superadditivity of metol and catechol

el wacho · Mar 17, 2009

hi everyone,

i have found very little on the net regarding this topic and would appreciate any information. here's what if found so far. my particular interest is in modifying the ratio between metol and catechol to enhance the characteristics of the catechol ( staining ) , though i would find any info interesting. thanks in advance.

Lowell Huff · Mar 17, 2009

Catechol is closely related to hydroquinone in structure and resembles it in activity and sensitivity to Bromide ions. In the presences of low quantities of developer preservative( Sodium Sulfite), catechol produces a residual stain image in those areas where development has occured. This stain image is effective in intensifying the lower image densities, making weak densities more effective. The oxidized product of catechol also tans gelatin in the image area. Catechol has been reported to be capable of image development even in the presence of considerable amounts of silver halied solvents, making this development agent suitable for combined developing and fixing solutions.

CBG · Mar 17, 2009

Gleaned from the net - the pairings I understand to work superadditively are:

Ascorbates - phenidone
Ascorbates - Metol
Ascorbates - p-aminophenol
Ascorbates - pyro
Ascorbates - Catechol
Catechol - - - phenidone
Glycin - - - - - phenidone
Hydroquinone - phenidone
Hydroquinone - Metol
Pyro - - - - - --phenidone
Pyro - - - - - - Metol

So if you find the metol / catechol pairing to be superadditive, you may be on to something newsworthy in our small world of film lovers.

I would be interested to hear if any of the more technically knowledgeable folks here see errors and omissions in my listing.

I do wonder if metol / pyro / catechol would have anything to offer, or if it is problematic. On a quick glance I don't see any such combination among the developer formulas I have saved.

Ian Grant · Mar 17, 2009

What about PPD

PPD - - Glycin
PPD - - Pyrocatechin (catechol)
PPD + Pyrocatechin = Meritol - - Metol
PPD + Pyrocatechin = Meritol - - Phenidone

Ian

sanking · Mar 17, 2009

Pyrocat-M, which was the base for -MC, contained Pyrocatechin + metol. In the -M formuila the metol appears to add to the synergy as the combination of pyrocatecin + metol is more active than pyrocatechin by itself. However, add too much metol and you reduce the stain.

You will also reduce the stain of Pyrocat-HD by adding too much phenidone, or by adding too much sulfite, or by adding too much ascorbic. Some people might want to do this since you get the tanning without the stain.

Sandy King

Alan Johnson · Mar 17, 2009

Pyrocat-M is metol-catechol:
www.apug.org/forums/forum37/25806-pyrocat-p-pyrocat-m.html
If the 2g/L phenidone in Pyrocat HD is replaced by 10g/L glycin,that works too,but the glycin is hard to dissolve.
It's probable the metol catechol is superadditive.This 2-solution Pyrocatechin developer from Developing by C.I.Jacobson 1948 has only Pyrocatecatechin but it needs to be a lot more concentrated:
(A) Pyrocatechin 10g ,Sodium Sulfite 20g ,water to 500ml
(B) Potassium Carbonate 60g ,water to 500ml
Mix A+B undiluted,develop 7 min.

gainer · Mar 17, 2009

HC110 for some time used catechol in place of hydroquinone. Both are dihydroxybenzene. There is little or no superadditivity without either sulfie or ascorbate. These combinations, from what I have seen, seem not to be so simple as we would like them to be. We know that D-76 without hydroquinone has about the same activity as proper D-76. Metol at pH 8.6 with sulfite is as active as Metol with ascorbate, but without sulfite. I wonder if it is proper to call MQ a superadditive pair when the activity of Metol is the same with or without hydroquinone when ther is no sulfite.

Alan Johnson · Mar 18, 2009

We know it is implied in some books that D-76 without HQ has the same activity as proper D-76,but it does not make sense to say that Metol and HQ are superadditive and that Metol alone gives the same development time as the superadditive mixture.
In Photographic Processing Chemistry by LFA Mason p 119, there is a graph of the time to reach a certain contast versus mol% metol at pH 10.6.Here the time for metol alone is about 2.5 times longer than that for the most superadditive mixture of metol and HQ
Granted the pH is a lot higher than D-76 and the concentration of sulfite is not stated but Mason's data seems to indicate superadditivity is in fact pretty much as expected.

gainer · Mar 18, 2009

Alan Johnson said:
We know it is implied in some books that D-76 without HQ has the same activity as proper D-76,but it does not make sense to say that Metol and HQ are superadditive and that Metol alone gives the same development time as the superadditive mixture.
In Photographic Processing Chemistry by LFA Mason p 119, there is a graph of the time to reach a certain contast versus mol% metol at pH 10.6.Here the time for metol alone is about 2.5 times longer than that for the most superadditive mixture of metol and HQ
Granted the pH is a lot higher than D-76 and the concentration of sulfite is not stated but Mason's data seems to indicate superadditivity is in fact pretty much as expected.

I didn't just say that off the top of my head. It came from 'The Theory of the Photographic Process"; Mees & James; 3rd edition, p.366-377. There is a graph that shows the effect of adding sodium sulfite, sodium ascorbate and hydroquinone, one at a time. I think I posted a copy of that graph here some time ago. The line showing the effect of hydroquinone alone on Metol without sulfite is dead flat, while each of the others rises to a maximum of roughly 7 times the activity as for Metol alone. I guess we could say that Metol and either Sulfite or ascorbate are superadditive, but not Metol and hydroquinone, at least at pH= 8.7.

