Regenerative Properties of Hydroquinone

Snapshot · Oct 14, 2007

Hi All,

I have a question on the regenerative properties of Hydroquinone (a.k.a. HQ). It's my understanding that HQ regenerates Metol in the presence of sodium sulfite. I also understanding it does not take on development agent properties until the solution it's in is at pH 10 or greater. At that point it becomes superadditive with Metol in terms of developing.

My question is, in a developer that contains metol but no sodium sulfite, does HQ have any regenerating properties at ph 9.5 or less?

Photo Engineer · Oct 14, 2007

HQ develops at pH values as low as 9.

HQ, when oxidized, reacts with sulfite to form HQ-sulfonate which is a weaker developing agent with about 1/5 the activity of HQ alone. The quinone intermediate is capable of side reactions with other reducing agents. Without sulfite present, the quinone hangs around and represses further development by HQ.

Offhand I just cannot remember the all of the details.

I would have to look it up to get those details. Since it is early Sunday morning, I guess I'll wait a bit.

PE

Alan Johnson · Oct 14, 2007

One theory is that metol is adsorbed on the grain surface and regenerated in situ by HQ.There is competition between sulfite ion and HQ to react with the adsorbed metol oxidation product.My reference did not say what happens in absence of sulfite. Walls & Attridge, Basic Photo Science.

Snapshot · Oct 14, 2007

The reason for my question is that I'm experimenting with a developer that does not contain sodium sulfite. The pH is in and around 9 so I'm wondering if there is any benefit to have HQ in the developer. If it doesn't develop nor does aid in regenerating Metol, then I see no purpose in having it in the formula. Is there something, perhaps, that I'm missing?

Photo Engineer · Oct 14, 2007

Sulfite is an antioxidant. That is the primary purpose in any developer or hypo solution, otherwise the solutions go bad fast.

Without sulfite, developers turn black and fixers form H2S gas and sulfur. (acidic fixers) Alkaline fixers are pretty much immune from this problem.

Sulfite also helps remove developing agents from the coating after development to prevent browning. They render the developing agents more soluable in water.

PE

Snapshot · Oct 14, 2007

Photo Engineer said:
Sulfite is an antioxidant. That is the primary purpose in any developer or hypo solution, otherwise the solutions go bad fast.

Without sulfite, developers turn black and fixers form H2S gas and sulfur. (acidic fixers) Alkaline fixers are pretty much immune from this problem.

Sulfite also helps remove developing agents from the coating after development to prevent browning. They render the developing agents more soluable in water.

PE

I see. Well, I was experimenting with dissolving Metol and Hydroquinone in some TEA, reasoning that sulfite isn't required. A variant of PC-TEA. Is sulfite still a recommended component in this scenario?

Photo Engineer · Oct 14, 2007

TEA is not water. The same reactions will not take place until you add water to the mix.

Eventually, I believe you will want some sort of antioxidant. There is another thread on this here that has been running about PC-TEA.

PE

Snapshot · Oct 14, 2007

Photo Engineer said:
TEA is not water. The same reactions will not take place until you add water to the mix.

Eventually, I believe you will want some sort of antioxidant. There is another thread on this here that has been running about PC-TEA.

PE

OK then, I'll take a look for the thread and have a read. Thank you for your guidance.

Alan Johnson · Oct 14, 2007

MCQ-TEA is mentioned here:
(there was a url link here which no longer exists)
There is one called QP-TEA I believe works.
MQ-TEA you suggest appears experimental, it may be staining.I have tried PCQ-TEA, the stain was hardly visible but I was told it might show up on a densitometer test.

gainer · Oct 14, 2007

"The Theory of the Photographic Process", 3rd edition, p.367, has a graph showing that hydroquinone with neither sulfite nor ascorbate does not regenerate Metol. Ascorbate with neither hydroquinone nor sulfite does regenerate Metol. Sulfite with neither ascorbate nor hydroquinone regenerates Metol, but the author states it is not by the same process as by ascorbate. The pH in these tests was held at 8.7.

