Sodium sulphite - which purity grade?

JPD · Jul 3, 2010

I will soon run out of Sodium sulphite. The last time I bought 5kg, and I think it is pro Analysi which is very pure. I'm not sure that I can buy from the same company this time, since the owner closed his shop and now is a full time photographer.

Which purity grade would you recommend? I have found a source for Purum that is ~98%, but below the pro Analysi.

nworth · Jul 3, 2010

Photo grade. Analytical reagents are overkill, but some sodium sulfite contains impurities that can be harmful photographically. Most practical and food grades work fine, however. If you can't get photo grade sulfite, you can try one of the others on a small sampling of non-critical work to make sure it works decently. Photo grade sulfite is usually supplied in anhydrous (or dessicated) form. Other grades may have different or uncertain hydration, and you have to take that into account in determining the quantity to use. If the hydration is not stated or is uncertain, don't use the material; you can't tell how much effective material you are measuring out.

JPD · Jul 3, 2010

The only source for photo grade I can find here in Europe is Calbe in Germany. Even if it's cheaper than the pro Analysi, the shipping would be high so it doesn't matter.

Here's the specifications for the pro Analysi Sodium sulphite from Merck. I don't think the chlorides and thiosulfates affect the results much. I had no problems.

Sodium sulfite
anhydrous for analysis EMSURE® Reag. Ph Eur

Assay (iodometric) 97.0 - 100.5 %
Identity passes test
Appearance of solution passes test
Insoluble matter ≤ 0.005 %
Free acid passes test
Titrable free base ≤ 0.03 meq/g
Chloride (Cl) ≤ 0.02 %
Thiosulfate (S2O3) ≤ 0.03 %
Heavy metals (as Pb) ≤ 0.0005 %
As (Arsenic) ≤ 0.0001 %
Cu (Copper) ≤ 0.0005 %
Fe (Iron) ≤ 0.001 %
Pb (Lead) ≤ 0.0005 %
Se (Selenium) ≤ 0.0004 %
Zn (Zinc) ≤ 0.001 %

JPD · Jul 5, 2010

I decided for some sulphite of the Puriss grade. It's between the Purum and pro Analysi.

BetterSense · Jul 5, 2010

I use the stuff TheChemistryStore.com sells in the 20lb bucket. It works.

Mahler_one · Jul 5, 2010

How would our colleagues in Europe obtain chemicals from The Chemistry Store?

JPD · Jul 5, 2010

BetterSense said:
I use the stuff TheChemistryStore.com sells in the 20lb bucket. It works.

Price:$ 31.00 / 25lb Pail. That's cheap and probably Technical or Practical grade. Makes me want to try a less pure grade than what I'm used to, like Purum just to see if it makes any difference.

JPD · Jul 5, 2010

Mahler_one said:
How would our colleagues in Europe obtain chemicals from The Chemistry Store?

We can't.

I know Calbe, www.calbe-chemie.de in Germany sell many raw chemicals for photographic use, like Sodium sulphite (Natriumsulfit) in the Fotopur grade. I don't know if they sell to individuals or retailers/industry only.

Ian Grant · Jul 6, 2010

Photo-grade Sodium sulphite (anhyd) should have a Heavy metal content (Pb) of less than 0.002%, and Iron 0,005%, Chloride of 0.05% is irrelevant, normal lab grade Sulphite which any laboratory chemical supplier will stock is fine for photographic use.

Technical grade Sodium sulphite usually available in 25kg bags is well within the Photo-grade specification.

Ian

JPD · Jul 6, 2010

Ian Grant said:
Photo-grade Sodium sulphite (anhyd) should have a Heavy metal content (Pb) of less than 0.002%, and Iron 0,005%, Chloride of 0.05% is irrelevant, normal lab grade Sulphite which any laboratory chemical supplier will stock is fine for photographic use.

Technical grade Sodium sulphite usually available in 25kg bags is well within the Photo-grade specification.

Good to know. I remembered the lengthy borax-discussion and thought that the purity of the sulphite was more important since a lot more of it is used in developers than borax. (Yes I know that borax is mined).

Gerald C Koch · Jul 15, 2010

See if you can find a supplier of food grade, its used in many things from sausage to wine.

Sodium sulphite - which purity grade?

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