Quinhydrones

To a cold aqueous solution of benzoquinone, add 1 drop of sulphurous acid solution (SOj-water) the solution turns deep green-brown owing to the intermediate formation of quinhydrone, CeH402,CeIl4(0H)2. Now add excess of sulphurous acid the solution becomes colourless owing to the formation of hydroquinone. Add a few drops of FeClj solution the reaction is reversed and the deep yellow colour (distinct from that of FeCl ) is restored. 

When a solution of, say, 1 g. of hydroquinone in 4 ml. of rectified spirit is poured into a solution of 1 g. of quinone in 30 ml. of water, qulnhydrone C,HA.C,H (0H)3, a complex of equimolecular amounts of the two components, is formed as dark green crystals having a gfistening metallic lustre, m.p. 172°. In solution, it is largely dissociated into quinone and hydroquinone. Quinhydrone is more conveniently prepared by the partial oxidation of hydroquinone with a solution of iron alum. 

Dissolve 100 g. of iron alum (ferric ammonium sulphate) in 300 ml. of water at 65°, Pour the solution, with stirring, into a solution of 25 g. of hydroquinone in 100 ml, of water contained in a 600 ml. beaker. The quinhydrone is precipitated in fine needles. Cool the mixture in ice, filter with suction, and wash three or four times with cold water. Dry in the air between filter paper. The yield of quinhydrone, m.p, 172°, is 15 g. It contains a trace of iron, but this has no influence upon the e.m.f, of the quinhydrone electrode provided that the washing of the crude material has been thorough. The quinhydrone should be stored in a tightly-Btoppered bottle. 

Stabilizers. Hydroquinone [123-31 -9] (4) is widely used in commercial resins to provide stabiHty during the dissolution of the hot polyester resin in styrene during the manufacturing process. Aeration of the styrene with oxygen (air) is required to activate the stabilizer, which is converted to an equiHbrium mixture of quinone and the quinhydrone (5) (11). At levels of 150 ppm, a shelf life of over 6 months can be expected at ambient temperatures. 

Quinhydrone [106-34-3] M 218.2, m 168 . Crystd from H2O at 65°, then dried in a vac desiccator. 1R,3R,4R,5R-Quinic acid (1,3,4,5-tetrahydroxy-cyciohexane-carboxylic acid) [77-95-2] M... 

Chin-hydrin, n. quinhydrine. -hydron, n. quinhydrone. -icin, n. quinicine. 

Catalysis plays an important part in the hydrazine/oxygen reaction. Copper salts were formerly added for this purpose, but in recent years certain organic substances, e.g. quinhydrone, have been employed and a number of proprietary activated hydrazines have been available. These are useful at low temperatures above 150°C scavenging rates with normal hydrazine are such that no great benefit is achieved by their use. 

Hg I Hg2Cl21 KCfsat, formie aeid quinhydrone, 0.05 M, sodium formate 0.25 M in formie aeid Pt... 

Some special redox electrodes. Within the group of redox electrodes, attention should be paid to the hydrogen and oxygen electrodes, and also to the quinhydrone electrode and its tetrachloro version. 

Quinhydrone electrode. If the equimolar compound formed by (benzo)quin-one (Q) and (benzo)hydroquinone (QH2), the so-called quinhydrone (Q2H2), is dissolved in water, it dissociates to the extent of about 90% into the two components. In conjunction with H+ ions and electrons an equilibrium is established on the basis of a completely reversible redox reaction ... 

As an appreciable amount of quinhydrone (50-100 mg) is added and for attainment of the redox potential a relatively small conversion is required, the situation of equimolarity still holds, so that... 

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