Question:
As part of a physics project I needed some pure water.I managed to get some autoclaved milli-q (=3Dmillipore?) water.

Now, my questions are:How pure has the water become?Exactly what happens with the water during the processes?What more could I have done to make sure that the water was as pure aspossible?What pH-value should I expect from the purified water?

If you know the answer to one or more of these questions, I would verymuch appreciate your help!

Answer: How much pure water? What does pure mean?

If you want it particle-free and sterile, run it through membrane filter(no surfactant!). If you want it mostly pure, distill, leavingnon-volatiles behind (except for misting). If you want it REALLY pure,multiple distillation or ion exchange (commercial still or ion exchangeloop) then a membrane filter polish and monitoring by resistance (18megaohm is good stuff).

CO2 from the air will move the pH all over the map - vacuum degas orpurge with filtered nitrogen or argon. Ultrapure water cannot bemeasured by a pH meter (electrode ion diffusion).

Would a gallon of chromatography water do it? A bottle of Perrier? (noorganics there, but inorganics and CO2 contaminate).

You could zone refine a column of ice.

The question is, what contaminants are objectionable?

If you want water and only water (what about isotope fractionation?)think about multiple serial vacuum sublimations of ice in a unitaryglassblown fused quartz apparatus. Think "breakseal" and "flame off."

Note that ultrapure water is remarkably corrosive. It doesn't stayultrapure for long. Semiconductor fabs use dynamic purification systems- the stuff never stops being purified. And even they have microbialcontamination problems (add a few ppm HF andthey go away.)

There is no way of telling if you have pure water without establishing whatyou mean by pure water. A Milli-q system is a general deionizing systemwhich removes ionic material. If it did not have an added organiccartridge, you may still have organic contaminants depending on the feedsource. You test the ionic activity through its conductivity. You want atleast 18 M-ohm water. If you autoclaved the water to sterilize it, youprobably have to use it immediately. With water, you define what is pure,there is no universal definition. As far as the pH, it will vary with ageafter it is removed from the system, generally it will be acidic due totaking up CO2 from the air.

The opinions of Dr. Boyter are provided for informational purposes only andshould not be used as advice. No warranty or expression of professionalismis implied.