Hydrate forms of sodium carbonate

[anyhuus]

I have some sodium carbonate and I am uncertain which hydrate form it is (monohydrate or crystalline). Is there some way of finding out? (e.g. specific gravity?)

 

[gainer]

1. Weigh some. Heat the bejeezes (this is a technical term you will find in the book "Caveman Chemistry" by Kevin M. Dunn) out of it and see how much weight it loses. The bejeezical vapor is water until you get it REALLY hot.

2. Weigh some, dissolve it in a known weight of water. Measure the resulting volume at 20 C. Now you can calculate the specific gravity of your solution because you know its weight and volume. Look in the Handbook of Chemistry and Physics for the table that lists things you can measure about sodium carbonate and see which percentage by weight or volume fits closest to what you have. Now you know the weight of anhydrous carbonate your solution per unit volume or per gram of solution weight.

 

[Gerald Koch]

There is a monohydrate and a decahydrate which is seldom seen today. The decahydrate was found as large clear ice like pieces which rapidly lost water to the air and became powdery.

You can weigh out a small amount, say 10 g, heat it and then reweigh it to determine which form you have. If it was the monohydrate the new weight will be 8.55 g. If it was the decahydrate the new weight would be 3.7 g.

 

[anyhuus]

Thanks for the quick answers.
What is the temperature I need to heat it to?

 

[avandesande]

Apparently it starts losing water at 100c.

http://www.jtbaker.com/msds/englishhtml/s3245.htm

 

[Donald Qualls]

For whatever it's worth, if you got it in white powdery form, it's almost certainly monohydrate. Both the anhydrous and decahydrate tend to convert to monohydrate in storage, gaining or losing water to/from the air; as a result, anything other than monohydrate also generally carries a price premium.

 

[Ole]

Take the lid off, put a piece of filter paper across the opening. Let stand in a well ventilated area for a month.

At the end of this, you have monohydrate. It makes no difference what you started with - anhydrous, hemihydrate, monohydrate or dekahydrate.

 

[anyhuus]

Well, what I got is soda from the grocery store and it is in a white powdery (actually more like sand) form. I tried to heat it in a stainless steel pan over the gas for about 5 minutes. The temperature raised considerably over 100 C. I could not really measure any weightloss (well, 10 grams had reduced to 9.9, but I think that is more down to measuring inaccuracy than loss of water). However, there were some quiet "snap-crackle-pop" sounds from the pan, but no visible vapour. I suppose it was the single water molecules i heard leaving the carbonate

Based on the feedback I got in this thread and some other threads on the forum, I will go forth under the assumption that it is monohydrate I have.
Thanks all for the feedback.

 

[anyhuus]

Ole, if you haven't guessed it already: what I'm going to use the carbonate for is the Beutler formula
Do you use photo grade carbonate from a chemical supplier, or simply soda from the shop down the street when you mix Beutler's?

 

[Ole]

I use soda from the nearest shop. It works just as well as pro Analysi potassium carbonate from VWR - I have tried both in Pyrocat-HD.

 

[anyhuus]

I use soda from the nearest shop. It works just as well as pro Analysi potassium carbonate from VWR - I have tried both in Pyrocat-HD.

.... and then you assume monohydrate and just do the maths to convert from anhydrous (sicc.) right away, or do you actually let the carbonate bottle be open for a month?

 

[Ole]

No - I just assumed that since it's cheap and has a VERY vague contents description ("60-100% Sodium Carobonate"), it would have to be monohydrate. And I'm careful about not closing the bottle properly

 

[Maine-iac]

Well, what I got is soda from the grocery store and it is in a white powdery (actually more like sand) form. I tried to heat it in a stainless steel pan over the gas for about 5 minutes. The temperature raised considerably over 100 C. I could not really measure any weightloss (well, 10 grams had reduced to 9.9, but I think that is more down to measuring inaccuracy than loss of water). However, there were some quiet "snap-crackle-pop" sounds from the pan, but no visible vapour. I suppose it was the single water molecules i heard leaving the carbonate

Based on the feedback I got in this thread and some other threads on the forum, I will go forth under the assumption that it is monohydrate I have.
Thanks all for the feedback.

