Rhodizonic acid
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| Names | |
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IUPAC name
5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone
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| Other names
dihydroxydiquinoyl
dioxydiquinone |
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| Identifiers | |
118-76-3 (dihydrate) 
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| 3D model (Jmol) | Interactive image |
| ChemSpider |
60401 
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| ECHA InfoCard | 100.003.888 |
| EC Number | 204-276-5 |
| MeSH | C005690 |
| PubChem | 67050 |
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| Properties | |
| C6H2O6 | |
| Melting point | 130 to 132 °C (266 to 270 °F; 403 to 405 K) |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is ?) |
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Rhodizonic acid is a chemical compound with formula C6H2O6 or (CO)4(COH)2. It can be seen as a two-fold enol and four-fold ketone of cyclohexene, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone.
Rhodizonic acid is usually obtained in the form of a colorless "dihydrate" C6H2O6·2H2O. The latter is actually 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, where two of the original ketone groups are replaced by two pairs of geminal diols. The orange to deep-red and highly hygroscopic anhydrous acid can be obtained by low-pressure sublimation of the dihydrate.
Like many other enols, rhodizonic acid can lose the hydrogen cations H+ from the hydroxyls (pK1 = 4.378 ± 0.009, pK2 = 4.652 ± 0.014 at 25 °C), yielding the hydrogenrhodizonate anion C6HO6− and the rhodizonate anion C6O62−. The latter is aromatic and symmetric, as the double bond and the negative charges are delocalized and evenly distributed over the six CO units. Rhodizonates tend to have various shades of red, from yellowish to purplish.
Rhodizonic acid has been used in chemical assays for barium, lead, and other metals. In particular, the sodium rhodizonate test can be used to detect gunshot residue (which contains lead) in a subject's hands, and to distinguish arrow wounds from gunshot wounds for hunting regulation enforcement.
Rhodizonic acid was discovered by Austrian chemist Johann Heller in 1837, by analyzing the products of heating a mixture of potassium carbonate and charcoal. For a while, rhodizonates were believed to have the formula C7O7 · Me3. The name comes from Greek ῥοδίζω (rhodizō, "to tinge red"), on account of the color of its salts.
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