The graph shows that the rate of development is approximately doubled when the concentration of either sulfite or ascorbate is about 0.025 gram molecular weight / liter, remaining essentially constant to at least 4 times that concentration.

Snapshot · Oct 14, 2007

So, it would appear that HQ does not regenerate metol (without sulfite) but can act as a developing agent in an aqueous solution with a pH of 9.0 to 9.5. Thanks for your input everyone.

dancqu · Oct 15, 2007

Snapshot said:
So, it would appear that HQ ... can act as a developing
agent in an aqueous solution with a pH of 9.0 to 9.5.
Thanks for your input everyone.

HQ in an aqueous solution of ph 9 to 9.5 and up and of
very low sulfite level will be a lith developer. Dan

Snapshot · Oct 15, 2007

dancqu said:
HQ in an aqueous solution of ph 9 to 9.5 and up and of
very low sulfite level will be a lith developer. Dan

How would that impact film development and print output if used as a standard B&W developer? Would it be undeveloped with contrasted highlights?

gainer · Oct 15, 2007

A lot depends on how much bromide you use. Without it, it will fog pretty badly IIRC. (It has been a while since I played with it.) Ascorbic acid behaves quite similarly. I'm not sure 9.5 pH will do the trick. That's only borax, and I seem to remember using NaOH or at least carbonate. In olden times, HQ was used alone and had a reputation as a high contrast developer. According to Hardy & Perrin, that reputation was due to the fact that it developed highlights first, and since development in those days was by inspection, the film was jerked before development was complete. You will find a number of warm tone print developers with HQ as the sole agent.

Photo Engineer · Oct 15, 2007

Looking over Mees and James again, it seems that they do mention that HQ developers with no sulfite give abnormal curve shapes with sharp toe and high contrast. Also, development is unexpectedly accelerated. So, if you do use a no-sulfite HQ developer watch out for high contrast and dense negatives if the data they show is still useful with modern films.

They explain the acceleration due to the fact that any quinone present reduces the induction period and development starts sooner.

They also describe the fact that HQ + sulfite or HQ + AA give somewhat similar results but for totally different reasons, and the same is true for Metol with sulfite and Metol with AA.

The interaction between Metol and HQ is a third mechanism which we all know as superadditivity. The ET (Electron Transport) methodology is not described as it was still under development at the time the book was written. They are somewhat different but similar. One relies on adsorption to the grain during development, and the other takes place in solution when the two agents interact.

There is much much more and many graphs on pH vs development, sulfite vs development, Metol + HQ vs development and agitation vs development. I suggest that you interested parties all hit the library and start at about page 355, and it really gets interesting at about 365.

PE

dancqu · Oct 16, 2007

Snapshot said:
How would that impact film development and print
output if used as a standard B&W developer? Would
it be undeveloped with contrasted highlights?

I don't recall a single instance of hydroquinone alone
being used to develop film to our usual normal pictorial
needs. It is used as the sole developing agent as a film
developer with lith films.

Also it is used as the sole agent when lith printing. The
only chemicals needed are hydroquinone, sodium sulfite,
and sodium carbonate. Many lith developer formulas
exist. Nearly all are more complex. Dan

gainer · Oct 16, 2007

dancqu said:
I don't recall a single instance of hydroquinone alone
being used to develop film to our usual normal pictorial
needs. It is used as the sole developing agent as a film
developer with lith films.

Also it is used as the sole agent when lith printing. The
only chemicals needed are hydroquinone, sodium sulfite,
and sodium carbonate. Many lith developer formulas
exist. Nearly all are more complex. Dan

You're just not old enough, Dan. Also, the notion of normal contrast changes considerably when you print by a POP method, which is self masking for contrast.

sanking · Oct 17, 2007

Photo Engineer said:
Looking over Mees and James again, it seems that they do mention that HQ developers with no sulfite give abnormal curve shapes with sharp toe and high contrast. Also, development is unexpectedly accelerated. So, if you do use a no-sulfite HQ developer watch out for high contrast and dense negatives if the data they show is still useful with modern films.

