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Re: pH of RO water (was APD V3 #749)

Hello Michael,

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!) 

I know of only one commonly available reference (the ASTM volume on
water) where this is spelled out in plain English (if only in a
footnote). I will not go into the technical reasons for it.

The pH of RO water probably will not be the same as that of starting
water, because the exclusion of ions by the RO membrane is not uniform
accross the periodic table and the residual ionic equilibrium will be
different from that in the source water. But, as you mentioned, the RO
water is not buffered, so it really does not matter: anything added to
it -- fish, plants, water, etc -- will change its pH.



> I think someone on one of the saltwater newsgroups explained that
> measuring the pH of RO water is almost impossible because there is so
> little in it.  Because it has basically no buffering the pH is very
> changeable and letting it equilbrate with the air will probably affect it.
> True however that the pure RO out of the filter would have the same pH as
> the tap going in.  BUt you can change that pH very easily due to no
> buffering.  Then add the buffering and you have the pH you want with the
> hardness, but no bad guys.
> This is a simple way of putting it but basically correct.  If someone
> would like to add please feel free as I am no expert.