Alan Johnson · Mar 18, 2009

Reason metol and hydroquinone are not superadditive in absence of sulfite may be that Ag+ needs an electron to develop it to silver and that electron originates from the reaction of HQ with SO3-.No sulfite, no development contribution from HQ.Thanks for the reference.

CBG · Mar 18, 2009

Ian, thanks for the addition of the ppd combinations! I had not seen those. CBG

Photo Engineer · Mar 18, 2009

Here are the general rules for superadditivity between developing agents. This was first proposed by G. I. P. Levenson and outlined more clearly by Haist.

1. A primary and a secondary developing agent must be present, each with a different activity.

2. The primary developing agent must adsorb strongly to the silver halide crystal.

3. This primary developing agent must have a relatively stable semiquinone after one electron transfer.

4. Regeneration of this primary developing agent must be completed by the secondary developing agent.

5. The semiquinone of the primary developing agent must be neutral or positively charged.

Schematically, this can be shown as follows:

1. Metol - 1 electron ----------------> oxidized Metol (methylquinoneimine)

2. Oxidized Metol + Sulfite --(slow)--> Metol Monosulfonate (this "decomposition" must be slow)

3. Oxidized Metol + HQ ------(fast)---> Metol + Qunone

4. Quinone + Sulfite ---------(fast)---> HQ Monosulfonate

Hope this helps quantify and explain better.

PE

gainer · Mar 18, 2009

Alan Johnson said:
Reason metol and hydroquinone are not superadditive in absence of sulfite may be that Ag+ needs an electron to develop it to silver and that electron originates from the reaction of HQ with SO3-.No sulfite, no development contribution from HQ.Thanks for the reference.

Unfortunatly, neither the combination MQS nor the combination MQA was tested in the quoted reference. Some have claimed that D-76 is as active without the Q, but may have lower capacity. The author's explanation of the graph is that sulfite keeps the oxidation products of Metol from inhibiting development, but ascorbate regenerates oxidized Metol. It cannot be deduced from the data shown that sodium ascorbate is a developing agent, but one can see that without the sulfite, hydroquinone does not regenerate the Metol, nor does it appear to have any effect on the Metol oxidation products.

Photo Engineer · Mar 18, 2009

Patrick;

See my post. HQ does not regenerate anything that depends absolutely on Sulfite, but oxidized Metol oxidizes HQ to the quinone. HQ + sulfite = nothing, but Quinone + sulfite = HQMS. HQMS is a less active developing agent that reacts at a ratio of about 5:1 in rate compared to HQ itself. Base is produced in this latter reaction.

Sulfite functions to drag the equillibrium to the right.

PE

gainer · Mar 18, 2009

PE's description of the chain of superadditive reaction requires sulfite. It appears that an ascorbate has properties of both hydroquinone and sulfite, not by name of course, but by function. We can use it as a developing agent somewhat akin to hydroquinone, and we can use it as an antioxidant somewht akin to sulfite. When we use it as a superadditive complement to a primary developer such as Metol, Phenidone, p-aminophenol, etc. , we can leave out the sulfite or put some in for the purpose of grain modification.

Photo Engineer · Mar 18, 2009

Dear All;

Please read: http://en.wikipedia.org/wiki/Le_Chatelier's_principle

This explains the role of sulfite ion in the post I made. It alters conditions by removing one of the reaction products and driving the equation to the right. The key is, and I stress, adsorption of one developing agent to the silver halide grain.

This is widely accepted as the source of superadditivity along with the other reasons given by Levenson and Haist in their independant publications and proven over and over again. HQ adsorbs poorly but Metol adsorbs well due to the pair of electrons on the nitrogen.

Now, in the case of PPD developers, by analogy, they adsorb well, but react with sulfite well too, so equation 2 above is (fast) not (slow). This changes the way PPDs react to superadditivity. And that is a whole 'nother story.

PE

Alan Johnson · Mar 19, 2009

In the case of Pyrocatechol,I wonder if oxidised metol can oxidise that to a quinone which is subsequently removed by reaction with sulfite.

gainer · Mar 19, 2009

Alan Johnson said:
In the case of Pyrocatechol,I wonder if oxidised metol can oxidise that to a quinone which is subsequently removed by reaction with sulfite.

Would the location of one OH make that much difference between two dihydroxybenzene molecules? I can say that hydroquinone works as well as pyrocatechol in Pyrocat MC, PC or HD. The stain color is different.

gainer · Mar 19, 2009

I surmise that the role of sulfite in superadditivity could be performed by other compounds, among which may be ascorbate. Whether the oxidation product that sulfite removes is regenerated or removed by ascorbate, the net result without hydroquinone seems to be about the same. I did learn once by experiment that the amount of sulfite required to get maximum activity out of the same amounts of M and Q as in D-76 as in D-76 was quite small.

Alan Johnson · Mar 19, 2009

Basic Photo Science by Walls & Attridge p197 gives the adsorbed form of metol as an ionized form of the free base CH3NHC6O-.This is oxidized by losing an electron to Ag+ -> Silver.To regenerate it requires an electron.It might come from ascorbate + 2OH- -> dehydroascorbate +2H20 +2 electrons.Just a guess.

superadditivity of metol and catechol

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