Hydroquinone anodic wave

Fig. 13. Reaction of 2,3-dimercaptopropanol with p-benzoquinone. Phosphate buffer pH 6.8, x 10 4M dimercaptopropanol, benzoquinone concentration (1) 0 (2) 0.59 (3) 1.15 (4) 1.74 (5) 2.3 X 10-4.M. Curves starting 0 V, S.C.E., 00 mV/absc h = 50 cm, full scale sensitivity 3.2 jjlA. Reaction between dimercaptopropanol and benzoquinone is fast, curves recorded immediately after mixing. Anodic wave of 2,3-dimercaptopropanol decreases and is replaced at very positive potentials by hydroquinone wave...

Besides extensive kinetic investigations, less detailed polarographic studies can provide useful information on the course of certain reactions. For example, in the reaction of 2,3-dimercaptopropanol with 1,4-ben-zoquinone (34), the anodic wave of 2,3-dimercaptopropanol vanishes with increasing concentration of quinone, when the ratio of thiol to quinone reaches 1 2 (Fig. 13). An anodic wave corresponding to oxidation of hydroquinone is also obtained. In the reaction, which is irreversible, neither cathodic disulphide waves nor anodic waves corresponding to monothiols appear. Thus an addition-reduction mechanism (14) can be postulated. 

Various other reactions. Some other enzymatic reactions, such as the catalysis of the oxidation of aldehydes and of xanthine [181], and oxidation of catechol and hydroquinone catalyzed by tyrosinose [182] have been described in the book [3]. The activity of cholinesterase is proportional to the increase of the anodic wave of thiocholine, which is produced by the hydrolysis of acetylcholine [183]. 

Because of the relatively easy oxidation of mercury, anodic waves are observed with the DME only for the strongest reducing agents such as hydroquinones, enediols (e.g., ascorbic acid), phenylhydroxylamine derivatives, and certain aldehydes. Numerous organic substances nevertheless yield anodic waves corresponding to mercury-salt formation, e.g., thiols and other derivatives of bivalent sulfur, amines, and some... 

The determination of hydroquinone in vinylic monomers and polymers, can be carried out in a methanol-benzene (1 1) mixture containing OT M ammonium acetate as a supporting electrolyte. Monomers can be mixed with the solvent in a 1 1 ratio and 0-00004 per cent of hydroquinone can be detected. In polymers, the solubility of which is about 0-2 g in 100 ml. of solvent, 0-03 per cent of hydroquinone is the lowest level. The method has been applied to methacrylates, vinylacetate, acrylonitrile, styrene and divinyl-benzene. For monomeric acids, such as methacrylic, the acid is first neutralized to phenolphthalein, and the anodic wave of hydroquinone is recorded in a phosphate-borate pH 7-0 buffer. 

Anodic catalytic waves for hydroquinone were observed at a rotating gold elec-tode, confirming the regeneration mechanism with a rate constant for Phenidone regeneration of 1.3 x 10 L mol s [57]. 

The absence of silver oxidation and/or reduction peaks is evidence for the electrochemical inactivity of the silver deposited on this carbon (in the form of metallic crystallites). The cyclic voltammogram recorded for the D—Ox carbon (Fig. 50, curve 2) exhibits two anodic peaks (fp., = +0.27 V, p,a = +0.77 V) due to the oxidation of adsorbed silver and surface hydroquinone-like groups, respectively. A single cathodic peak (Ep,) = +0.16 V) is due to the reduction of quinone-like surface groups according to Scheme 19. The large cathodic reduction wave confirms the presence of adsorbed silver cations and their reduction... 

The cyclic voltammograms of all the carbons carrying preadsorbed silver, recorded in dilute nitric acid solution (Fig. 51), exhibit a Ag"/Ag" couple (cathodic wave < +0.4 V and an anodic response in the +0.40-0.60 V potential range), as well as the electroactive quinone/hydroquinone-like surface system ( p, s +0.50 V p.a2 = +0.90 V). The presence of distinctly shaped anodic silver oxidation peaks indicates the partial solution of sorbed (deposited) metal. An almost sixfold higher anodic peak for D—Ox carbon confirms the partially ionic form of the adsorbed silver. 

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