Monohydrous vs Anhydrous

Could somebody clarify what is a monohydrous raw chemical vs an anhydrous one.

Also, is there a way to use a conversion scale when weighting them to use one for another?

The reason I am asking is that I am wanting to substitute some Sodium carbonate anhydrous in place of sodium carbonate monohydrous.
I cannot find the later.

Thank you for explaning and clarying.

Best

Dee

Chemical compounds take up varying amounts of water when they form crystals. Anhydrous means without water - there is no water associated with the crystal form. Monohydrate means there is one molecule of water associated with each molecule of the compound in the crystal form. Pentahydrate (as in sodium thiosulfate pentahydrate) means there are five molecules of water associated with each molecule of the compound in the crystal form. The water adds weight to the crystal form and affects the amount you need to use in compounding formulas. For instance, you need 17 percent more sodium carbonate monohydrate than anhydrous sodium carbonate to get the same results. But some crystal forms of a compound are more stable than others. Anhydrous sodium carbonate may pick up water from the air, so the monohydrate form is usually preferred to give consistent results. Similar factors affect other chemicals. You can calculate the amount of a compound needed to substitute for another form by calculating the molecular weight of both crystal forms, including the water molecules, and using the proportional amount.

nworth said:

But some crystal forms of a compound are more stable than others. Anhydrous sodium carbonate may pick up water from the air, so the monohydrate form is usually preferred to give consistent results. Similar factors affect other chemicals. .

Click to expand...

..perhaps it's good to mention that for Sodiumsulphite it is the other way around: if you have the hydrated form it will slowly turn into SodiumSULPHATE, and although in photographic terms the sulphate itself is not a problem, less or no sulphite definitely is..that is: that is how I understood matters, I am sure I will be corrected if this is not true..

Best,

Cor

The difference between the Hydrate and the anhydrous version is purely the amount of molecules of water bound up with the active material you adding.

In practice it means that you will add less anhydrous material than the hydrate to get the same working strength of solution

Fwiw: If you see the term "dessicated" as a description, that also means "anhyrdous." i.e. "dry"

Many formula have been converted in publications like the Photo Lab Index, DCB and other US publications from the original Anhydrous weights to the Monohydrate form.

This is due to availability of the different forms US/Europe. In Europe the Anhydrous form is manufactured & available, the monohydrous is rarely found.

Ian

Thank you all for your comments.

As it happens, yesterday evening p178 of Tim Rudman's toning book jumped out to my face... and I muttered: "Dooh".

I have now a better understanding of these words.

I wondered why both forms, anhydrous and monohydrate were mixed up in cook books, and why wet printers did not make their life easier by sticking up to one form only.

Best

Dee

Can any one says, If I use mono hydrous instant of anhydrous as a ingredients are used by manufacturers of ORS. Can harmful to people.