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The washing soda sold in supermarkets can either be sodium carbonate decahydrate or sodium carbonate monohydrate. The Arm and Hammer "Super Washing Soda" is the monohydrate.

 

Both the monohydrate or the decahydrate will work perfectly well in a developer provided you adjust the quantity used for the water content. It is pointless to go to trouble to obtain anhydrous sodium carbonate then add it to water.

 

I suspect that sometimes anhydrous is used for accurate weight. You can dry it just before measuring, if you don't trust that someone hasn't left the bottle open.

 

In the case of photography, weighing so accurately should not be needed.

-- glen

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Using the monohydrate of sodium carbonate, instead of anhydrous, causes only a 15% shortfall in the active chemical - not really a big deal for a developer formula.

 

OTOH, using the decahydrate gives you only 37% of what's needed. The decahydrate is what's usually supplied in cheap bags of 'washing soda' or 'soda crystals'.

 

Downloading the maker's MSDS for the product should tell you exactly what you've got.

 

Once you know, the following corrections can be made:

Multiply the weight of monohydrate by 1.17 to give the equivalent anhydrous quantity.

Multiply the decahydrate weight by 2.7 to give the anhydrous equivalent.

 

Or you can bake any form in an oven at 100 degrees C for an hour or so to be sure you have the anhydrous salt.

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Or you can bake any form in an oven at 100 degrees C for an hour or so to be sure you have the anhydrous salt.

 

Based on the bottles of primary standard grade Na2CO3 I have, I'd advise 140ºC to be sure you're actually getting anhydrous, although as you said it doesn't really matter. You would never use this $200, 500g bottle for photographic purposes...this has a very specific purpose to standardize the concentration of acid solutions.

 

IMG_5702.thumb.jpg.473b530b6459d74f0eb8cf6853545948.jpg

 

(BTW, when buying lab reagents for uses where precision/accuracy isn't critical, I typical will buy the most hydrated form available. The reason for that is that-even though I have to use more-I know that the hydrated form probably is exactly what it says it is).

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I'd advise 140ºC to be sure you're actually getting anhydrous,

 

- Duly noted Ben.

I didn't want to advise any higher for fear of getting into decomposition territory...... again!

 

If I may get your professional advice. Is a 10% w/v sodium hydroxide solution stable in the proverbial well-stoppered bottle? Maybe not a glass bottle, because I seem to remember there's an interaction.

 

Was thinking of using the caustic solution as the 'B' part of a long-life two-part phenidone-ascorbate developer.

Edited by rodeo_joe|1
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If I may get your professional advice. Is a 10% w/v sodium hydroxide solution stable in the proverbial well-stoppered bottle? Maybe not a glass bottle, because I seem to remember there's an interaction.

 

We buy 50% w/v commercially, and it has a virtually infinite shelf life-in a plastic bottle.

 

I routinely store concentrations from .1M on up to 6M(~16%), and consider them shelf stable for a couple of years at least. There's a POSSIBILITY of them picking up humidity and going down in concentration, but even at 10%(starting) it wouldn't be any appreciable amount.

 

You are right to avoid glass-it will etch and otherwise dissolve it-the effect is more pronounced at higher concentrations, and 10% is getting into what I'd consider dangerous. By all means avoid any kind of ground glass joint, as you will likely "freeze" it.

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We buy 50% w/v commercially, and it has a virtually infinite shelf life-in a plastic bottle.

 

I routinely store concentrations from .1M on up to 6M(~16%), and consider them shelf stable for a couple of years at least. There's a POSSIBILITY of them picking up humidity and going down in concentration, but even at 10%(starting) it wouldn't be any appreciable amount.

 

You are right to avoid glass-it will etch and otherwise dissolve it-the effect is more pronounced at higher concentrations, and 10% is getting into what I'd consider dangerous. By all means avoid any kind of ground glass joint, as you will likely "freeze" it.

 

- Thanks for that Ben. I was hoping for several months shelf-life at least. The urge to shoot film is very infrequent these days.

 

Plus I thought I might have imagined the caustic soda/glass interaction.

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