Catechol-Borax Complex

[Raghu Kuvempunagar]

It is said that Catechol forms a complex with Borax and this results in a reduction in the activity of Catechol based developers that also contain Borax. If this is indeed the case then I'm curious to know why MCM-10, a well-known Meritol developer, then have both Catechol and Borax? Couldn't the reduction in activity be achieved in MCM-10 by using lower amount of Catechol?

Interestingly, MCM-10 also has Tribasic Sodium Phosphate which is to be added before Borax while making the stock solution. Does this, i.e. the addition of TSP and the order of adding TSP and Borax, have any effect on the formation of Catechol-Borax complex?

 

[Rudeofus]

That complex may not be a bad thing. There exists also a complex between ascorbate and borate ion, and I personally believe, that this complex contributes to the good properties of XTol and DS-10. Imagine this: you have a low effective concentration of ascorbate or catechol, but the pool keeps getting replenished from the complex.

The reason for the peculiar mixing sequence may lie in faster dissolution of ingredients rather than the final product. What exactly do you mean with "stock solution"? Just the regular formula listed here, or a concentrate? BTW the formula listed here has Borax and TSP listed in reverse order. The mixing order may be not that important after all.

 

[Raghu Kuvempunagar]

That complex may not be a bad thing. There exists also a complex between ascorbate and borate ion, and I personally believe, that this complex contributes to the good properties of XTol and DS-10. Imagine this: you have a low effective concentration of ascorbate or catechol, but the pool keeps getting replenished from the complex.

Very interesting insight! I have always wondered why the Catechol in MCM-100 doesn't get quickly oxidized. Could it be that apart from keeping the effective concentration of Catechol low, the complex is also protecting it from oxidation? Or at least the complex is itself less prone to oxidation and thereby preserves Catechol? Plausibly this is why MCM-100 has a reasonable shelf-life despite containing Catechol and a strong alkali.

The reason for the peculiar mixing sequence may lie in faster dissolution of ingredients rather than the final product. What exactly do you mean with "stock solution"? Just the regular formula listed here, or a concentrate? BTW the formula listed here has Borax and TSP listed in reverse order. The mixing order may be not that important after all.

By stock solution, I meant the developer itself. MCM-100 is generally not used diluted.

 

[Rudeofus]

Very interesting insight! I have always wondered why the Catechol in MCM-100 doesn't get quickly oxidized. Could it be that apart from keeping the effective concentration of Catechol low, the complex is also protecting it from oxidation? Or at least the complex is itself less prone to oxidation and thereby preserves Catechol? Plausibly this is why MCM-100 has a reasonable shelf-life despite containing Catechol and a strong alkali.

One factor for the long shelf life may well be the high amount of sulfite in the developer. Complex formation may also be a factor, but I have no data on that. Note, that e.g. very strong iron complexes prevent Fenton reaction, while weak complexes actually accelerates it.

I just noticed, that Kodak was not too fond of borates in their developer formulas (toxic for citrus fruit plants), but that XTol (and most other ascorbate developers from other sources) seem to use borates as main alkali. I have, so far, not seen an explanation for this, and even PE could not help me on this back then.

 

[relistan]

... but interestingly (at least partially for environmental/regulatory reasons) Fotoimpex indicated they opted for a different type of alkali/buffer rather than borate. Just thought this might be of interest.

Very interesting. Regarding the regulatory concern: in Europe borax is on the "Substances of Very High Concern" list already and it seems likely more restrictions will come. As you say It may just be ADOX hedging their bet about regulation and also trying to do better for the environment.

 

[Raghu Kuvempunagar]

it is also said that borates should not be used with pyrogallol. Nevertheless PMK seems to work fine.

On a related note, there is this metaborate substitute proposed by @mrred. Though it has Borax (0.35g/l of developer working solution), it works fine with Pyrocat-HD. But it also has 0.75/l Sodium Hydroxide which could be making the difference. So what exactly happens with catechol-borate complex formation when a strong alkali is present? Is the complex formation inhibited by the strong alkali?

 

[Alan Johnson]

https://www.journal.csj.jp/doi/pdf/10.1246/bcsj.52.3005
See equation (1) above, in more alkaline solutions I believe the reaction will be driven to the right so this favors the stability of the catechol - borate complexes noted in equations (2) and (3).

 

[Rudeofus]

On a related note, there is this metaborate substitute proposed by @mrred. Though it has Borax (0.35g/l of developer working solution), it works fine with Pyrocat-HD. But it also has 0.75/l Sodium Hydroxide which could be making the difference. So what exactly happens with catechol-borate complex formation when a strong alkali is present? Is the complex formation inhibited by the strong alkali?

Literature claims, that complex formation between Catechol and Boric Acid is weak at pH < 8 and at pH > 11. If Pyrocat HD has a pH between 8 and 11, complex formation should be strong.

Reference: "Electrochemical study of catechol–boric acid complexes" by Mohammad Rafiee and Davood Nematollahi

 

[Raghu Kuvempunagar]

Literature claims, that complex formation between Catechol and Boric Acid is weak at pH < 8 and at pH > 11. If Pyrocat HD has a pH between 8 and 11, complex formation should be strong.

Reference: "Electrochemical study of catechol–boric acid complexes" by Mohammad Rafiee and Davood Nematollahi

This probably explains why Pyrocat HD doesn't work well with metaborate but works fine with higher pH borax-lye concoctions.

 

[Rudeofus]

My impression is, that the coordination chemistry between Catechol and Borate is a bloody mess. Dependent on pH different species are around for both constituents, and Borate switches from a 3-coordinated ligand to a 4-coordinated ligand at some higher pH. Both Alan's and my reference draw lots of curves but few conclusions. There are reported differences between pH 9.23 and 11, but I saw no quantitative statement in either reference. Drawing conclusions for a developer appears to be a wild guess at best.

Pat Gainer provided a recipe for making Sodium Metaborate from Lye and Borax: 14.5 grams Lye + 69 grams Borax. So yes, a formula specifying 0.35g/l Borax and 0.75g/l Lye is indeed more alkaline than one with 1 g/l Sodium Metaborate. This excessive alkalinity does more than just break up the Catechol-Borate complex, though. God knows, what the pH of this concoction is, and how stable and repeatable it may be across the globe. I do know, that I get quite a bit of fizzling, when I put some of my moderately fresh Potassium Hydroxide into Acetic Acid.