Superadditivity developing agents

All of the above developing agents have unique characteristics, and some have a special purpose. The shortening of the list of modern developing agents has more to do with ease of manufacture, storage, and shipping than it does with their usefulness. The superadditive effects of hydroquinone, metol, Phenidone, and ascorbic acid (see discussion of superadditivity later in this chapter) has also added to their popularity with manufacturers. 

The phenomenon known as superadditivity plays an important role in many film developers. Superadditivity occurs when the combined result of two developing agents is greater than either one of them working alone. 

We know it as Vitamin C, an anti-oxidant essential to human life. Photographers think of it as a secondary developing agent in superadditive pairs such as Metol-Ascorbate or Phenidone-Ascorbate. These have been known for years but recently made popular by Kodak XTOL film developer. 

Hydroquinone and A -methyl-/j-aminophenol (Metol) form a superadditive mixture which was shown by Tausch and Levenson [47] to involve the consumption primarily of hydroquinone with the preservation of Metol. This led to the regeneration theory proposed by Levenson, that Metol was acting as the developing agent at the silver halide surface and that oxidized Metol was reduced back to Metol by hydroquinone as outlined in Eqs. (30)-(33). 

Electochemical measurements by Jaenicke and co-workers [56] indicate two possible mechanisms by which superadditivity could arise. In the first, a developer showing irreversible oxidation becomes reversible in the presence of a second developing agent. In the second, when the mixed potential is in the limiting current region of the anodic reaction, superadditivity occurs if the limiting current rises because the number of electrons delivered per molecule of a first developer, for example hydroquinone, is increased by the addition of a second developer, such as Phenidone. This could happen with hydroquinone development in the presence of sulfite because the reaction product, hydroquinone monosulfonate, is a poor developer by itself but in the presence of Phenidone it is activated. 

Effective superadditive mixtures result from combinations of l-phenyl-3-pyrazolidinone (Phenidone) [48] and its derivatives with hydroquinone, ascorbic acid, /7-hydroxyphenylaminoacetic acid, hydroxylamines, pyrogallol, and other agents. In general, superadditivity is a phenomenon primarily used in black-and-white developers but it has also been observed in color development [49]. 

Willems also showed, in the same article, that some compounds that formed stable radicals would also catalyze silver bleaching by persulfate. He suggested that these compounds acted as electron transfer agents, performing as a shuttle for electrons from the silver to the persulfate thus a superadditive mechanism, similar to that seen in development, could operate. Kobayashi et al. [141] observed a similar effect with Af,7V,Af, 7V -tetramethyl-/j-phenylenediamine but also showed that this compound was destroyed by the action of persulfate, and benzoquinone was formed. It was this quinone that acted as the electron transfer agent in aged solutions. 

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