Developer superadditive

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. 

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. 

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. 

Used alone in sodium carbonate/sulfite solutions, it is very fast but extremely soft working and is only capable of producing negatives of low contrast. In combination with hydroquinone it produces developers with superadditivity that are even more efficient than MQ developers. 

Additionally, while the first oxidation product of hydroquinone, mono-sulphonate, forms an almost inert system with metol, it has a superadditive effect with Phenidone, increasing developing power. 

When used correctly, pyro creates a stained image of unprecedented tonal scale, especially in the high values. In combination with either metol or Phenidone, it exhibits superadditive characteristics. It produces an overall stain on the negative, which adds to the contrast of the silver image. The stain is a desirable part of a properly developed pyro image. 

Thus density superadditivity is only an apparent superadditivity even though the sum of the silver densities for the separate developers A and B appears to be less than that for the A + B curve. This is because developer B would not have an induction period in the presence of developer A and so would contribute to the overall development almost immediately, giving the broken curve A + B which is merely additive. In kinetic terms, true superadditivity occurs when the rate of growth of density or silver for the combined A with B developer is greater than the sum of the rates of growth of density or silver for the A developer and the B developer separately. 

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). 

Thus superadditivity can be viewed as a sequence of two types of electron transfer reaction, one which is heterogeneous between Metol and the latent image, resulting in silver development, and one which is homogeneous between hydroquinone and... 

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]. 

Superadditivity has been observed in both chemical and physical development [52] thus the presence of silver halide is not a necessary condition for its occurrence but is likely to modify its detailed course. This would appear to rule out the charge-barrier theory in its original form, although charge effects might also occur at silver as well as at silver halide surfaces. 

This indicates that superadditivity arises by the removal of an inhibiting species in Phenidone oxidation, probably the Phenidone radical as in case 1 above. Lee and Miller [53] showed that development of silver halide by the Phenidone radical generated in a flow system was much slower than that by Phenidone itself by a factor of about 24, which agreed very closely with the results of Levenson and Twist [52c], and Shiao and Dedio [54] also found that superadditivity of ascorbic acid and a Phenidone derivative (MHP) was explained by scavenging the MHP radical, thus eliminating any inhibition. A similar inhibition by oxidation products of development was proposed to explain the behavior of ascorbic acid physical developers [55]. 

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. 

Hamano et al. [112a] extended their electrode model to include superadditivity and the effects of halide ions and quaternary salts on development. 

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. 

Lee WE (1977) The rate of development part 2—Superadditivity. In James TH (ed) The Theory of the photographic process, 4th edn. Macmillan Publishing Co Inc, New York Collier Macmillan Publishers, London... 

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