another sodium carbonate question

[Colin Graham]

If you need 1.17 times the amount of monohydrate to equal anhydrous, why is it that monohydrate cost almost twice that of anhydrous? I'm assuming that it's because it's more difficult to manufacture and keep in the monohydrated state, but if anyone can correct me I'd appreciate it.

 

[gainer]

That is hard to understand. The monohydrate is, I believe, the most stable state. If you leave either the monohydrate or the higher hydrates exposed, they will eventually get to be monohydrated. When you buy monohydrated, you are getting 17% water. If you are paying more, buy the anhydrous, which is without water of crystallization.

 

[gainer]

I meant to say "If you leave either the anhydrous or the higher hydrates exposed"

 

[Colin Graham]

Thanks Patrick. Now it really doesnt make any sense! I havent checked every supplier, just a few... Maybe they just want to discourage people from using monohydrate? Hmmm. Not a big deal but I was curious.

 

[Photo Engineer]

The anhydrous costs more money to make due to the drying process which requires a lot of heat and control to drive off the extra water but not destroy the carbonate.

PE

 

[Ole]

There's also the small matter of purity. I discovered that locally, sodium thiosulfate (dekahydrate) costs about the same in 100g, 1kg ang 25kg packages. Same stuff, different purity, different price.

 

[Colin Graham]

The anhydrous costs more money to make due to the drying process which requires a lot of heat and control to drive off the extra water but not destroy the carbonate.

PE

But what I was saying is the monohydrate is twice as expensive as the anhydrous, which is suprising

 

[Photo Engineer]

But what I was saying is the monohydrate is twice as expensive as the anhydrous, which is suprising

Oh yes, but I didn't add that the making of the hydrate and ensuring its purity is quite difficult as well. As pointed out above, you have to be careful that you don't end up with 'slush' or the anydrous compound.

Drying totally is one thing, but precisely drying to the monohydrate is even more tricky or you get intermediates.

Sorry that was not clear.

PE

 

[gainer]

Maybe it boils (pun intended) down to the facts that more energy is required to make the anhydrous but the endpoint is probably more easily identified while adding back just the right amount of water to make the monohydrate requires more control, especially if you want technical or higher grade. I'm a little puzzled as to why the water content of sodium carbonate would be a concern. It seems to me that a saturated solution, or a solution with a certain specific gravity, at a certain temperature would be preferred for precise laboratory use. For most of our photographic uses, the difference between anhydrous and monohydrated would not be noticed. You can make your own anhydrous sodium carbonate by heating baking soda. Its also fun to watch the little volcanos form. Use a stainless steel or Pyrex saucepan. It will be pretty pure because the baking soda you get from the market is meant for human consumption.

 

[Photo Engineer]

Patrick;

You can overheat sodium carbonate and get sodium oxide which then rehydrates back to sodium hydroxide. This is one of the control problems in making the anyhdrous and then the second is rehydrating to the monohydrate.

If you are not careful....

NaCO3 + too much heat -> CO2 + NaO
2NaO + H2O -> 2NaOH

And, if you do it in an aluminum pan, hydrogen gas is liberated with possible dire consequences as the above reacts with the aluminum. This is why I don't suggest doing it on the stove or any other way by a non-expert.

Otherwise, I agree and you said it well above.

PE

 

[dancqu]

... why is it that monohydrate cost almost
twice that of anhydrous?

Almost twice? More than twice from Photographer's
Formulary. Something I did not know. Silica Gel, calcium
chloride and Drierite are three desiccants that will likely
work to keep the anhydrous dry.

An easy check is to weigh the carbonate before and after
exposure and/or before and after drying. I do not know
the drying temperature. It may be quite high. Dan

 

[Colin Graham]

Oh yes, but I didn't add that the making of the hydrate and ensuring its purity is quite difficult as well. As pointed out above, you have to be careful that you don't end up with 'slush' or the anydrous compound.

Drying totally is one thing, but precisely drying to the monohydrate is even more tricky or you get intermediates.

Sorry that was not clear.

PE

Thanks for the clarification. Sorry, I've been in a bit of a fog today...I wonder why? :->

 

[Gerald Koch]

While it's possible to heat sodium carbonate and have it release carbon dioxide, it must be heated to past its melting point of 851 C.

 

[gainer]

Patrick;

You can overheat sodium carbonate and get sodium oxide which then rehydrates back to sodium hydroxide. This is one of the control problems in making the anyhdrous and then the second is rehydrating to the monohydrate.

If you are not careful....

NaCO3 + too much heat -> CO2 + NaO
2NaO + H2O -> 2NaOH

And, if you do it in an aluminum pan, hydrogen gas is liberated with possible dire consequences as the above reacts with the aluminum. This is why I don't suggest doing it on the stove or any other way by a non-expert.

Otherwise, I agree and you said it well above.

