Scum when selenium toning

[Marco Buonocore]

Every now and then I will get a "scum" when selenium toning, and more often with higher alkaline sepia toning.

I know it can be removed with a quick bath in 3% acetic acid. But that's an extra step ; an extra wash. The less I have to handle a print, the less likely it is to get damaged.

I understand that this "scum" is caused by hard water, is that correct? If I were to make my toner up with distilled water would that help? Is there something I could add to the water like EDTA that would help?

It's not a real problem on small prints, but on larger work it's a headache.

Thanks!

 

[Rudeofus]

If that scum can easily be removed with 3% Acetic Acid, then yes, water hardness is the likely culprit. You can use EDTA or Calgon (Hexametaphosphate) to prevent scum formation.

One thing that wonders me is why your toners don't have such an agent in them already. Either you have excessively hard water, or you self mix without such a sequestering agent, or something else is amiss.

BTW: don't waste money on EDTA free acid, Disodium-EDTA or Tetrasodium-EDTA is what you want. Add 1-2 grams per liter to your working solutions.

 

[Bob Carnie]

Marco

I have seen the scum with sepia , I do remember Kodak Brown toner was impossible not to get scum with Toronto water.

When mixing Hypo clear as Rudeofus mentions I add Hexametophosphate.. I have about a 200 year supply if you want to come and scoop some from me. It stopped the problem immediately

Bob

 

[Marco Buonocore]

Thanks guys, really great information.

My selenium is KRST, sepia is from scratch.

I make my hypo clear myself ; just 1 tbsp sodium sulfite per litre of water. I guess that is the culprit? Thinking back, the problem probably started when I started doing this.

Bob, I'll take you up on your offer and grab some of that hexometophosphate. If it works I'll buy a bunch on my next chemistry order.

Thanks!

 

[Rudeofus]

My selenium is KRST, sepia is from scratch.

Since sepia toners are very alkaline, you definitely need to add a sequestering agent. Hexametaphosphate suffers from hydrolysis in very alkaline environment, but this takes days and your sepia toner bath is single session anyway I suppose. I am surprised that a packaged product like KRST needs extra ingredients, but I don't know your Toronto's water.

I make my hypo clear myself ; just 1 tbsp sodium sulfite per litre of water. I guess that is the culprit? Thinking back, the problem probably started when I started doing this.

Yes, Sodium Sulfite is quite alkaline and will precipitate Calcium Carbonate.

The reason for scum formation in alkaline environment is simple: in neutral environment you have mostly Bicarbonate anions, and Calcium Bicarbonate is somewhat soluble, therefore there can be quite a bit of Calcium in your water. If you raise pH (i.e. add OH-), Bicarbonate will turn into Carbonate (HCO3- + OH- <----> CO3-- + H2O), Calcium Carbonate is quite insoluble and will form the precipitate you observed on your prints.

 

[pdeeh]

. Hexametaphosphate suffers from hydrolysis in very alkaline environment, but this takes days

does this explain why my (Sodium carbonate heavy) paper developers will still throw a precipitate after a few weeks even though I have used hexametaphosphate?

 

[Rudeofus]

does this explain why my (Sodium carbonate heavy) paper developers will still throw a precipitate after a few weeks even though I have used hexametaphosphate?

I haven't seen it in paper developer, but Tetenal's E6 FD does this after some storage. I'm not sure that water hardness could account for the amount of precipitate I get, and preliminary tests with a subset of the ingredients didn't produce the precipitate.

In other words: I don't know ...

 

[Marco Buonocore]

Rudeofus,

It's a post like yours that really makes me appreciate APUG. Fantastic information, and I think I understand the situation much better now. Thank you!

 

[Marco Buonocore]

Further to Rudi's comment regarding sodium sulfite alkalinity/calcium precipitation, this is why most home mix HCA formulas without a sequestering agent include 1-2g sodium bisulfite/metabisulfite to lower the pH of the usual 20g/l sodium sulfite solution. This is seen in commercial formulas as well, in addition to sequestering agents. The pH values for Ilford Washaid and Kodak HCA are 8 and 7, respectively.

Hello Michael. The reason I never added sodium bisulfite/metebisulfite to my hypo clear was based on something I read in the Darkroom Cookbook - the instructions were I could leave it out if I wanted to "improve the paper's gloss". Not going to lie, I wasn't too concerned with the gloss, I was just more interested in making my life a bit easier. Live and learn!