In the past few months I've recrystallized a lot of calcium acetate, whether it's from eggshells or seashells, or rocks (my new preferred source
because they don't smell). And what I noticed is that it evaporates to form 2 very distinct crystals, which I've designated as the alpha and beta
variants.
Calcium Acetate Alpha (CaAc α)
- puffy, almost like popcorn.
- fluoresces under ultraviolet light.
- tends to absorb impurities in the brine, causing discoloration.
Calcium Acetate Beta (CaAc β)
- forms spiny, needle-like crystals, sometimes growing freely or else entrapped in larger masses.
- exhibits no fluorescence
- tends to eject impurities in the brine, resulting in very pure white crystals.
[1][2]
Calcium acetate alpha often possesses geode-like cavities of the beta morph. I'm not really sure how it generates these hollow shells, maybe through
some sort of weird capillary pumping mechanism like the way that saltwater climbs the edge of a glass as it evaporates.
[1][2][3]
In these images you can see radial patterns in the beta morph. In the larger of the two "craters" there's a big cluster of the alpha morph growing
right in the center.
The criss-crossing interwoven spikes of the beta morph remind me a lot of rapidly-crystallized solutions of supersaturated sodium acetate.
[1]
A high-purity sample so you can see how both polymorphs are normally white.
When viewed under ultraviolet light, the alpha morph fluoresces while the beta morph does not. I have no idea how this happens and it was the main
reason I was absolutely convinced they must be two different chemicals.
This conviction was broken when I tried to isolate the two materials, but found that no matter how many times I separated and recrystallized them I
always ended up with dishes containing both crystals.
I also tested if it could've been magnesium acetate, which I made from epsom salt, but I found that magnesium acetate doesn't crystallize, it just
turns to goop in a similar fashion to nickel acetate.
--
The conditions under which the crystal structures diverge is unclear but has something to do with the water level of the evaporation dish. Other
determining factors such as temperature and impurities are unknown. Further testing required.
clearly_not_atara - 3-3-2022 at 09:17
Is it possible that one of them is Ca(OAc)2*xH2O and another is Ca(OAc)2*yH2O*zHOAc?SnailsAttack - 4-3-2022 at 04:54
Is it possible that one of them is Ca(OAc)2*xH2O and another is Ca(OAc)2*yH2O*zHOAc?
you mean maybe the calcium acetate is complexing with some leftover vinegar in solution? I think that could definitely be possible. if the beta morph
is absorbing vinegar that might explain why it tends to forms last, since vinegar has a somewhat higher boiling point than water.macckone - 4-3-2022 at 12:08
Sodium acetate forms a diacetate, I would assume calcium would do the same if there is excess acetic acid.AJKOER - 9-3-2022 at 10:50
"Calcium Acetate, Ca(C2H3O2)2, crystallises from hot solutions in thin needles with 1 molecule, from cold solutions with 2 molecules of water of
crystallisation. The transition point is at 84° C. At 20° C. 100 grm. of water dissolve 34.73 grm. of anhydrous calcium acetate, and at 100° C.
29.65 grm."
And also apparently, at times indeed an acid salt presence:
"With acetic acid and calcium nitrate a crystallised double compound is formed, and an acid salt, Ca(C2H3O2)2.C2H3O2, is obtained by treating solid
calcium hydroxide with concentrated acetic acid."
Note: my observation is that Atomistry is a collection of dated (no copyright active) chemical journal accounts.SWIM - 9-3-2022 at 11:25
YOU AGREE THAT THE INFORMATION IS PRESENT ON AN "AS IS" BASIS AND THAT atomistry.com IN NO WAY WARRANTS THE INFORMATION OR ANY OF ITS RESULTS AND IS
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So it seems to actually be mostly from a series of lectures by one Sergey Ruzheinikov, born November 1973, a research associate from Sheffield U. At
least he was until 2012 or so. Can't find later references to him on their site.
The atomistry site says he holds the copyright on these lectures.
