Developing agents hydroquinone

This is shown clearly in the data of Lee and James [43] in Figure 12, in which the sum of the separate rates of development increases to a maximum for a given ratio of the two developing agents, hydroquinone and Phenidone. 

CgH,oN20. Colourless crystalline solid, m.p. 121 °C. Made by reacting phenylhydrazine with ethyl acrylate to obtain the hydrazide which cyclizes to the product. Its major commercial importance is as a photographic developing agent, being used particularly in conjunction with hydroquinone. 

Hydroquinone [123-31 -9] represents a class of commercially important black-and-white chemical reducing agents (see Hydroquinone,RESORCINOL, AND catechol). The following scheme for silver haUde development with hydroquinone shows the quantitative importance of hydrogen ion and haUde ion concentrations on the two half-ceU reactions that describe the silver—hydroquinone redox system ... 

The requirements of a developer moiety for incorporation into a dye developer are well fulfilled by hydroquinones. Under neutral or acidic conditions hydroquinones are very weak reducing agents and the weakly acidic phenoHc groups confer tittle solubility. In alkali, however, hydroquinones are readily soluble, powerful developing agents. Dye developers containing hydroquinone moieties have solubility and redox characteristics in alkali related to those of the parent compounds. 

Another positive-working release by cyclization, illustrated by equation 5, starts with an immobile hydroquinone dye releaser (8), where R = alkyl and X is an immobilizing group. Cyclization and dye release take place in alkaU in areas where silver haUde is not undergoing development. In areas where silver haUde is being developed, the oxidized form of the mobile developing agent oxidizes the hydroquinone to its quinone (9), which does not release the... 

Silver catalysis of the reduction of silver ions appears to be a necessary condition for normal development. The reaction of developing agents including several types of chemical compounds, e.g., hydroquinone, p-aminophenol, hydroxylamine, catechol, and p-phenylenediamine, are known to exhibit this catalysis to a high degree. 

Equation (11) is in good agreement with the established dependence of development rate upon bromide ion concentration for hydroquinone and ferro-oxalate development. It does not account for the dependence of rate on the concentration of the developing agent. It is in adequate... 

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. 

Developing agents for film can be used for paper and vice versa. In practical terms, some developing agents are more advantageous to use in one process than another. For example, ppd is better suited for film development than for paper development, while chlorhydroquinone is better suited for paper than film. Hydroquinone, metol, ascorbic acid, and Phenidone are well suited for either film or paper. This is an additional reason they are preferred by manufacturers. It is worthwhile to familiarize yourself with the brief description of each developing agent in Appendix 3 Pharmacopoeia. 

Most general-purpose developers in use today are metol-hydroquinone-based (MQ) rather than Phenidone-hydroquinone-based (PQ). This is in part because Phenidone has not been around that long as a developing agent, and also because Phenidone is more expensive. However, because of the allergic reaction to metol suffered by many photographers (Appendix 3 Pharmacopoeia), it is sometimes desirable to substitute Phenidone. 

To overcome these drawbacks, developing agents of high activity and short induction periods are required. Phenidone and hydroquinone are suitable monobath developing agents. A combination of these two generally leads to higher speed, improved contrast, and maximum density. 

Even though it has been in use since 1880 pyrocatechin (catechol) is one of the most overlooked developing agents. The working properties of catechol are very much like those of hydroquinone and it can even be used as a substitute for hydroquinone in many formulae, though it works more rapidly than hydroquinone. 

There are other developing agents, chief among them hydroquinone, that will produce a tanned image when used in a formula similar to D-175 (Formulas Extreme Compensating Developers D-175 Tanning Developer). Even so, pyro/cat are the two most commonly used for this purpose. 

Developing agents. The most common print developing agents in use today are metol or Phenidone, plus hydroquinone. In recent years glycin has regained some popularity. 

Notes A derivative of hydroquinone, chlorhydroquinone was once a mainstay for warm-tone prints. Unfortunately, it is becoming increasingly hard to find, not because it is not useful as a warm-tone developing agent, but because it is both expensive and dangerous to manufacture. 

Uses As a developing agent, usually with hydroquinone, but sometimes by itself, as in Kodak D-23. 

Developers containing metol as the sole developing agent are not widely used, but metol with hydroquinone provides the most widely used developer combination. 

Uses High-energy accelerator or alkali sometimes used to activate low-energy developing agents (e.g., hydroquinone). 

Uses As an ecologically friendly developing agent, usually in combination with metol or Phenidone. It can also be used as a substitute for hydroquinone. Start with 1.8 times the weight of the hydroquinone. Sodium isoascorbate is not acidic so the alkali does not have to be increased. 

Hydroquinone (H2Q) is another developing agent and is commonly used in black-and-white processes, usually in conjunction with an auxiliary developing agent such as Metol (A -methyl-p-aminophenol), which is also called Elon, or Phe-nidone (l-phenylpyrazolidin-3-one) (see Figure 6), a heterocyclic agent (see Section 8.2.5). The overall reaction of hydroquinone with silver halide is shown in Eq. (15). In fact hydroquinone undergoes two sequential one-electron transfer steps, with an intermediate semiquinone (S ) and with p-benzoquinone (Q) as the final product. 

The nucleus or development center in physical development can be described as a dual electrode on which the reduction of silver ion to silver and the oxidation of developing agent take place simultaneously. Electrochemical measurements of silver physical development in a hydroquinone/Phenidone physical developer with silver ion complexed with thiocyanate proceed as a catalytic electrode process [39]. 

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. 

It is proposed that the semiquinone is an active developing agent, estimated by Umberger [64] to be 10 times as active as the hydroquinone dianion. The low sulfite level in Lith developers allows the p-benzoquinone (Q) to build up, which boosts the semiquinone concentration via the dismutation reaction. The increased semiquinone level accelerates development which produces more p-benzoquinone and semiquinone according to Eqs. (46), (47),... 

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