Sodium Carbonate: Monohydrate versus Anhydrous

Snapshot

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Hi All,

I have some sodium carbonate that I want to use in some home brewed chemstry. However, the formulas call for sodium carbonate, monohydrate and all I have is the anhydrous version. I believe that I would need to use 20% of the required chemical if I use anhydrous version. Is this correct? Anyone have any experience in using these chemicals interchangeably?

Thanks in advance for your responses.
 

srs5694

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My notes say that if you've got anhydrous sodium carbonate and your formula calls for monohydrate, you should multiply the weight by 0.833. Using 20% of the specified amount (0.20 multiplication factor) is way off. I suspect the 20% figure is a two-fold corruption: (1) You've specified 20% of the required chemical when you mean (or your source meant) 20% more of the required chemical (that is, multiplying the source value by 1.20, or 120%); and (2) the 120% figure is for converting a formula that specifies anhydrous when you've got monohydrate -- the opposite conversion of what you want to do.

My notes, incidentally, are based on this table.[/i] It only has the conversion going backwards from what you want to do, but reversing it is just a matter of taking the reciprocal.
 
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Snapshot

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Ooops... you are absolutely correct. I meant to indicate that I would use 80% of the chemcial. My math skills are seemingly subpar as of late. It was my understanding that I'm to use 5/6 the amount called for sodium carbonate (monohydrate) when using the anhydrous version (which equals your 0.8333 number).

For example, if a formula calls for 50 grams of sodium carbonate (monohydrate), I could instead use 41.85 grams of sodium carbonate (anhydrous) as a substitute. Hopefully, I got this right.
 
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Ole

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Snapshot said:
For example, if a formula calls for 50 grams of sodium carbonate (monohydrate), I could instead use 41.85 grams of sodium carbonate (anhydrous) as a substitute. Hopefully, I got this right.
Click to expand...

Yes, that's correct.
 

gainer

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The ratio pf molecular weights is 124/106 mono/anh. Most uses of sodium carbonate need not be accurate to closer than 1% and many need not be closer than 10%. Anhydrous carbonate tends to become monohydrated by taking moisture from the atmosphere. If you really need acuracy, you should dessicate it or heat it above 100 C until the weight quits changing.

Many times, a formula that was originally contrived using avoirdupois units is converted to metric or vice versa with much greater precision than is required, and in all probability with much greater precision than the original measurements were made. What began as "some" sodium carbonate becomes 28.35 grams when 25 or 30 would do as well simply because the amount was originally specified as 1 ounce, and was probably not weighed any closer than 1/20 ounce.
 
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Snapshot

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It sounds like that getting the exact correct amount of sodium carbonate (with regards to anhydrous versus monohydrate) may not be critical as I thought. In fact, it my not be pratically achievable in non-lab conditions. Thanks for clarifying this for me.
 

Photo Engineer

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The problem comes from anhydrous sitting around in an opened container for any length of time in a humid area. It picks up water.

The monohydrate is more stable on the shelf, as you then know just what you are getting.

So, a rule of thumb is that if the anhydrous is in the form of pure individual particles (lumps, pellets or crystals) then it is good, but if it is in hard cakes and difficult to break up, your carbonate is going bad = turning into monohydrate.

PE
 

phritz phantom

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i just tried crystal soda from the drug store for the first time and this works great. you have to double the amount in comparison to anhydrate (x2,13, exactely).
since it there's still a lot of water in the carbonate, so i was pleasantly surprised how esy it did dissolve. i always had problems getting the anhydrous stuff completely into solution, works like a charm with crystall soda.
also cheap and available round the corner.
 

Mike Wilde

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Cheap carbonate source in North America

Arm and Hammer washing soda, as sold in grocery stores/hardware stores/ department stores in Canada (kind of hard to find - it turns up at my Sobeys) is carbonate, and cheap, and there is no shipping to deal with like when I order from the photo chemical supplier.

The drag is that it has a perfume agent added to it, although the smelll isn't really offensive.

When I mix up agfa 100/D72/ Dektol type print developers I store them in old white plastic Ilford print deevloper bottles. The perfume agent forms a scum in the developer that floats to the top and sets up rather firm after a few days. So when it is time to use the bottle , I give the storage bottle a little squeze, slice the scum clump off with a finger into the garbage can, and proceed to pour the required amount of developer into a graduate.

The developer retains a slight perfumed smell, but it seems to function in the same way as the photo supplier sourced carbonate. I now keep the photo supplier sourced carbonate for negative developers.

Interstingly enough, I discovered the last time I was returning from a car travel holiday in the USA, that the Arm and Hammer washing soda sold in the USA (at least at the time I bought it a few years ago) does not have the perfume agent. So my wife bought the candy bars and junk food that doesn't show up on Canadian store shelves, and I bought 2 boxes of washing soda.
 
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Snapshot

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This is useful to know. It certainly saves a trip to Nymoc.
 

srs5694

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FWIW, I'm another who uses this product (the non-perfumed US version) as a sodium carbonate source. I bought a box two years ago and it's still going strong. I have seen cautions, though, that using odd sources such as this increases uncertainty, and therefore risk -- contaminants like the perfume agent you mention in the Canadian version could affect the quality of the developer. Personally, I've never had problems, but that doesn't mean somebody else won't.
 

Maine-iac

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Gainer's reply fits my experience: "Many times, a formula that was originally contrived using avoirdupois units is converted to metric or vice versa with much greater precision than is required, and in all probability with much greater precision than the original measurements were made. What began as "some" sodium carbonate becomes 28.35 grams when 25 or 30 would do as well simply because the amount was originally specified as 1 ounce, and was probably not weighed any closer than 1/20 ounce."

In practice, since anhydrous carbonate, once opened, will eventually become hydrated, there's no real difference that I've been able to observe in any developer formula I've used, and I've concocted many. I just buy Arm & Hammer Washing Soda in 2 lb. boxes at the supermarket and it works fine with any formula.

Larry
 
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