Litmus is the famous acid-base pH indicator of chemistry: red in acid solution
and blue in alkali.
The chemistry involved is initially the same as for orcein. The same precursors in the lichens are hydrolysed to orcinol which is oxidised by air in the presence of ammonia to give phenoxanone derivatives. Under the more alkaline conditions, further oxidation and polymerisation occurs to give litmus. The chemistry was finally understood after the work of Hans Musso as described on the orcein page.
Litmus has been made in The Netherlands at least since the 16th century.
In 1940 this seeming monopoly came to an end when Johnsons of Hendon
in the UK started producing it.
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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 are ground in a solution of sodium carbonate and ammonia. Stir the lichens from time to time and the colour 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.
More about litmus from King's
American Dispensatory published in 1898
and from Mrs M Grieve - a modern herbal originally published in 1931
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