Rhodizonic acid, sodium salt rhodizonate

Rhodizonic acid sodium salt (5,6-dihydroxycyclohex-5-ene-l,2,3,4-tetraone di-Na salt)... 

Sodium rhodizonate (0-5 %). Dissolve 0-5 g sodium rhodizonate (rhodizonic acid, sodium salt C6OeNa2) in 100 ml water. The solution decomposes rapidly. 

Sodium rhodizonate (rhodizonic acid, sodium salt) reagent... 

Synthesis. Benzenehexol is available only from laboratory reagent suppliers. The simplest laboratory preparation involves the aeration of the glyoxal—bisulfite addition product in sodium carbonate solution at 40—80°C, isolation of the sodium salt of tetrahydroxybenzoquinone, followed by acidification to obtain the free tetrahydroxy-p-benzoquinone in about 8% yield the latter is reduced with stannous chloride in boiling dilute hydrochloric acid solution to benzenehexol (77) in 77% yield (261). A similar procedure affords dipotassium rhodizonate (80) in good yield (262). 

H. S. Isbell, H. L. Frush, and Z. Orhanovic, Oxidation of sodium salts of alduronic and glyculosonic acids by sodium peroxide, Carbohydr. Res., 36 (1974) 283-291 Oxidation of certain cyclic carbonyl compounds with alkaline hydrogen peroxide, ibid., 43 (1975) 93-100 H. S. Isbell, Concurrent oxidation and reduction reactions of cyclohexanehexone, rhodizonic acid, and tetrahydroxy-l,4-benzoquinone with hydrogen peroxide, ibid., 39 (1975) C4-C7. 

By condensing the sodium salt of rhodizonic acid with o-phenylenedi-amine a quinone was obtained as early as 1888, but the originally proposed structure as a 2,3-dione (1888CB1227) was shown to be incorrect, the compound being in fact 295 (62LA131). 

Sodium rhodizonate test Barium salts yield a reddish-brown precipitate with sodium rhodizonate (see under Barium, Section III.31, reaction 7). Sulphates and sulphuric acid cause immediate decolourization because of the formation of insoluble barium sulphate. This test is specific for sulphates. 

Strontium salts react similarly to barium salts with sodium rhodizonate. However, strontium rhodizonate is easily soluble in dilute hydrochloric acid in the cold, whereas the barium compound is converted into an insoluble bright red acid salt. It must be noted that small amounts of barium rhodizonate prepared in a test tube in neutral solution, and then treated with hydrochloric acid, seem to dissolve, because the red barium salt is formed in a fine state of division and is hard to see. If, however, the reaction is carried out on paper, the brown-red stain of barium rhodizonate is turned to a... 

The brown aqueous solution of sodium rhodizonate (I) produces colored precipitates of basic lead rhodizonates from neutral or slightly acid lead solutions. Violet Pb(Ce0fl) Pb(0H)2 H20 is thrown down in neutral solution scarlet-red 2 Pb(C 0e) Pb(0H)2 H20 is produced from weakly acid solutions. The deep color of these lead salts probably arises from the fact that the organic anionic component contains five double bonds, as seen in II. o o... 

Barium salts give a red-brown precipitate with rhodizonic acid or sodium rhodizonate (see page 119). The precipitate, produced on paper, is stable toward dilute hydrochloric acid, whereas sulfates and sulfuric acid cause immediate decolorization owing to the formation of insoluble barium sulfate. This fact can be applied to give a sensitive and specific test for sulfates. 

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