Hi guys let me add my two cents as a chemist.
In the old days double distillation was done to ensure a better job in
removing the last traces of solutes carried out into the condensate by means
of steam carry-over and also to dissipate the dissolved gases.
However, if one follows good laboratory practices this second step is
totally unnecessary.
Firstly, once the evaporation begins, one should reduce the amount of heat
applied just to keep the boiling going, otherwise one increases the risk of
steam carry over.
Secondly, one needs to add evaporation beads to avoid tumultuous boiling ,
another possible source of contamination.
Thirdly, one needs to start with about one third to one half more water than
what is required to avoid evaporating the tailings (where the contaminants
concentrate as the evaporation progresses)
It is important to check the thermometer. If the raw water is contaminated
the contaminants may be steam distilled into the condensate; the temperature
reading will confirm that.
One begins to collect distilled water only when the thermometer reflects 100
C. and stops collecting when there is 1/3 left of the original volume.
Finally, if the system is cooled with tap water make sure you turn the cold
water before the evaporation begins to avoid steam from reaching the
condensate and make sure there are no leaks of the cooling water into the
condensate.
That was forty years ago I hope I did not forget anything.
Cheers
Frank
--------------------------------------------------
From: "M. G. Devour" <[email protected]>
Sent: Monday, August 16, 2010 7:27 PM
To: <[email protected]>
Subject: Re: CS>twice distilled water
Dear PT,
What I am interested in is how water that has been distilled can still
be producing contaminants? It was my understanding that once it was
distilled it is contaminant free.
People used to use double distilled water when it had to be absolutely
as pure as possible. Nowadays, there are (big and expensive) systems of
cartridges used in lab and industrial settings that produce extremely
pure deionized water that is appropriate for those applications.
For us, double distilled is the most approachable way to get to that
level of purity... though you should ask yourself if you *really* need
that purity or if it's overkill.
Why wouldn't the first distillation cycle get everything?
Well, to start with, we've already talked about tossing off the first
bits of water from the distiller to get rid of any volatiles that came
off with the water and remain dissolved in the distillate.
Then we learned to leave behind the last of the water, rather than
boiling it off completely. This means that the resevoir doesn't need to
boil dry and get hotter in order for the temperature switch to
automatically shut down the heater. That makes it easier to clean out,
I'm sure, but *also* means that some of the higher boiling point
contaminants don't get a chance to evaporate and join with the last of
the condensate to contaminate the batch.
Both of these methods allow you to capture the purest portion of the
output stream.
Then there's not filling it all the way, which avoids the obvious
problem of boiling water *splashing* into the condenser.
The thing is that all of these processes are continuous... All the
volatiles don't come off at the same temperature or infinitely quickly,
and all of the possible contaminants have a perhaps small, but finite
and measurable evaporation rate at whatever temperature you're
operating. So, the whole while the water is being evaporated, at least
some traces of these other things are also making it across to the
output.
So a second cycle of distillation, just as carefully done as the first
(and *without* the output filter! <grin>) will simply take those
contaminant levels down by *another* factor of however many orders of
magnitude they came down with the first cycle, or at least close.
I hope that makes sense?
Peace,
Mike D.
[Mike Devour, Citizen, Patriot, Libertarian]
[[email protected] ]
[Speaking only for myself... ]
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