Litmus

Last updated
Litmus powder Lackmus.jpg
Litmus powder
Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components 7-hydroxyphenoxazone.svg
Chemical structure of 7-hydroxyphenoxazone, the chromophore of litmus components

Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. In an acidic medium, blue litmus paper turns red, while in a basic or alkaline medium, red litmus paper turns blue. In short, it is a dye and indicator which is used to place substances on a pH scale.

Contents

History

The word "litmus" comes from an Old Norse word for “moss used for dyeing”. [1] About 1300, the Spanish physician Arnaldus de Villa Nova began using litmus to study acids and bases. [2] [3]

From the 16th century onwards, the blue dye was extracted from some lichens, especially in the Netherlands.

Natural sources

Parmelia sulcata Parmelia sulcata.jpeg
Parmelia sulcata

Litmus can be found in different species of lichens. The dyes are extracted from such species as Roccella tinctoria (South American), Roccella fuciformis (Angola and Madagascar), Roccella pygmaea (Algeria), Roccella phycopsis , Lecanora tartarea (Norway, Sweden), Variolaria dealbata, Ochrolechia parella , Parmotrema tinctorum , and Parmelia . Currently, the main sources are Roccella montagnei (Mozambique) and Dendrographa leucophoea (California). [2]

Uses

Litmus paper after being used Litmus paper.JPG
Litmus paper after being used

The main use of litmus is to test whether a solution is acidic or basic, as blue litmus paper turns red under acidic conditions, and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5–8.3 at 25 °C (77 °F). Neutral litmus paper is purple. [2] Wet litmus paper can also be used to test for water-soluble gases that affect acidity or basicity; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia gas, which is alkaline, turns red litmus paper blue. While all litmus paper acts as pH paper, the opposite is not true.

Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions, the solution is red, and under alkaline conditions, the solution is blue.

Litmus(pH indicator)
below pH 4.5above pH 5.3
4.55.3

Chemical reactions other than acid–base can also cause a color change to litmus paper. For instance, chlorine gas turns blue litmus paper white; the litmus dye is bleached [4] because hypochlorite ions are present. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.

Chemistry

The litmus mixture has the CAS number 1393-92-6 and contains 10 to around 15 different dyes. All of the chemical components of litmus are likely to be the same as those of the related mixture known as orcein but in different proportions. In contrast with orcein, the principal constituent of litmus has an average molecular mass of 3300. [5] Acid-base indicators on litmus owe their properties to a 7-hydroxyphenoxazone chromophore. [6] Some fractions of litmus were given specific names including erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein, and leucazolitmin. Azolitmin shows nearly the same effect as litmus. [7]

A recipe to make litmus out of the lichens, as outlined on a UC Santa Barbara website says: [8]

Details are difficult to find because the processes were kept secret.

This summary of a modern manufacturing procedure is from The Vanishing Lichens, D H S Richardson, London, 1975. The lichens (preferably Lecanora tartarea and Roccella tinctoria) are ground in a solution of sodium carbonate and ammonia.

Stir the lichens from time to time and the color changes from red to purple and finally blue after about four weeks. The lichens are then dried and powdered. At this stage the lichens contain partly litmus and partly orcein pigments. The orcein is removed by extraction with alcohol, leaving the pure blue litmus. It is marketed as blue lumps, masses, or tablets, after mixing with colorless compounds such as chalk and gypsum. Litmus paper is paper impregnated with this substance.

Mechanism

Red litmus contains a weak diprotic acid. When it is exposed to a basic compound, the hydrogen ions react with the added base. The conjugate base formed from the litmus acid has a blue color, so the wet red litmus paper turns blue in an alkaline solution.

Related Research Articles

<span class="mw-page-title-main">Dye</span> Soluble chemical substance or natural material which can impart color to other materials

A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and may require a mordant to improve the fastness of the dye on the fiber.

A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, a pH indicator is a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+) in the Arrhenius model.

<span class="mw-page-title-main">Soil pH</span> Measure of how acidic or alkaline the soil is

Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions in a solution. In soils, it is measured in a slurry of soil mixed with water, and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils and very strongly alkaline soils are rare.

