Metol-hydroquinone developer

The black and white negative in each layer is developed using conventional developers of the metol/hydroquinone or phenidone/hydroquinone type... 

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. 

There are a number of differences between Phenidone/Hydroquinone (PQ) and Metol/ Hydroquinone (MQ) print developers, but none of them are significant enough to cause one to be preferred to the other. Perhaps the most important difference is the longer and more consistent working life of a PQ developer. This is because Phenidone is less sensitive to bromide so the build-up of bromide in solution has little effect on its activity. However, it should be noted that the organic restrainers found in high pH PQ developers are carried off by the print. While this will not affect the activity of the developer it will have some affect on the overall contrast of succeeding prints. 

Notes At one time, pyro was a universally used developer. It was even available in prepackaged formulas from companies such as Eastman Kodak. Unfortunately, it has been almost entirely replaced by metol-hydroquinone- and Phenidone-hydroquinone-type developers. The main reasons are that pyro stains, it must be handled carefully, and it does not keep well in solution. In other words, it is not as commercially viable as other forms of developers. 

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. 

Combining metol with hydroquinone can, in some formulas, result in a developer that is fester than metol alone and produces contrast equal to, or greater than, that of hydroquinone alone. Developers of this kind are known as MQ (for Metol and Quinol).2 The optimum... 

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. 

Hydroquinone alone is not largely used, but in combination with metol (MQ) or Phenidone (PQ), it is among the most popular developers. By varying the relative quantities of metol and hydroquinone and adjusting the quantities of sulfite and carbonate, almost any desired contrast or rate of development can be obtained. 

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

Phenidone-based developers keep better than those based on metol. The reason is the oxidation product of Phenidone is more efficiently regenerated by hydroquinone than is metol. 

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. 

Notes Sodium ascorbate is usually in combination with metol or Phenidone. It may be used as a substitute for ascorbic acid in film developers. It may also be used as a substitute for hydroquinone. Try using 1.8 times the weight of the hydroquinone. The alkali does not have to be increased because sodium ascorbate is not acidic. 

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). 

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... 

Aniline is converted into hydroquinone, a photographic developer in black and white photography used with metol (p-methylaminophenol). Production of hydroquinone commences with oxidation of aniline by manganese dioxide in aqueous sulfuric acid at 0-5 °C,... 

The dihydric phenols are speciality products widely used industrially and commercially. Hydroquinone and its relative, 4-methylaminophenol (metol) are the main chemicals used for development in black and white photography. Butylated hydroxyanisole, (BHA) mainly 2,6-di-tert-butyl-4-methoxyphenol (5), an important food antioxidant is synthesised from 2,4,6-tri-t-butylphenol by reaction in methanolic solution with chlorine to give 2,4,6-tri-t-butyl-4-methoxy-... 

Metol (Table 1) has been the most important black-and-white film developer since 1895 and is a known skin irritant and sensitiser. The use of hydroquinone (Table 1) as a black-and-white film developer is increasing, and it is normally used together with Metol... 

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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