Problems with distilled and deionised water

Simon Moore <simon.moore [at] hants__gov__uk> writes

>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! ...

To respond to Simon Moore's questions about distilled and deionized
water, I consulted a friend who is an engineer and who wrote quite a
bit about the measurement of pH as part of his dissertation.  Here
are his responses:

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

No. See below.

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

Not that I know of.

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

Dissolved ions cause conductivity.

>    4.  Does the same apply to distilled water?

No.

The basic problem you've come across is that pH actually makes a
poor measure of the purity of water.  This is how the resistivity of
the water came to be adopted as the measure of the quality of output
of deionizing systems.  For pH measured with a meter, the basic
problem is that the ionic strength of pure water is too low for the
electrodes to have the proper response in a timely manner.  If
you're measuring pH this way, you may have noticed that the signal
drifts for some time before settling down.  The number observed is
probably more an artifact of the low ionic strength than a
reflection of the properties of the water.  Also, distilled water
tends to have a pH of around 5.5, as takes up carbon dioxide from
the air during condensation, and thus has some carbonic acid
present.

If you're measuring with indicators, or indicator-impregnated paper,
you're operating outside the bounds of my knowledge and I'll leave
it for someone else to answer.

Standards agencies such as ASTM publish specifications for various
grades of water for laboratory use:

    <URL:http://www.astm.org/
        cgi-bin/SoftCart.exe/
        DATABASE.CART/REDLINE_PAGES/D1193.htm?E+mystore>

    **** Moderator's comments: The above URL has been wrapped for
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They're also an excellent jumping-off point for learning more.

Robert H. Woodman, Ph.D., via
Emily K. Bell
Senior Library Associate/Conservation
Wellesley College
Wellesley, MA USA
781-283-3595


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                  Conservation DistList Instance 21:18
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