Litmus test (chemistry)

Litmus test (chemistry)

Overview
Litmus is a water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-soluble mixture of different dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s extract
Extract
An extract is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol or water. Extracts may be sold as tinctures or in powder form....

ed from lichens, especially Roccella tinctoria
Roccella tinctoria
Roccella tinctoria is a species of fungus in the genus Roccella, homotypic synonym of Lecanora tinctoria Czerwiak., 1849. It was first described by Augustin Pyramus de Candolle in 1805. It has the following varieties:...

. It is often absorbed onto filter paper
Filter paper
Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

 to produce one of the oldest forms of pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

, used to test materials for acidity. Blue litmus paper turns red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 under acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic conditions and red litmus paper turns blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

 under basic
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 (i.e. alkaline) conditions, with the color change occurring over the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 range 4.5-8.3 at 25 °C. Neutral litmus paper is purple.
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Encyclopedia
Litmus is a water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-soluble mixture of different dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s extract
Extract
An extract is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol or water. Extracts may be sold as tinctures or in powder form....

ed from lichens, especially Roccella tinctoria
Roccella tinctoria
Roccella tinctoria is a species of fungus in the genus Roccella, homotypic synonym of Lecanora tinctoria Czerwiak., 1849. It was first described by Augustin Pyramus de Candolle in 1805. It has the following varieties:...

. It is often absorbed onto filter paper
Filter paper
Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

 to produce one of the oldest forms of pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

, used to test materials for acidity. Blue litmus paper turns red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 under acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic conditions and red litmus paper turns blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

 under basic
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 (i.e. alkaline) conditions, with the color change occurring over the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 range 4.5-8.3 at 25 °C. Neutral litmus paper is purple. Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions the solution is red, and under basic conditions the solution is blue.

The litmus mixture has the CAS number 1393-92-6 and contains 10 to 15 different dyes. Most of the chemical components of litmus are likely to be the same as those of the related mixture known as orcein
Orcein
Orcein, also archil, orchil, lacmus, Citrus Red 2, 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...

, but in different proportions. In contrast with orcein, the principal constituent of litmus has average molecular weight of 3300. Acid-base indicators on litmus owe their properties to a 7-hydroxyphenoxazone chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...

. Some fractions
Fraction (chemistry)
A fraction in chemistry is a quantity collected from a sample or batch of a substance in a fractionating separation process. In such a process, a mixture is separated into fractions, which have compositions that vary according to a gradient. A fraction can be defined as a group of chemicals that...

 of litmus were given specific names including erythrolitmin
Erythrolitmin
Erythrolitmin is the active ingredient extracted from the Litmus lichen, used in chemistry as a pH indicator. The erythrolitmin molecule is related to the orceins, and consists essentially of several phenoxazone and orcinol residues.-Interaction with acids:The intense coloring of the molecule is...

 (or erythrolein), azolitmin, spaniolitmin, leucoorcein and leucazolitmin. Azolitmin shows nearly the same effect as litmus.

History


Litmus was used for the first time about 1300 AD by Spanish alchemist Arnaldus de Villa Nova
Arnaldus de Villa Nova
Arnaldus de Villa Nova was an alchemist, astrologer and physician....

. From the 16th century on, the blue dye was extracted from some lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s, especially in the Netherlands.

Natural sources



Litmus can be found in different species of lichens.The dyes would be extracted from such species as Roccella tinctoria
Roccella tinctoria
Roccella tinctoria is a species of fungus in the genus Roccella, homotypic synonym of Lecanora tinctoria Czerwiak., 1849. It was first described by Augustin Pyramus de Candolle in 1805. It has the following varieties:...

(South America), Roccella fuciformis (Angola and Madagascar), Roccella pygmaea (Algeria), Roccella phycopsis, Lecanora tartarea (Norway, Sweden), Variolaria dealbata, Ochrolechia parella, Parmotrema tinctorum and Parmelia
Parmelia
Parmelia may refer to:* Parmelia , the barque in which were transported the first settlers of the British colony of Western Australia in 1829* Parmelia , a large genus of lichens with global distribution...

. Currently, the main sources are Roccella montagnei (Mozambique) and Dendrographa leucophoea (California).

Uses


The main use of litmus is to test whether a solution is acidic or alkaline. Wet litmus paper can also be used to test water-soluble gases; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 gas, which is alkaline, colors the red litmus paper blue.

Chemical reactions other than acid-base reaction can also cause a color-change to litmus paper. For instance, chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas turns blue litmus paper white – the litmus paper is bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

ed. This reaction is irreversible and therefore the litmus is not acting as an indicator in this situation.

See also

  • Nitrazine strips
    Nitrazine
    Nitrazine or phenaphthazine is a pH indicator dye.-Nitrazine test:As the disodium salt, it is used in pH testing strips for medical applications. It is also used in civil engineering to determine the carbonation spread in concrete structures and therefore assess the state of the rebar's...

     - Measures pH about 4.5 to 7.5 with more precision
  • pH indicator
    PH indicator
    A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

  • Phenolphthalein
    Phenolphthalein
    Phenolphthalein is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions...

  • Bromothymol blue
    Bromothymol blue
    Bromothymol blue is a chemical indicator for weak acids and bases. The chemical is also used for observing photosynthetic activities or respiratory indicators .Bromothymol blue acts as a weak acid in solution...

  • Methyl orange
    Methyl orange
    Methyl orange is a pH indicator frequently used in titrations.It is often used in titrations because of its clear and distinct colour change. Because it changes colour at the pH of a mid-strength acid, it is usually used in titrations for acids...

  • Universal indicator
    Universal indicator
    A Universal indicator is a pH indicator composed of a blend of several compounds that exhibits several smooth colour changes over a pH value range from 1-14 to indicate the acidity or basicity of solutions. Although there are a number of commercially available universal pH indicators, most are a...

    - Modern paper measures pH 1 to 14 with distinct colors for each. The color chart is usually included with the strips.