Good ol' Arm & Hammer Washing Soda is all the carbonate you'll ever need for any formula that calls for it. Never fails. I've never worried about whether I had decahydrate or monohydrate, though Ole's explanation about leaving the lid open now explains it for me. But whatever, I've never had a failure in a formula due to the carbonate.

Larry

 

[Mike-D]

Based on how it acts in developers like D61a and D72 I treat A&H washing soda like monohydrate. If you use the quantities that you would need if it were decahydrate the developer becomes way too energetic.

Mike D

 

[jim appleyard]

Odd, I treat the A&H Washing Soda as if it were decahydrate. I convert all my formulas from anhydrous and mono to deca and measure the appropriate amount. I've never had a problem either and I'm doing just the opposite from whaqt Mike is doing. Wonder why?

 

[Wayne]

A&H washing soda is monohydrate. I called and pressed them on this topic several years ago and they were very adamant that it was monohydrate.

 

[srs5694]

FWIW, Ryuji Suzuki identifies A&H Washing Soda as the monohydrate form based on his own tests on this page.

 

[Gerald Koch]

If you want higher purity sodium carbonate you can decompose Arm & Hammer baking soda (sodium bicarbonate) to sodium carbonate. The baking soda is USP grade and of much higher purity than the washing soda. Just place the baking soda in a glass or stainless steel pan and bake it at 350 F for 1 hour with occassional stirring. The resulting product is quite fluffy and is anhydrous sodium carbonate. This is what I use for the Beutler formula.

 

[Mike-D]

Odd, I treat the A&H Washing Soda as if it were decahydrate. I convert all my formulas from anhydrous and mono to deca and measure the appropriate amount. I've never had a problem either and I'm doing just the opposite from whaqt Mike is doing. Wonder why?

I suppose it is not all that critical. If a developer is more or less active, as would happen by changing the amount of alkalai, then you compensate by changing time etc. Its not exactly linear. I have a 1936 copy of Kodak's "Elementary Photographic Chemistry". It says that for D-76 if you increase the amount of borax ten times you cut the development time in half.

Mike D

 

[pnance]

Why not just use Soda Ash from a pool supply store? Less other ingredients. Label says less than 2% other on my pail.

 

[Gerald Koch]

Why not just use Soda Ash from a pool supply store? Less other ingredients. Label says less than 2% other on my pail.

I would worry what the other 2% is!

 

[pnance]

I wonder what the 40-10% is in the Arm & Hammer Washing Soda? I know I smell perfume, see soap suds and lord only knows what else!

 

[Wayne]

I wonder what the 40-10% is in the Arm & Hammer Washing Soda? I know I smell perfume, see soap suds and lord only knows what else!

Well it IS washing soda, so suds are not unexpected. Makes a great laundry additive if your water is too hard to get sudsy with normal detergent. They make it w/o the perfume though, I suggest looking for the unscented, straight carbonate version.

 

[gainer]

I would worry what the other 2% is!

The stuff I get claims it to be "inert ingredients". I can't think of very many ingredients that would be inert in a swimming pool and active in a developer that aren't already in most tap water. It could be simply water. The carbonate as they package it may be very close to anhydrous. It could be sand, but 2% sand would surely show in the mixing. The only sediment I have seen is from the reaction with calcium and magnesium in my well water. It is not there when I use distilled or rain water or some EDTA.

 

[Gerald Koch]

Years ago when I used A&H Washing Soda for print developer it threw down a considerable amount of tan sediment. I tested this and it consisted, in part, of hydrated ferric oxide. I was put off by the amount of iron in the product and stopped using it. Iron is not the best thing to have as a contaminate in developers. I now buy my carbonate from The Chemistry Store or make it from USP baking soda.