litmus

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litmus,

organic dye usually used in the laboratory as an indicator of acidity or alkalinity (see acids and basesacids and bases,
two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water. Properties
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). Naturally pink in color, it turns blue in alkali solutions and red in acids. Commonly, paper is treated with the coloring matter to form so-called litmus paper. Litmus is extracted, chiefly in the Netherlands, from certain lichens (see archilarchil
or orchil
, blue, red, or purple dye extracted from several species of lichen, also called orchella weeds, found in various parts of the world. Commercial archil is either a powder (called cudbear), a pasty mass (called archil), or a drier paste (called persis).
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), which are mashed, treated with potassium carbonate and ammonia, and allowed to ferment. The resulting product is mixed with various colorless substances, such as chalk or gypsum, and is sold in dark blue lumps, masses, or tablets. The active component of litmus, i.e., the part sensitive to acids or bases, is called erythrolitmin.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Litmus

 

a dye extracted from certain species of lichen (for example, Roccella tinctoria).

The composition of litmus is complex and has not yet been fully established. The pigment component is the weak acid azolitmin, which yields blue salts. Litmus is used as an indicator: it turns red in an acid medium and blue in an alkaline medium. Test papers impregnated with the dye are generally the form used.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

litmus

[′lit·məs]
(materials)
Blue, water-soluble powder from various lichens, especially Variolaria lecanora and V. rocella ; turns red in solutions at pH 4.5, and blue at pH 8.3; used as an acid-base indicator. Also known as lacmus; lichen blue.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

litmus

An organic chemical indicator of acidity or alkalinity; is red in color for pH values below 4.5 and blue above 8.3.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

litmus

a soluble powder obtained from certain lichens. It turns red under acid conditions and blue under basic conditions and is used as an indicator
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005