[3dem] Precipitation observed after making freeze substitution cocktail containing 5% water

[Reinhard Rachel]

>>> 19.12.2019 at 20:41:
> I would agree with the 7-8% stock concentration for UA. Even at that 
> concentration, it would take a while to dissolve in methanol. Additionally, I 
> would filter it with a 0.22 micron filter, make aliquots and then freeze in 
> LN2. Dont know if LN2 was overkill but i have never seen cloudy precipitation 
> in my FS mixes and i was using 3% OS and 0.5% UA along with some TA and 
> glutaraldehyde. 

with some delay, here is a brief comment from my side.
some people do not mention the solvent, in their comments (or in the M&M in the papers). 
in theory, you can push the stock solution in pure water to 7-8% , but ...
this depends whether you have CO2 free water (UO2 carbonate might precipitate, first!). The CO2 / carbonate anion is one of our enimies, in these solutions. Just keep this in mind. 
Thus, you may boil Water (= get rid of CO2) before solving the UAc. Or use freshly distilled (!!) water. Yes, distilled, not the Millipore stuff: no, never. -- And if you like, you may add minute amounts = a mini drop of glacial acetic acid (which tries to keep the pH such that the CO2 is favored against the bicarbonate anion and the carbonate!)

and solubility depends on the temperature (dissociation constants depend on the temperature! physical chemistry). 
Thus, I do not recommend to put such stock solutions into the cold room, below 10 C, or fridge. 
Nor does it make sense to freeze them ... why? make small amounts and use them up. Freezing:  you enforce the stuff to precipitate ... and, for the solution at room temp.: If you know of a bug that starts growing in a stock solution of UAc 5% or higher, please let me know; I would be interested (I am also microbiologist). 

we and others have tested: UAc dissolves nicely in MeOH, up to 20%. This does not last that long; I would make small amounts, only, which are needed within a week. - but this is easy to do. 

Have fun, and to all of you: 
merry Christmas - a happy New Year, and all the best! 
Reinhard


-- 
Prof. Dr. Reinhard Rachel
University of Regensburg
Centre for EM / Anatomy
Faculty of Biology & Preclin. Med.
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D-93053 Regensburg - Germany
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