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> Could someone help me in understanding the properties of distilled
> water and deionized water? Someone I spoke with said that deionized
> water was corrosive. It would seem to me that it would be less
> corrosive since it doesn't contain the ions or minerals that other
> water has. Enlighten me.
G'day; Distilled water might contain small amounts of other volatile
substances, but very little minerals. For instance if one distils
say, beer, without the elaboration of special fractionating columns,
the distillate will contain water of course, but also ethyl alcohol,
ethyl acetate, butyl alcohol, and so on - a whole swag of small
amounts of other volatile substances. Thus distilled tap water will
contain a small amount of volatile contaminants, but will be
reasonably pure for most purposes.
Water is de-ionised by passing it through glass columns containing
deionising resins, and the output will contain no metal salts, acids
or alkalis or other substances which ionize when dissolved in water.
But it may - and probably will - contain substances which do not
ionize.
If extra pure water is required, then distilled water is used to
dissolve a small amount of potassium permanganate, a little alkali is
added, and the liquid is boiled and re-distilled using a good
fractionating (separating) column. The permanganate oxidizes any
organic material in the water, and the distillate is very pure.
For ultra pure water, the water obtained by re-distillation over
alkaline permanganate is passed through deionising columns. This will
also remove carbon dioxide. BUT! as soon as the water is allowed
to contact the atmosphere it will start to dissolve carbon dioxide,
and this will ionize at once. Water thus containing carbonic acid is
corrosive; this is how caves are formed in limestone country.
I used to work with pure waters at one time, and the ionization is
measured by passing a small electric current through it via platinum
electrodes to measure resistance. Distilled water had a resistance
of 58 thousand ohms per square centimetre of electrode at a distance
of one centimetre apart. Deionised water had a resistance of 4
million ohms.
For most purposes, either distilled or deionised water is
satisfactory.
If pure water is wanted, then don't get it from a garage,( who sells
it to top up batteries.) Many tend to fill the bottle with either tap
or rain water. I have proved this! Get either distilled water or
deionised water from the local pharmacy. If you need a lot, then get
a deionising unit from a laboratory supplier, or from a specialist in
purifying water. Most photographic processing labs use deionised
water, and have a small unit on the premises. -Yes, I know it's a bit
long - but you did ask to be enlightened! Cheers -- John Burgess;
johnb AT ts.co.nz of Mapua Nelson NZ
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