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Guy Macon's adventures with ultrapure water


Guy Macon

Jan 1, 1970
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(Keeping in mind that I am an electronics engineer with quite
limited knowledge of chemistry and some experience in designing
low-cost low-performance resistivity meters...)
I am after water with an R>18[Mohm/cm]

Water over 18.2Mohm/cm@25C doesn't exist. At room temperature
it spontaneously forms H+ and OH- Ions (and H3O+ Ions?
My memory fails me on that one).

Also, at what temperature? Ultrapure changes resistivity
4.5% per degree C @25C.
Also what is the best way of storing ultra pure water

It doesn't exist. You need to purify it on the spot. Ultrapure
water will dissolve anything it possibly can, and you want to give
it as little time leaching chloride from plastics and dissolving
metals and glass as possible. Making pure water is difficult,
and keeping it that way is impossible
Should I dispense it in to several small bottles with pipette lids?

No. To stay even close to 18.2Mohm/cm@25C you must start with
vacuum degassed ultrapure water and then never let it contact

O2 dissolved in the water makes it better at attacking metals
(and lowering the resistivity) and dissolved CO2 will make
carbonic acid, which then attack the metal. You will get
lots of CO2 in the water even though the air doesn't have
much because CO2 dissolves so well, and ultrapure water has
little or no buffering capacity.

The pH of pure water is around 7 if I recall correctly, but
after being exposed to the air for a while the dissolved CO2
gives you a pH reading closer to 5. This will drop your
resistivity to somewhere in the 7 to 12 Mohm/cm@25C range.
Please note that measuring the pH of ultrapure water with
a pH sensor will not work right. Too little conductivity.

If this doesn't frighten you enough, do a Google search
on [ oligotrophic ultrapure ]. Better get some superbright
UV lamps!
Quartz Triple distilled water

You are aware that distillation *concentrates* some impurities,
aren't you? I don't know what the "Quartz" is all about, but
it sounds like a great way to get dissolved silicon oxide in
your ultrapure water.