PE

I started with NaHCO3 which I heated in a stainless steel pan (I did specify either stainless steel or Pyrex) to liberate CO2. I think it would have taken a lot more heat than I could concentrate on the stove top to decompose the resulting Na2CO3. The pot would have been at about red heat. I checked the weight accurately before and after the outgassing appeared to have ceased and it matched calculations. I did this at a time when the nearest place to get photo quality carbonate was 50 miles away and it cost about 5$ a pound, and I wanted it yesterday. In 1994 5$ was worth what 10$ is now.

I appreciate your knowledge of chemistry in general and of photographic chemistry in particular. If we were standing next to each other, I would probably accuse you of being Monkish, after the obsessive-compulsive fictional detective Adrian Monk. I'm sure you would respond in kind and we would go flip for soft drinks. We did a lot of this banter at NASA. I doubt they even have Coke machines anymore. Siigh!

 

[Photo Engineer]

Patrick;

That should have been:

Na2CO3 + too much heat -> CO2 + Na2O.

See. My knowledge or attention is slipping.

We had a Pepsi machine at Kodak and Pepsi dispensers in the cafeteria. The machine kept the cans so cold that when you popped them, the release of CO2 froze the Pepsi and you had to let it stand a bit before you could drink it.

I'm really not OC except where photographic myths or misconceptions take place, and there really are too many of them nowdays.

I do like Adrian Monk (Tony Shaloub) but preferred his previous assistant. He was great in Galaxy Quest playing the clueless actor who played the clueless engineer.

PE

 

[gainer]

Now I know I'm old. We still got our Coke in glass 6 oz. bottles. For a nickel. When the price went to 6 cents because of a state tax, the machine's price went to a dime. Beer in cans came later, before soft drinks, and the cans required an opener and were tin coated steel.

 

[juan]

But the best part of the machines with the 6 oz. glass bottles was what we called "pulling" for Cokes. Each local bottling company produced bottles with the name of the local city on the bottom. These were the days of returnable bottles, so the various bottles were spread across the country.

In our game, everyone would buy a Coke from the machine, turn the bottle over, and read the city where the bottle was produced. The person who pulled the city farthest away got the price of his Coke back from each person participating.

The game actually required some basic knowledge of geography, and, of course, since there was no map available, led to a lot of arguments.

As far as sodium carbonate, I just use Arm and Hammer washing soda
juan

 

[gainer]

I can't even get A&H Washing soda here. I'm in the middle of West Virginia, in the country which comprises most of our State. Go figure. I would think there would be lots of uses for it here. Maybe it's also used, like lye, for nefarious purposes.

 

[juan]

Here in Florida, it's sold in Publix supermarkets but not in any other that I've found. At one time, it was sold by Amazon, but I don't see it there anymore. It turns up in a search there as being mentioned in books about meth manufacture, so I guess the nefarious purposes speculation is correct.
juan

 

[Maine-iac]

As far as sodium carbonate, I just use Arm and Hammer washing soda
juan

Me too. It's served as my sodium carbonate in all the developers I've mixed over the past 30 years. Never gave me a problem. (Except when, like Pat Gainer, I lived in a place where it was unavailable, like France, par example. But I solved that by just buying a box or two every trip back to the U.S. and lugging it back with me.)

The side benefit is that I always have a strong cleaning agent around when I need it.

Larry

 

[Reinhold]

Remember all those "Drink Coca-Cola" signs painted on barns and drugstore walls? (10¢, In The Glass, At The Fountain). Simple pleasures for simple folk...

I've got a 2lb can of Sun brand "Pool & Spa PLUS", 98% sodium carbonate sitting on my desk as I type this. $4.95. "To increase the pH of water". Got it in the Hot Tub supply department of a local Big Box (Target? Wal-Mart?).

 

[Photo Engineer]

Remember all those "Drink Coca-Cola" signs painted on barns and drugstore walls? (10¢, In The Glass, At The Fountain). Simple pleasures for simple folk...

I've got a 2lb can of Sun brand "Pool & Spa PLUS", 98% sodium carbonate sitting on my desk as I type this. $4.95. "To increase the pH of water". Got it in the Hot Tub supply department of a local Big Box (Target? Wal-Mart?).

We have been here before, over and over and over.

What is the other 2%, and is it harmless in photographic processing?

PE

 

[Gerald Koch]

The major impurity in sodium carbonate is often sodium chloride since sodium carbonate is produced by the Solvay process which utilizes sodium chloride.

 

[Reinhold]

Sorry for 'buttin in again, but I had so much fun playing with that barn picture that I had to try a pseudo sepia color, using Photoshop's Duotone controls...


Ron...

No contest. I'm not a hardcore brewer of developers, but I occasionally "tweak" a paper developer or toner. If that 2% fouled it up, I'd just dump it and reach for the more expen$ive yellow bottle...


Reinhold

www.classicBWphoto.com

 

[Photo Engineer]

Gerald, Reinhold, that sodium chloride is a mild antifoggant for chloride and chlorobromide papers.

PE