Litmus
Litmus is obtained from certain lichens blue-violet dye. The aqueous solution possesses the property, depending on whether it is combined with acidic or alkaline substances ( ie, depending on the pH) to change their color. They therefore serve as an acid - base indicator in chemistry. Litmus is a deep blue mixture with the CAS number 1393-92-6.
- 4.1 In the past and today
- 4.2 litmus test
- 4.3 litmus milk
- 4.4 Structure Determination
- 4.5 Declaration on the indicator effect of litmus
- 4.6 Materials used 4.6.1 Orceine
- 4.6.2 azolitmin and Leucazolitmin
Name and history
The name comes from the Indo-European: leg is called " dribble " and Mus stands for " porridge ". The name goes back to the historical production of litmus, which was obtained from a mash of lichens and water, which was allowed to drain.
Litmus was first used in 1300 by the physician and alchemist Arnald of Villanova as a chemical reagent, ie as a substance that is used for the detection and identification of another substance. Since the 16th century, won the blue dye on a larger scale (mainly in the Netherlands as Lakmoes ) from different lichen species. After the secret of the manufacture of litmus was aired from lichens gray colored lichen Lecanora tartarea and Rocella tinctoria were carried out under the name of mountain moss and moss clip as an export item from the Netherlands to other industrialized countries.
Natural Occurrence
Litmus is found in nature as that used with him orcein dye in different lichen species. Were used for dye extraction formerly mostly Roccella tinctoria (grows on rocks Macaronesia (Cape Verde, Canary Islands, Madeira, Azores ) and the west coast of South America ), Roccella Fuciformis (origin usually Angola and Madagascar), Roccella pygmaea (Algeria), Rocella phycopsis, Lecanora tartarea (Norway, Sweden), Variolaria dealbata (Pyrenees and Auvergne ), Ochrolechia parella ( entire Atlantic coast of northwestern Europe ), Parmotrema tinctorum ( also Canaries ), and various species of Parmelia ( widespread on many deciduous trees ). Main sources today Roccella montagnei (Mozambique ) and Dendrographa leucophoea ( California).
Production
It was a long time trying to keep the details of the manufacturing process for economic reasons, secret, in order to create a monopoly. The powdered lichens are for several weeks in a soda ( sodium carbonate) or potash (potassium carbonate ) and ammonia alkalized solution was allowed to stand with occasional stirring. As previously stood as a chemical ammonia not available, obtain the necessary proportion of ammonia by addition of urine. The urea contained in urine is converted by enzymes as catalysts in ammonia. While the powdered lichens are in the solution, its color gradually changes from red to blue. The blue at the end of the implementation process mixture is dried and ground into powder. In this stage of the manufacturing process, the powder contains some litmus and partially orcein - color pigments. With alcohol on the orcein attributable, ruby color component is dissolved out, leaving behind the deep blue litmus. Pressed and partially mixed with gypsum and chalk, it comes as easily crumbling molding compound on the market. Litmus is from a specific color component - the azolitmin - manufactured and sold as an acid - base indicator with similar characteristics litmus.
Today's industrial production
In 1758 the two British chemist G. Gordon and Cuthbert Gordon patented the industrial production of Cudbear ( English name for orcein ) and litmus from lichens, but it took until 1940 before the British company Johnson of Hendon (later Johnsons Photopia Ltd. ) the production began.
In the production of lichens are first milled, mixed with water by the addition of lime, potash and ammonium carbonate solution ( ( NH4) 2CO3 ) and allowed to ferment in an air stream. The solution changes color to brown and violet after about three weeks to a deep blue. The slurry is stirred formed through a sieve and mixed with chalk, gypsum into small cubes. After drying, they can be further processed; they contain large amounts of orcein and only about 5-8 % of the litmus dyes. The orcein can be easily separated due to its good alcohol solubility with ethanol.
The use and structure
Then and now
Litmus was formerly particularly in the Netherlands, the so-called blueing of washing and dyeing of stimulants ( wine, baked goods, liqueurs, cheese, make-up and sugar paper) use. By light blue staining, yellowing of the air is equalized. For the textile dyeing litmus is unsuitable because of its color changes in acids and alkalis.
Litmus test
Today litmus is used exclusively as an acid - base indicator in chemistry and biology. At pH values less than 4.5 litmus turns red at values greater than 8.3 between blue and violet. It is mainly used in two forms: in an aqueous solution as the litmus tincture and as litmus paper. In the latter, there are paper strips impregnated with weakly acidic or alkaline litmus tincture ( test paper ). The main constituent of litmus is polymeric in construction ( of several elements ) from 7- hydroxy-2 -phenoxazine chromophores, which explains his relationship with the crimson dye orcein.
Litmus milk
With 2.5 to 7 % litmus staggered, 10 % skim milk or skim milk powder and water at a suitable concentration is used in microbiology as a litmus milk. This is to determine if bacteria can utilize lactose (milk sugar ) was used. For this, the litmus milk is inoculated with the bacterium. For color change by acidification of blue ( alkaline) to red ( acidic) has been proven that the bacteria can ferment lactose. For better handling, the milk is often gelled with agar.
Structure determination
The German chemist Hans Musso (1925-1988) examined in the years 1955 to 1965, the structure of the lichen dyes Orseille, orcein (also called Cudbear ) and the litmus dyes and also published about 25 articles. Based on column chromatographic separation methods he was able to detach most of the dyes and determine their structure.
Declaration on the indicator effect of litmus
The cause of the color change of the litmus from blue to purple to red when changing the pH is extremely complex and is made up of several factors:
- Main cause of the color change in uptake or release of protons ( halochromism ) is the reversible (ie reversible ) formation of phenols from red blue phenolates.
- Furthermore, the Oxazinsystem can (in this case a cyclic arrangement with regard to stagnant oxygen and nitrogen atom) are protonated or deprotonated, ie, it can protons accumulate on the connection or eliminate again.
- In addition also changes nor the tone of the characteristic of litmus red and blue colors, if you are strong or weak acids or bases to litmus. The results from the stepwise addition or removal of other protons.
Due to the size of the molecule more phenolic groups and oxazine nitrogen atoms for this addition or loss of protons are available. In addition, therefore, arise in the pH range 5.6 to 8 mixed systems with a yellow-orange to blue- green color.
Materials used
The litmus mixture contains about 10-15 different substances. Were extracted successfully next orcein Erythrolein (also Erythrolitmin ) azolitmin, Spaniolitmin, Leucoorcein and Leucazolitmin. Azolitmin as a pure substance has approximately the same effect as the indicator mixture.
Orceine
In Lackmusgemisch the two Orceine α - Hydroxyorcein C18H17O5N and α - Aminoorcein C18H18O4N2 are mainly contained. These arise ( in addition to more than 10 other Orceinen ) of the dyes of lichen by treatment in ammonia -containing alkaline solution under oxidation. In the majority of Lackmusherstellung orcein separated.
Azolitmin and Leucazolitmin
Azolitmin is mainly responsible for the color change dye with the molecular formula C18H10O10N. It arises from Orceinen by oxidation. Leucazolitmin is a colorless derivative of Azolitmins and obtainable therefrom by reduction.
Safety
In English-language sources litmus is classified as irritating to ( mucus ) skin and eyes as well as "probable at higher temperatures flammable ". EU or German guidelines are not available.