jsc
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Registered: 16-3-2011
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How do I measure hydrates?
Hi, I am struggling with dealing with hydrates. A lot of compounds have multiple hydrated forms. For example, sodium carbonate N2CO3 has a
decahydrate, heptahydrate, monohydrate, etc., all of which have different molecular weights.
Questions:
(1) what is the best way of dealing with this? guess at the most likely form and use those numbers? heat the material and drive off the water?
(2) is it likely that a given hydrate will be all of one type, or could you have monohydrate mixed with decahydrates and stuff?
(3) how easy is it to turn crystalline hydrates into their annhydrous form? do you just heat the stuff up to 100C and, bam, you're done, or is it more
complicated than that?
(4) do hydrates just keep adding water from the atmosphere naturally or does it require special conditions? For example, lets say I have N2CO3
monohydrate and leave it sitting out. Will it just automagically start turning into decahydrate?
(5) MOST IMPORTANT QUESTION: What is the best source for getting comprehensive information about the hydrates of common compounds and how they formed,
etc?
Thanks for any assistance.
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bbartlog
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Registered: 27-8-2009
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In re question 3): Different hydrates dehydrate at different temperatures, and some of them can't really be thermally dehydrated as they decompose
before all the water is driven off.
In re question 1): for those that can be dehydrated at reasonable temperatures (< 250C in my case, your situation may vary...) I prefer to
dehydrate them. For example, Na2CO3 is easily dehydrated pretty completely. For others, I don't know; I generally try to estimate the degree of
hydration for things like CaCl2. If it's important to get a particular hydrate, or know for sure what it is you have, you can crystallize from aqueous
solution at a controlled temperature; in that case the crystals should all be of a particular hydrate form.
In re question 2): certainly they can be mixed. Leave a bag of some hygroscopic compound exposed to air and of course the stuff on top becomes more
hydrated.
And indeed, most all things that have hydrates are hygroscopic to some degree or other. You can generally assume that the hotter you have to heat it
to drive off the water, the more it will want to absorb water from the air.
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