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!  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?

There is no direct relationship between pH and resistivity.
Resistivity is the inverse of conductivity. The conductivity of
solutions depends on the concentration of ALL ions in solution. This
by itself does not uniquely specify the conductivity of the
solution, because it does not say anything about the presence of
other ions that may affect the electrical conductivity.

As a rule of thumb, though, resistivity is inversely proportional to
ion concentrations. Thus high ion concentrations lead to low
resistivity (tap water), and small ion concentrations lead to high
resistivity (distilled water).

Storage of deionized or distilled water in plastic containers is
also influencing ion-content/pH. Measuring the pH prior each use is
recommended.

Angela Meincke
Associate Conservator
Museum of Fine Arts, Boston


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                  Conservation DistList Instance 21:18
                 Distributed: Wednesday, August 1, 2007
                       Message Id: cdl-21-18-001
                                  ***