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Re: pH of RO water

Hello Joseph,

You are, obviously, right that the dissociation constant of pure water
is 10^-14. But do you think that a *hobbyist* can determine such a
dissociation constant in the fish-room? I don't think so!

As I mentioned in my original post, The ASTM volume on water (I think
that it is volume 18 A & B, but am not certain) in the section on
measuring pH, gives that warning about measuring pH of pure water. Right
now I don't have access to that work, so can not cite the exact page
reference. The classic reference is Clark's "Determination of Hydrogen
Ions", from the 1920's.

If we take a medium-grade distilled water, resistivity of, say, 1 megohm
or better, then in a colorimetric determination one drop of an indicator
solution will be the dominant source of hydrogen ions. You will get a
measurement, but it will be that of the resulting diluted indicator
solution, not of the original water.

An electronic pH meter is just a high impedance Volt meter. If the
resistance of the medium is too high, and one is using a *glass*
electrode, (not quinhydrone/sat. calomel half-cells) -- the reading will
drift -- because the instrument can not reliably measure the 59
mV/decade that it is required to do, to measure pH. This is easily
provable, if one has access to *fresh* (without dissolved CO2) 10 megohm
(or better) water then the drift is very obvious. (The quinhydrone/sat.
calomel half-cells are not something that a hobbyist would want to try

I know of one way to circumvent this problem but, again, that is not
something a hobbyist should try in his/her fishroom (because of danger
of contamination and subsequent false measurements): Per 25 mL of sample
add 1 drop of a 10 mM KCl solution of 99.9999% (6N) purity, prepared in
an acid-washed Teflon volumetric flask, using 10 megohm (or better)
water. It will lower the resistivity sufficiently, that a "research
grade" (very high impedance) pH meter, equipped with a quality glass
electrode, will be able to measure it with some assurance of accuracy
and precision.

Hope this explains it somewhat.



> Date: Mon, 4 Jan 1999 08:43:25 -0500 (EST)
> From: joseph sachleben <jsachleb at chemistry_ohio-state.edu>
> Subject: Re: pH of RO water
> George Slusarczuk wrote:
> You are absolutely right! Even among chemists it
> is a very little known fact, that the pH of distilled (and RO) water can
> not be measured. Let me correct that: It CAN be
> measured, but the value is meaningless! (That  does NOT mean that
> distilled water has "no pH" -- just that we can not mesure it using
> methods available to a hobbyist -- and that includes
> "research grade" electronic pH meters!)
> *****************************************************
> Hi George,
> I think this argument is wrong. The pH of "pure water", Distilled
> water not allowed to come into contact with anything that might
> leach into the water, is 7. This is because the disassociation constant
> of water at room temperature is 10^-14. This can be measured. The
> dissociation constant of water can be measured. I'm curious as to
> what the technical problems are that you mention.
> Joe Sachleben
> jsachleb at chemistry_ohio-state.edu