Aurin (C.I. 43800), sometimes named rosolic acid or corallin is an
organic compound, forming
yellowish or deep-red crystals with
greenish metallic luster. It is practically insoluble in water, freely soluble in
alcohol. It is soluble in strong acids to form yellow solution, or in aqueous
alkalis to form
carmine red solutions.
Due to this behaviour it can be used as
pH indicator with pH transition range 5.0 - 6.8. It is used as an intermediate in manufacturing of
dyes.
Synthesis
Aurin was first prepared in 1834 by the German chemist
Friedlieb Ferdinand Runge, who obtained it by distilling coal tar. He named it Rosölsäure or Rosaölsäure (red oil acid).[2][3] In 1861, the German chemists
Hermann Kolbe and
Rudolf Schmitt presented the synthesis of aurin by heating oxalic acid and
creosote (which contains phenol) in the presence of concentrated sulfuric acid.[4] (Gradually, chemists realized that commercial aurin was not a pure compound, but was actually a mixture of similar compounds.[5][6])
Aurin is formed by heating of
phenol and
oxalic acid in concentrated sulfuric acid.
Safety
Aurin may cause eye, skin, and respiratory tract irritation. Ingestion and inhalation should be avoided.
^In 1859, the German-English chemist
Hugo Müller (de) (1833–1915) found that rosolic acid could be produced simply by mixing phenol and calcium carbonate and then exposing the mixture to air for a prolonged time. Müller, Hugo (1859).
"Note on rosolic acid". Quarterly Journal of the Chemical Society. 11 (1): 1–5.
doi:
10.1039/QJ8591100001.
^See the papers of the English chemist Richard S. Dale and the German-English chemist
Carl Schorlemmer and of the German chemist
Heinrich Fresenius (de) (1847–1920).
A review of findings about aurin up to 1872 appeared in: Fresenius, H. (1872).
"Ueber das Corallin" [On corallin]. Journal für praktische Chemie. 2nd series (in German). 5 (4): 184–191.
doi:
10.1002/prac.18720050117.
Fresenius, H. (1872).
"Corallin". Journal of the Chemical Society. 25: 705–706.
^Thorpe, Thomas Edward, ed., A Dictionary of Applied Chemistry, 2nd ed. (London, England: Longmans, Green, and Co., 1913), vol. 5, "Triphenylmethane colouring matters,"
p. 551. From p. 551: "The researches of Dale and Schorlemmer, Zulkowski, and others have shown that common aurin consists of a mixture of a number of substances, which, in a pure state, are well crystallised."