Keeping Sodium Carbonate in anhydrous form

avortex · Jan 19, 2017

As sodium carbonate monohydrate (the one I prefer to use) costs an arm and a leg in my country compared to anhydrous, I decided to switch to the later one.

How can I keep it in its anhydrous form and avoid the monohydrate conversion with time and moisture?
Will silica gel do the trick?

Gerald C Koch · Jan 19, 2017

Just keep it in a glass jar with a good air tight lid.

avortex · Jan 19, 2017

Thanks! So no silica gel needed. Cool!

darkroommike · Jan 20, 2017

I'd say it may depend on your local humidity, but a glass jar with a good lid is a start. You can also re-dehydrate sodium carbonate in a low oven in a stainless steel pan with an oven proof handle. 200 deg. F. 95-100 C. is close enough you're not baking a cake.

bernard_L · Jan 20, 2017

As sodium carbonate monohydrate (the one I prefer to use) costs an arm and a leg in my country

That is just the pH-plus powder for pools. Rainwater is acid (atmospheric CO2). Maintaining the pool water pH requires an alkali: sodium carbonate monohydrate. Either buy a 5-kg bucket from a pool supply store, or beg/buy 1kg from someone with a pool.

avortex · Jan 20, 2017

bernard_L said:
That is just the pH-plus powder for pools. Rainwater is acid (atmospheric CO2). Maintaining the pool water pH requires an alkali: sodium carbonate monohydrate. Either buy a 5-kg bucket from a pool supply store, or beg/buy 1kg from someone with a pool.

I asked several Ph Plus companies, and one of them told me its anhydrous. The rest have no idea about what's the form of the carbonate they're selling.
On drugstores, 1Kg of anhydrous cost 3€ and is widely available. The only place that sells monohydrate charges 14€. That's too much!

avortex · Jan 20, 2017

darkroommike said:
I'd say it may depend on your local humidity, but a glass jar with a good lid is a start.

Humidity is pretty high here, living near the sea. Perhaps I'll put some silica gel bags inside the glass jar.

Wayne · Jan 20, 2017

Do you have something like this in your country's supermarkets? It's monohydrate and pretty darned cheap and available in any grocery store in the US. I see Amazon won't ship overseas but you might have the equivalent perhaps in another brand?

Rudeofus · Jan 20, 2017

Wayne said:
Do you have something like this in your country's supermarkets? It's monohydrate and pretty darned cheap and available in any grocery store in the US. I see Amazon won't ship overseas but you might have the equivalent perhaps in another brand?

An MSDS for this product can be found here and lists Sodium Carbonate with CAS number 497-19-8, which is the anhydrous form. The monohydrate form has CAS number 5968-11-6.

Wayne · Jan 20, 2017

Rudeofus said:
An MSDS for this product can be found here and lists Sodium Carbonate with CAS number 497-19-8, which is the anhydrous form. The monohydrate form has CAS number 5968-11-6.

There seems to be some disagreement on this point, for example (there was a url link here which no longer exists)
I called Arm & Hammer some years ago and posted their reply, but I don't remember with 100% certainty what it was. Since I have been seen on APUG saying it is both anyhdrous and monohydrate at different times, the one thing I'm sure of is that my memory can't be trusted on this. However somewhere in the archives here my original post after calling them should be found, but I couldn't find it and quickly grew bored with the search. Maybe it was on LFPF or even rec.photo.darkroom

David Lyga · Jan 20, 2017

I really do not understand why simple washing soda is so expensive in Spain. Here, in the USA, any large grocery store carries Arm and Hammer Washing Soda, which is precisely sodium carbonate, mono-hydrate. This lack rather intrigues me and all I can think of is that there is some 'environmental' factor which precludes it being offered in Spain. - David Lyga

darkroommike · Jan 20, 2017

There's also pool supplies. pH Plus is sodium carbonate.

Gerald C Koch · Jan 20, 2017

darkroommike said:
I'd say it may depend on your local humidity, but a glass jar with a good lid is a start. You can also re-dehydrate sodium carbonate in a low oven in a stainless steel pan with an oven proof handle. 200 deg. F. 95-100 C. is close enough you're not baking a cake.

You can use the same technique to make anhydrous sodium carbonate from sodium bicarbonate. Bicarbonate of Soda USP or equivalent should be available just about anywhere. To be on the safe side I always use 300F as the temperature and heat it for one hour with occasional stirring. The temperature should be over 212F. BTW you cannot break sodium carbonate down any further. So don't worry about over heating it.

NaHCO3 --> Na2CO3 + CO2 + H2O

BTW, a few minutes exposure to humid air while compounding a developer is NOT going to significantly change the anhydrous salt. Then too carbonate forms a buffer and the risk is further reduced. Don't worry. Be happy!

