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Re: [APD] RO/pH question

On Sat, 15 May 2004 13:30:17 -0500 (EST), Russell Edward Vance <revance at indiana_edu> wrote:

I recently purchased a RO/DI filter and just hooked it up. It is a 6 stage
filter with 3 prefilters a 75gpd RO membrane, DI filter and a final carbon
post-filter. It has not run long enough to get all the sediment out of the
post carbon filter and DI unit, so the product water still has a TDS of
10ppm. This must be partly due to the sediment in those last two
filters because the water coming out of the RO membrane is lower
than 10ppm. Somehow the water still has a pH of 9! I don't understand how
almost pure water can still have such a high pH. The KH is 0 (or at least
my test indicates 0... I am sure it isnt exactly 0) and the general
hardness is also very close to 0. Does anybody know why this is and how I
can correct it? Thanks.


A quote off this (http://www.lenntech.com/pH-and-alkalinity.htm) page:
"Did you know that the pH of Coca-Cola is about 2? And did you know that it is useless to measure the pH of RO-water or demiwater? Both demiwater and RO-water do not contain any buffer ions. This means that the pH can be as low as four, but it can also be as high as 12. Both kinds of water are not readily usable in their natural form. They are always mixed before application!"

Basically RO water has effectively no buffering capacity - it is 'dead' water with almost nothing in it, which makes it very reactive and susceptible to huge pH swings from a tiny change in acidity or alkalinity. To prepare RO water for use, simply add an appropriate buffer such as Seachem Equilibrium to your requirements.

Andrew McLeod
thefish at theabyssalplain_freeserve.co.uk

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