Home made washaid question

[henk@apug]

I am going to make my own washaid with 1 tablespoon of sodium sulfate and a pinch of sodium metabisulfite for 1l of water.
I would like to prepare that in advance.
Anyone has an idea of the shelf life for this working solution (unused). Thx.

 

[koraks]

sodium sulfate

Sodium sulfite, not sulfate!

Anyone has an idea of the shelf life for this working solution (unused).

In a full, stoppered bottle - weeks, months. Maybe not quite years. The mode of deterioration is that the sulfite oxidizes under influence of dissolved oxygen - and guess what it oxidates into...that's right, sulfate! In an open vessel, oxygen will keep dissolving into the water and this will just keep oxidizing sulfite until none is left. Also, if you use a bottle and take some out once in a while and leave air on top, the air will be replaced by fresh air (including 16% oxygen) every time you open the bottle.

Overall, I wouldn't worry about it and just keep a jar of sulfite at hand. Whenever you need wash aid, you dissolve a spoonful in your tray/jar of water. It takes only a minute or so to dissolve. The dry sulfite will last indefinitely. The metabisulfite isn't even needed.

You could make a concentrate (e.g. ca. 10% sulfite solution), but really - what purpose does it serve if you can just keep 100% sulfite around easily?

 

[Doremus Scudder]

It's easy to mix on the spot, as needed, The sulfite dissolves easily. Be aware that sodium metabisulfite has a limited shelf life, even dry, so needs to be fresh in order to do its pH balancing job. However, it's not all that necessary, as Koraks points out. You can mix a wash aid with just sulfite; it's just not quite as efficient.

Once the sulfite is in solution, it starts degrading due to oxidation. Yes, keeping it in a full, stoppered bottle will help, but it will still slowly oxidize. If you want to ensure freshness, mix your wash aid on the spot and use it one session.

Best,

Doremus

 

[Steve Goldstein]

As others have suggested, just mix it when you need it. I’ve been doing so for years, 20 grams (not critical) per liter. Sodium sulfite dissolves easily in room temperature water. Mixing it as needed adds maybe 2 minutes to my setup time.

I stopped adding Sodium metabisulfite years ago when my bottle ran out - I never liked the whiff of stink when opening that bottle!

 

[retina_restoration]

I am going to make my own washaid with 1 tablespoon of sodium sulfate and a pinch of sodium metabisulfite for 1l of water.
I would like to prepare that in advance.
Anyone has an idea of the shelf life for this working solution (unused). Thx.

Not SULFATE, it's Sodium SULFITE.
And don't store it once it's been mixed in water — make it fresh every time. (You can use it for one working session/day)

 

[koraks]

Be aware that sodium metabisulfite has a limited shelf life, even dry

I've heard this before, but so far, I've not noticed this myself. The oldest metabisulfite I have on stock is over 10 years old. It's the same as the day I bought it; a free-flowing powder. There must be some basis to what you said, but I honestly don't know.

 

[BHuij]

...just keep a jar of sulfite at hand. Whenever you need wash aid, you dissolve a spoonful in your tray/jar of water. It takes only a minute or so to dissolve. The dry sulfite will last indefinitely. The metabisulfite isn't even needed.

This is what I've been doing for years. Couple of tablespoons or so of sulfite in a ~1.8ish liter juice bottle I repurposed. I don't think it needs to be all that precise. Dump it in, shake the bottle up for 30 seconds at the start of a print session, use once after toning, discard.

 

[Milpool]

Lowering the pH to roughly neutral (as in commercially prepared products) was essentially a compromise both for pH stability and to make the HCA most compatible with the widest variety of materials as well as hardening fixers. All other things being equal the neutral sulfite solution is a little less efficient.

It's easy to mix on the spot, as needed, The sulfite dissolves easily. Be aware that sodium metabisulfite has a limited shelf life, even dry, so needs to be fresh in order to do its pH balancing job. However, it's not all that necessary, as Koraks points out. You can mix a wash aid with just sulfite; it's just not quite as efficient.

Once the sulfite is in solution, it starts degrading due to oxidation. Yes, keeping it in a full, stoppered bottle will help, but it will still slowly oxidize. If you want to ensure freshness, mix your wash aid on the spot and use it one session.

Best,

Doremus

 

[john_s]

Lowering the pH to roughly neutral (as in commercially prepared products) was essentially a compromise both for pH stability and to make the HCA most compatible with the widest variety of materials as well as hardening fixers. All other things being equal the neutral sulfite solution is a little less efficient.

So does that mean that the simplest method (just sodium sulphite) is actually better than any of the embellished more complicated versions (e.g. with metabisulphite, citrate, EDTA...)?

 

[Steve Goldstein]

Not necessarily - it depends how hard your tap water is (assuming you use tap water). Citrate and EDTA are helpful if you have hard water as I understand it, so if you don't include these and have hard water it might even be worse. They're benign in non-hard water. Remember, Kodak et al design their chemistry to work for everyone everywhere, regardless of how soft or hard their tap water is.

In my case sulfite alone works fine.

 

[Milpool]

No. My point (poorly made) is only that if you’re scratch mixing a HCA, whether or not you choose to include the bisulfite (metabisulfite) is a minor issue.

Unless a hardening fixer was used there should be little practical difference between an alkaline or neutral sulfite-based HCA. Concerning the effects of emulsion swell on diffusion rates I have seen arguments in both directions. I assume what Doremus Scudder refers to above is the shorter diffusion path at a neutral pH.

The other compounds in the Kodak or Harman products are there basically to deal with a broad range of water quality, in particular water hardness - to prevent calcium sulfite precipitation. If I remember correctly the Harman product also contains a surfactant.

So does that mean that the simplest method (just sodium sulphite) is actually better than any of the embellished more complicated versions (e.g. with metabisulphite, citrate, EDTA...)?

 

[Doremus Scudder]

I've heard this before, but so far, I've not noticed this myself. The oldest metabisulfite I have on stock is over 10 years old. It's the same as the day I bought it; a free-flowing powder. There must be some basis to what you said, but I honestly don't know.

Koraks,

I was having problems with PMK and have traced it to both/either old sodium metabisulfite and an old sodium metaborate solution. I've corrected both, but during my research found that metabisulfite in powdered form has a "shelf life" of somewhere from 5 months to 5 years, depending on grade and storage conditions. Reagent grade seems to last longest, and is typically advertised with a shelf life of 5 years. Being as I have an old Kodak jar of metabisulfite from probably the 1990s, I thought it might be wise to buy new

As for the metaborate (to digress a bit), it appears that even though a saturated solution of sodium metaborate has a rather long shelf life, it is susceptible to oxidation over time. When degradation begins, there is usually a precipitate, so watching for this helps. I mix smaller amounts of PMK A now and try to use them quickly.

Best,

Doremus

 

[RalphLambrecht]

As others have suggested, just mix it when you need it. I’ve been doing so for years, 20 grams (not critical) per liter. Sodium sulfite dissolves easily in room temperature water. Mixing it as needed adds maybe 2 minutes to my setup time.

I stopped adding Sodium metabisulfite years ago when my bottle ran out - I never liked the whiff of stink when opening that bottle!

with alkaline fixers, there is much less need for it in the first place.

 

[M Carter]

Tim Rudman stated "4 hour tray life" for a heaping teaspoon of sulfite to a liter of water. Not sure how accurate that is, but I've never spent more than 4 hours washing prints!

It is a great way to manage HCA, the stuff is cheap by the pound (eBay or mail order) and is one of the safer chems we use - it's a food preservative after all.