Gerald C Koch · Jan 20, 2017

David Lyga said:
I really do not understand why simple washing soda is so expensive in Spain. Here, in the USA, any large grocery store carries Arm and Hammer Washing Soda, which is precisely sodium carbonate, mono-hydrate. This lack rather intrigues me and all I can think of is that there is some 'environmental' factor which precludes it being offered in Spain. - David Lyga

One of our British members once commented that washing soda was practically available on any street corner mom and pop store. So why is Spain so different?

darkroommike · Jan 20, 2017

washing soda also known as soda de sal [Sosa]
sodium carbonate also known as carbonato de sodio
ain't Google wonderful?

Wayne · Jan 20, 2017

darkroommike said:
washing soda also known as soda de sal [Sosa]
sodium carbonate also known as carbonato de sodio
ain't Google wonderful?

Yes but it still doesn't tell you whether it's anhydrous or monohydrate. Even if washing soda left the factory anyhdrous, wouldn't it quickly become monohydrate in those pasteboard boxes?

Wayne · Jan 20, 2017

Rudeofus said:
An MSDS for this product can be found here and lists Sodium Carbonate with CAS number 497-19-8, which is the anhydrous form. The monohydrate form has CAS number 5968-11-6.

I found my old post on rec.photo.darkroom. When I called Arm & Hammer in April 2001 they told me their washing soda is monohydrate, and they did check with their technical team to get that answer. That's what I've always used it as. Maybe they use the wrong number on their MSDS? Their washing soda hasn't changed in any detectable way...

avortex · Jan 20, 2017

Yes, as I've commented before, sodium carbonate anhydrous is very cheap and widely available in Spain.
My only concern was with its progressive transformation into monohydrate, but as stated here, keeping it in a sealed glass jar would do the trick.
I've always used the monohydrated version, but it's so expensive that I just decided to switch to anhydrous...

Thanks everybody for your answers!

By the way, someone told me that monohydrate is the most common form in the USA and the anhydrous in Europe.

David Lyga · Jan 21, 2017

Definitely, the mono is FAR more common in the USA. - David Lyga

darkroommike · Jan 21, 2017

Wayne said:
Yes but it still doesn't tell you whether it's anhydrous or monohydrate. Even if washing soda left the factory anyhdrous, wouldn't it quickly become monohydrate in those pasteboard boxes?

From what I've read on various European caffenol sites a lot of the washing soda in Europe is anhydrous and comes in plastic resealable bags. So if you keep the bag sealed...?

Wayne · Jan 21, 2017

darkroommike said:
From what I've read on various European caffenol sites a lot of the washing soda in Europe is anhydrous and comes in plastic resealable bags. So if you keep the bag sealed...?

I wonder if resealable plastic bags keep humidity out of a hygroscopic compound that well. They might keep your sandwich dry for a couple days but I suspect they are far from impermeable.

Another question the OP could ask, since he actually prefers mono, is there an easy way to know when anydrous has become mono? If there is, and if happens in a reasonably short amount of time, he could just buy a bunch and let it sit out and hydrate, and then not worry about keeping it air tight.

or does it hydrate beyond mono in those conditions?

darkroommike · Jan 21, 2017

Wayne said:
I wonder if resealable plastic bags keep humidity out of a hygroscopic compound that well. They might keep your sandwich dry for a couple days but I suspect they are far from impermeable.

Another question the OP could ask, since he actually prefers mono, is there an easy way to know when anydrous has become mono? If there is, and if happens in a reasonably short amount of time, he could just buy a bunch and let it sit out and hydrate, and then not worry about keeping it air tight.

or does it hydrate beyond mono in those conditions?

Not a sandwich bag but a more heavy duty bag, but then again I'll be it makes little difference for washing out your dirty underwear.
There are three common forms, the anhydrous powder, monhydrate also powder, and a decahydrate crystal. Google and the CRC Handbook say the decahydrate effloresces into the monohydrate.

pdeeh · Jan 21, 2017

I'd be slightly surprised if - excepting those who have a fully calibrated workflow backed up by densitometry - anyone could tell the difference between results from a developer made with pure anhydrous carbonate and those from one made with carbonate that's a mixture of anh and mono (for example)

Fwiw, I've never seen supermarket washingsoda in the UK other than Decahydrate.

Martin Rickards · Jan 21, 2017

Sodium carbonate anhydrous is also known as soda ash and is the main ingrediant, along with sand, for the manufacture of glass.

Gerald C Koch · Jan 21, 2017

pdeeh said:
I'd be slightly surprised if - excepting those who have a fully calibrated workflow backed up by densitometry - anyone could tell the difference between results from a developer made with pure anhydrous carbonate and those from one made with carbonate that's a mixture of anh and mono (for example)

Fwiw, I've never seen supermarket washingsoda in the UK other than Decahydrate.

My gawd, I remember my grandmother using the decahydrate more than half a century ago. BIG ice-like lumps that would quickly form a coating of the white monohydrate. Very costly shipping water about.

Keeping Sodium Carbonate in anhydrous form

Privacy & Transparency

Privacy & Transparency