<span class="mw-page-title-main">Base (chemistry)</span> Type of chemical substance

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century.

<span class="mw-page-title-main">Phenolphthalein</span> pH indicator that turns pink in basic solution

Phenolphthalein ( feh-NOL(F)-thə-leen) is a chemical compound with the formula C20H14O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.

<span class="mw-page-title-main">Bromothymol blue</span> pH indicator

Bromothymol blue is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH. A common use is for measuring the presence of carbonic acid in a liquid. It is typically sold in solid form as the sodium salt of the acid indicator.

<span class="mw-page-title-main">Stop bath</span>

Stop bath is an acidic solution used for processing black-and-white photographic films, plates, and paper. It is used to neutralize the alkaline developer, thus halting development.

<span class="mw-page-title-main">Congo red</span> Chemical compound

Congo red is an organic compound, the sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid). It is an azo dye. Congo red is water-soluble, yielding a red colloidal solution; its solubility is greater in organic solvents. The use of Congo red in the textile industry has long been abandoned, primarily because of its carcinogenic properties, but it is still used for histological staining.

<span class="mw-page-title-main">Methyl orange</span> Chemical compound

Methyl orange is a pH indicator frequently used in titration because of its clear and distinct color variance at different pH values. Methyl orange shows red color in acidic medium and yellow color in basic medium. Because it changes color at the pKa of a mid strength acid, it is usually used in titration of strong acids in weak bases that reach the equivalence point at a pH of 3.1-4.4. Unlike a universal indicator, methyl orange does not have a full spectrum of color change, but it has a sharp end point. In a solution becoming less acidic, methyl orange changes from red to orange and, finally, to yellow—with the reverse process occurring in a solution of increasing acidity.

<span class="mw-page-title-main">Universal indicator</span> Indicator that works over a wide range of pH

A universal indicator is a pH indicator made of a solution of several compounds that exhibit various smooth colour changes over a wide range pH values to indicate the acidity or alkalinity of solutions. A universal indicator can be in paper form or present in a form of a solution.

<span class="mw-page-title-main">Red cabbage</span> Cabbage cultivar

The red cabbage is a kind of cabbage, also known as Blaukraut after preparation. Its leaves are coloured dark red/purple. However, the plant changes its colour according to the pH value of the soil due to a pigment belonging to anthocyanins. In acidic soils, the leaves grow more reddish; in neutral soils, they will grow more purple, while an alkaline soil will produce rather greenish-yellow coloured cabbages. This explains the fact that the same plant is known by different colours in various regions. It can be found in all of Europe, throughout the Americas, in China, and especially in Africa.

Erythrolitmin is the active ingredient extracted from the Litmus lichen, used in chemistry as a pH indicator. Erythrolitmin is related to the orceins, and consists essentially of several phenoxazone and orcinol residues.

<span class="mw-page-title-main">Orcein</span>

Orcein, also archil, orchil, lacmus and C.I. Natural Red 28, are names for dyes extracted from several species of lichen, commonly known as "orchella weeds", found in various parts of the world. A major source is the archil lichen, Roccella tinctoria. Orcinol is extracted from such lichens. It is then converted to orcein by ammonia and air. In traditional dye-making methods, urine was used as the ammonia source. If the conversion is carried out in the presence of potassium carbonate, calcium hydroxide, and calcium sulfate, the result is litmus, a more complex molecule. The manufacture was described by Cocq in 1812 and in the UK in 1874. Edmund Roberts noted orchilla as a principal export of the Cape Verde islands, superior to the same kind of "moss" found in Italy or the Canary Islands, that in 1832 was yielding an annual revenue of $200,000. Commercial archil is either a powder or a paste. It is red in acidic pH and blue in alkaline pH.

