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Mark Lipton
 
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Ian Hoare wrote:

>>and that it is usually used to remove volatile acidity, or to decrease alcohol concentration to below 14 percent for
>>tax purposes. In America, that is --

>
>
> Really? This is an entirely different usage from that practiced here, and
> I'm not clear how this can be done. Sounds like a question for Mark!!!


Err... It is?? I'd imagine that Andy is thinking of another type of
concentrator, the name of which escapes me, that works by evaporation.
Since VA is by its nature volatile, you are essentially distilling it
out of solution, though by all rights you'd also lose alcohol.

>
> Trying to keep technicalities to a minimum, reverse osmosis is like
> operating a filter. You take a semi permeable membrane, which lets small
> molecules through but not larger ones, and force the liquid being treated
> through it. It's most common usage is for desalination, where it allows H2O
> through but not Na+ and Cl-. I would be utterly astonished if a "normal"
> semi permeable membrane could allow volatile acidity (acetic acid CH3C00H)
> and alcohol (ethanol, C2H5OH) through _without_ letting through the water in
> much larger quantities. I have given the chemical formulae, because in
> general the more complex the structure (and the higher the atomic weight of
> the elements composing the molecule) the larger it is, and the more
> difficult it will be to pass them through the pores of the filter. Now it's
> obvious that these two chemicals with two carbon atoms each will be bigger
> than water with only one oxygen atom. So forgive me if I am doubtful about
> whether you're right.


RO is basically dialysis run under pressure, so it uses exclusion based
on molecular size primarily, as you've so ably described.

Mark Lipton