Why 3 or more developing agents?

<p>Here is the MSDS for Tetenal Ultrafin liquid developer. <br>

http://www.tetenal.com/sidaopener_uk.htm?file=sidauk/100152.pdf<br>

As you can see, it contains Metol, Hydroquinone and Phenidone. Why not just MQ or PQ?</p>

<p>Why not? Manufacturers have to somehow differentiate their products. Personally, I normally use D-23, so I think that one developing agent is plenty :-)</p>

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<p >Metol is the common name for para-methylaminophenol sulfate. Metol is soft-working (low contrast) developer that has a low stain potential. Metol is often combined with hydroquinone the common name for 1,4-benzenediol, a harder-working (high contrast) developer. The combination produces good shadow detail with higher contrast. Phenidone is the common name for 1-phenyl-3-pyrazolidone, a non-staining developing agent that work well in areas of the film that suffer from under exposure.</p>

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<p >Developers are largely derivatives of benzene originally a byproduct of coal. All oxidize and return to their coal tar roots. Thus exhausted developers have staining properties. Developers are blends of different developing agents. The differences from formula to formula are often quire subtle. </p>

 

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<p>While I'm sure Tetenal know what they're doing, I can't help but remember the quote in Anchell and Troop's 'The Film Developing Cookbook' by the estimable Bob Schwalberg: "One developing agent is best, two is okay, three is very suspect and four, the guy is definitely a jerk."</p>

<p>The simple Metol + sulphite developers are OK. I think that the Windisch Metol-Sulphite developer is an older formula than D-23.<br>

MQ developers were the most common type until Phenidone and it`s derivatives came along. But why use both Metol and Phenidone with Hydroquinone in the same developer, rather than just an MQ or PQ developer?</p>

<p>Chris Waller said: <strong><em>"While I'm sure Tetenal know what they're doing, I can't help but remember the quote in Anchell and Troop's 'The Film Developing Cookbook' by the estimable Bob Schwalberg: "One developing agent is best, two is okay, three is very suspect and four, the guy is definitely a jerk."</em> </strong></p>

<p>I have that book and the comment has stuck in my mind ever since I read it. I`m sure that there must be some logical explanation behind using more than two agents, but I have yet to find one that satisfies my curiosity.</p>

<p>Phenidone is often use for a speed increase. Could this be their objective. I've never used Tetenal devs; does this one offer a speed increase?</p>

<p>Maybe the formula was originally PQ, then the bean counters decided to replace part of phenidone with metol to lower the production costs. :-P</p>

<p>Seriously though, phenidone and metol are quite different agents. Despite its reputation as a mere metol replacement, phenidone is actually different and gives different results. I wouldn't be too surprised if metol and phenidone worked well together and complemented each other, though I must say I haven't happened upon such a formula yet.</p>

<p>510-Pyro contains three developing agents (pyrogallol, ascorbic acid, and phenidone). Phenidone is supperadditive with Pyro and ascorbic acid, while ascorbic acid is only additive with pyro. But ascorbic acid plays another role in the developer, as a preservative in place of sulfite, and a moderator of general stain, while also adding to the developing capacity of the developer. If the phenidone was left out of the formula, the resulting developer would be slower working, produce less than full emulsion speed, and have less developing capacity. If the ascorbic acid was left out, the resulting developer would oxidize rapidly, produce higher levels of general stain, and have less developing capacity. If the pyro was left out, the resulting developer would not be a staining/tanning developer, would be slower working, and have less developing capacity. 510-Pyro is a unique developer, and would not retain its unique characteristics without all three agents in their given proportions. It might seem complicated, but considering it meets the requirements of developing agent, preservative, restrainer, alkali and solvent with only four ingredients, three of which contribute directly to the reduction of silver halide, in a single, concentrated solution with excellent keeping properties, I'm satisfied with the formula, and 510-Pyro is easily my favorite developer. </p>