PE

I don't know if Mees and James mention pyrocatechin but my experience is that it acts very much like hydroquinone. When used with no sulfite and with no secondary reducer development is slow to a point, but then accelerates quickly to produce a lot of contrast.

A couple of years ago, when I developed the Pyrocat-P and Pyrocat-M formulas, I also experimented with a pyrocatechin + ascorbic acid formula. Initial results looked very interesting,but when I ran a set of BTZS curves I found that there was a extreme range of CI within a short period of development time. I don't remember the exact data but as I recall CI went from 0.30 to 1.0 with development times of eight to twelve minutes. This is a hair trigger response and certainly not ideal for developing film for pictorial purposes.

What I found was that ascorbic, unlike metol and phenidone, has no synergism with pyrocatechin and it functions much like a restrainer in combination with it.

Sandy King

Photo Engineer · Oct 17, 2007

Sandy, Mees and James go through all of the phenolic developing agents to some extent or another.

But I may have stated it poorly when I described HQ + and - sulfite.

Without sulfite, HQ gives a high contrast image with fast development rates, but with sulfite, HQ gives normal contrast images with slower development rates. This entire effect is related to induction time, that is the time between HQ contacting exposed grains and the start of silver production.

As for AA vs Metol, there is little mention. Most of the comparison is AA and Sulfite. All of these interact with HQ and each other, but each interaction is by a different mechanism chemically, and has a different result.

PE

Ian Grant · Oct 17, 2007

Combing through developer formulae I've come across 3 Agfa formulae which use Pyrocatechin instead of Hydroquinone, these developers should behave as Sandy indicates.

The first two are high contrast graphics developers (no sulphite), the third has sulphite and is a high speed rapid X-ray developer. Until I can verify the formulae I'll not post them, one is Agfa 79, which is very different to Agfa Ansco 79 but both are extreme contrast developers

An interesting combination is Pyrocatechin and PPD, which is the basis of Meritol which was a Johnson's proprietary developing agent.

Ian

gainer · Oct 17, 2007

sanking said:
What I found was that ascorbic, unlike metol and phenidone, has no synergism with pyrocatechin and it functions much like a restrainer in combination with it.

Sandy King

It appears from recent pyrocat developers that in fact, ascorbic acid in very small concentrations is a suitable replacement for sulfite for the purpose of stimulating the synergism between Metol, p-aminophenol, or Phenidone and pyrogallol, pyrocatechin or hydroquinone. In larger concentrations, it can also take the place of pyrocatechin or hydroquinone in non-staining developers without sulfite.

sanking · Oct 17, 2007

gainer said:
It appears from recent pyrocat developers that in fact, ascorbic acid in very small concentrations is a suitable replacement for sulfite for the purpose of stimulating the synergism between Metol, p-aminophenol, or Phenidone and pyrogallol, pyrocatechin or hydroquinone. In larger concentrations, it can also take the place of pyrocatechin or hydroquinone in non-staining developers without sulfite.

I agree. Ascorbic in small amount does effectively replace sulfite in stimulating synergism between pyrocatechin + metol, phenidoine and p-aminophenol.

But pyrocatechin + ascorbic acid alone, unlike Pyrocatechin + metol or phenidone or p-aminophenol, does not appear to be a synergistic combination.

Sandy

gainer · Oct 17, 2007

Yes, I think pyrocatechin + ascorbic acid is more like pyrocatechin + more pyrocatechin at pH<11 or so. Higher than 11, either one could make a lith developer. We know that ascorbic acid is an antioxidant that does things in the human body that sulfite cannot do, and is a developing agent in the darkroom which sulfite is not, even at high pH. It appears that at least some part of the synergism between two agents at low pH is due to antioxidant action, as the rate of development of Metol can be doubled by sulfite at 0.05 M concentration at pH=8.7. The same change of rate is obtained at the same pH by 0.05 M ascorbic acid.

Regenerative Properties of Hydroquinone

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