<span class="mw-page-title-main">Blotting paper</span> Absorbent paper

Blotting paper is a highly absorbent type of paper used to absorb ink or oil from writing material, particularly when quills or fountain pens were popular. It could also be used in testing how much oil is present in products. Blotting paper referred to as bibulous paper is mainly used in microscopy to remove excess liquids from the slide before viewing. Blotting paper has also been sold as a cosmetic to aid in the removal of skin oils and makeup.

<span class="mw-page-title-main">Ethnolichenology</span>

Ethnolichenology is the study of the relationship between lichens and people. Lichens have and are being used for many different purposes by human cultures across the world. The most common human use of lichens is for dye, but they have also been used for medicine, food and other purposes.

<span class="mw-page-title-main">Turnsole</span>

Turnsole, katasol, or folium was a dyestuff prepared from the annual plant Chrozophora tinctoria.

<span class="mw-page-title-main">Toluidine blue</span> Chemical compound

Toluidine blue, also known as TBO or tolonium chloride (INN) is a blue cationic (basic) dye used in histology and sometimes clinically.

<span class="mw-page-title-main">Orcinol</span> Chemical compound

Orcinol is an organic compound with the formula CH3C6H3(OH)2. It occurs in many species of lichens including Roccella tinctoria and Lecanora. Orcinol has been detected in the "toxic glue" of the ant species Camponotus saundersi. It is a colorless solid. It is related to resorcinol, 1,3-C6H4(OH)2.

<i>Chrozophora tinctoria</i> Species of flowering plant

Chrozophora tinctoria is a plant species native to the Mediterranean, the Middle East, India, Pakistan, and Central Asia. It is also present as a weed in North America and Australia.

<span class="mw-page-title-main">Glossary of dyeing terms</span>

Dyeing is the craft of imparting colors to textiles in loose fiber, yarn, cloth or garment form by treatment with a dye. Archaeologists have found evidence of textile dyeing with natural dyes dating back to the Neolithic period. In China, dyeing with plants, barks and insects has been traced back more than 5,000 years. Natural insect dyes such as Tyrian purple and kermes and plant-based dyes such as woad, indigo and madder were important elements of the economies of Asia and Europe until the discovery of man-made synthetic dyes in the mid-19th century. Synthetic dyes quickly superseded natural dyes for the large-scale commercial textile production enabled by the Industrial Revolution, but natural dyes remained in use by traditional cultures around the world.

References

  1. "Litmus". 7 October 2021.
  2. 1 2 3 Neupert, Manfred (January 31, 2013). "Lackmus". Römpp Lexikon Chemie (in German).
  3. Surber, Christian; Humbert, Philippe; Abels, Christoph; Maibach, Howard (2018). Surber, Christian; Abels (eds.). pH of the Skin: Issues and Challenges - Current Problems in Dermatology. Basel: Karger Medical and Scientific Publishers. p. 3. ISBN   9783318063851 . Retrieved 1 May 2023. In the early 14th century, the Spanish scholar, Arnaldus de Villa Nova (1235–1311), began to use litmus for studying acids and bases. This compound, extracted from a lichen, had been used as a dye since at least the time of the Vikings, but he was the first person known to use it as a test of acidity.
  4. O'Leary, Donal (2000). "Chlorine". The Chemical Elements. Archived from the original on 2008-12-21.
  5. Beecken, H.; E.-M. Gottschalk; U. v. Gizycki; H. Krämer; D. Maassen; H.-G. Matthies; H. Musso; C. Rathjen; Ul. Zdhorszky (2003). "Orcein and Litmus". Biotechnic & Histochemistry. 78 (6): 289–302. doi:10.1080/10520290410001671362. PMID   15473576. S2CID   41944320.
  6. H. Musso, C. Rathjen (1959). "Orcein dyes. X. Light absorption and chromophore of litmus". Chem. Ber. 92 (3): 751–753. doi:10.1002/cber.19590920331.
  7. E. T. Wolf: Vollständige Übersicht der Elementar-analytischen Untersuchungen organischer Substanzen, S. 450–453, veröffentlicht 1846, Verlag E. Anton (Germany).
  8. "Students ask questions, UCSB scientists answer them". UCSB ScienceLine. 2002.