Developing agents metol-hydroquinone

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. 

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. 

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

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. 

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). 

All patients should be tested with Metol and hydroquinone, as these are included in both black-and-white and colour processes. Colour-developing agents should, however, be tested only in those patients involved in colour film processing. Many of the substances are potent allergens, and some (particularly PBA-i) may induce patch-test sensitisation. 

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