As someone pointed out, the purity of water is often measured by its
conductance (or resistance). Someone else wondered what "the numbers"
are. Here's the rule-of-thumb I typically use:
First, we're going to talk about the resistance or conductance of water
between electrodes with an area of 1 sq cm and separated by one
centimeter. Typically these would be platinum electrodes and typically
the measurement would be made with AC at a frequency of about 1000
Hz... but for our purposes here, this is too much detail. The
important part is the 1 sq cm electrodes at 1 cm spacing.
Under these conditions, a resistance of 2 megohms is pretty good water
that can be used for most chemical analyses. When the deionizer in our
chemistry lab is providing 2 megohm water, we're happy. Once it gets
lower than 2 megohms we think about changing the resin... if it drops
below 1 megohm we definitely change the resin. For extremely sensitive
analytical chemistry (sensitive to extraneous ions) we might prefer
water that is 5-10 megohms, but that is more difficult to get. A
generally-accepted limit for "intrinsic water" is about 18 megohms.
People in the semiconductor industry try to get water to the 18 megohm
level if it will be used in semiconductor processing.
Thus, if we want to talk about whether water is a good conductor or
not, then we should use the intrinsic limit of about 18 megohm
resistance (or 56 nanomhos conductance) across a one-centimeter block
of water. That's definitely not a good conductor.
If we want to talk about your everyday "distilled" or "deionized" water
we might be talking about a resistance of 1 or 2 megohms. That also is
not a good conductor.
If we're talking about tap water, or river water, or the water in a mud
puddle... who knows. It could be quite a good conductor. But in all
the cases less than 18 megohms, it's the impurities acting as the
charge carriers, not the water itself.
By the way, simply finding that the resistance of your water is near 18
megohms does not mean it is pure. It could have organic chemicals or
bacteria that could goof up your chemistry (your experiments, or your
body, or both). Measuring water purity by its resistance/conductance
only measures to what extent you have removed the ionizable impurities.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817