Problems with distilled and deionised water

My knowledge of liquid conductivity physics is hazy and my memories
of mhos have almost disappeared.

Many of us frequently have to work with water that has been
processed through a deioniser or some sort of purifying process.  On
occasions I check these waters to ensure that pH remains close to
neutral, as this factor can adversely affect many aspects of my
work.  I have occasionally noticed low pH readings for distilled
water (down to 3.2) and for DI water down to 3.5!  In the case of
the DI water this could just be due to the deionising cylinder
requiring replacement or contamination with micro-organisms perhaps
(cf. algae). DI water is supposed to have a resistivity of c. 18
megaohms and tap water has barely any reading on this scale.

Four questions.

    1.  Does this somehow affect pH by increasing hydrogen ion
        concentration?

    2.  Is there a sliding scale that relates megaohm resistivity to
        pH?

    3.  If dissolved ions cause the resistivity, how come the M-ohm
        readings are inversely high for DW and low for tap water?

    4.  Does the same apply to distilled water?

(Excuse my ignorance on this subject)

Simon Moore, MIScT, FLS, ACR,
Senior Conservator of Natural Sciences.
Hampshire County Council
Recreation and Heritage Department,
Museums and Archives Service,
Chilcomb House, Chilcomb Lane,
Winchester SO23 8RD, UK
+44 1962 826737


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                  Conservation DistList Instance 21:17
                  Distributed: Saturday, July 21, 2007
                       Message Id: cdl-21-17-017
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