form the soluble silver cyanide complex ion. Upon the addition of more silver, the solution becomes turbid because insoluble silver iodide forms. Iodide is therefore an indicator for excess silver ion in this reaction.
Another kind of indicator is the adsorption indicator, the best-known representative of which is the dye fluorescein. Fluorescein is used to detect the completion of the reaction of silver ion with chloride ion, the colour change occurring in the following manner. After a quantity of silver large enough to precipitate all the chloride has been added, additional silver ion is partially adsorbed on the surface of the particles of silver chloride. Fluorescein also is adsorbed and, in combining with the adsorbed silver ion, changes from yellow-green to red.