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rhodizonic acid

n. (context organic compound English) A colorless crystalline substance obtained from potassium carboxide and from certain quinones, which forms brilliant red, yellow, and purple salts.

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Rhodizonic acid

Rhodizonic acid is a chemical compound with formula CHO or (CO)(COH). 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 "di hydrate" CHO·2HO. 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.

 Elizabeth Patton, Robert West (1970), New aromatic anions. VIII. Acidity constants of rhodizonic acid.  J. Phys. Chem., volume 74 issue 12, pp 2512–2518. 

 Dario Braga, Gianna Cojazzi, Lucia Maini and Fabrizia Grepioni (2001), Reversible solid-state interconversion of rhodizonic acid H2C6O6 into H6C6O8 and the solid-state structure of the rhodizonate dianion C6O62 – (aromatic or non-aromatic?). New J. Chem., volume 25, pages 1221 - 1223. 

Like many other enols, rhodizonic acid can lose the hydrogen cations H from the hydroxyls (pK = 4.378 ± 0.009, pK = 4.652 ± 0.014 at 25 °C),

 R. I. Gelb, L. M. Schwartz, D. A. Laufer (1978), The structure of aqueous rhodizonic acid. J. Phys. Chem., volume 82 issue 18, pp. 1985–1988.

yielding the hydrogenrhodizonate anion CHO and the rhodizonate anion CO. 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.

 Robert A. Chalmers and Geoffrey M. Telling (1967), A reassessment of rhodizonic acid as a qualitative reagent. Microchimica Acta, Volume 55, Number 6, pages 1126-1135. 

In particular, the sodium rhodizonate test can be used to detect gunshot residue (which contains lead) in a subject's hands,

 Vincent J. M. Di Maio, ''Gunshot wounds: practical aspects of firearms, ballistics, and forensic techniques, 2nd edition. CRC, 1998. ISBN 0-8493-8163-0. p. 341.

and to distinguish arrow wounds from gunshot wounds for hunting regulation enforcement.

 Ronald L. Glover (1981), Detecting Lead in "Arrow" Wounds in Deer Using Rhodizonic Acid Wildlife Society Bulletin, Vol. 9, No. 3, pp. 216-219. Online version accessed on 2009-07-30.