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Re: water filter

On Mon, 5 Oct 1998, Jay Dike wrote:

> > > I recently got a PUR water filter,  it is one of the half liter
> > >  pitchers.  I was cynical when I saw this at first.  However, after
> > >  putting water that comes out of the tap with a PH of 8-9 and have
> it
> > >  come out with a PH of 6.2 I have become a definate believer. 
> Does any
> > >  one know what is in this thing? All it says is that it contains a
> > >  "pleated microfiber"
> > 
> I remember testing water that been through a Brita filter (filter in a
> pitcher model).  My tap water has a pH > 7.6, ~1 dGH, and ~12-14 dKH
> (after going through an ion exchange water softener).  The pH went
> down to something like 6.2.  I don't remember the exact number but I
> remember it was a "large" change.

Curiosity struck.  My wife keeps a Brita pitcher in the 'fridge, so I
poured a glass and let it sit out for a few hours to warm up (the test
kits don't give valid results on cold water), then measured it's
parameters and the parameters of fresh tap water at about the same
temperature.  I found 5 degrees alkalinity from the Brita and 6 degrees
from the tap water.  The Brita water had a pH of 7.4 or so, and the pH of
the tap water was about 8. 

I was surprised, particularly by the change in pH.

The CO2 concentrations implied by these numbers (according to Pauli
Hopea's chart at the Krib) is 1.8 ppm in the tap water and 6 ppm in the
Brita water - a difference of 4.2 ppm. The 1 degree difference in the
alkalinity amounts to about 22 mg/l of bicarb, which contains as much 
carbon as about 16 mg/l of CO2. 

It would be fairly easy to explain these changes if the cation exchange
resin in the Brita filter was charged originally with hydrogen ions.  The
hydrogen would exchange for cations in solution (sodium, magnesium,
calcium or potassium).  Once it was in solution the hydrogen ion would
react with the bicarb, converting some (say, 22 mg/l or so) to CO2+water. 
Excess CO2 would dissipate rather slowly from cold water sitting in a
still pitcher in a refrigerator.  It sat about and warmed up a little
before I tested it, and by then the CO2 dropped down to 6.  The 
freshly-filtered water might have had a pH of 6.8 or so. 

The pH difference isn't very significant because it changes with the CO2
content; it's really only the 1 degree change in alkalinity that makes any
difference to the use of this water in an aquarium, and that 1 degree 
doesn't (to me) justify the expense of using the filter for water treatment.
Maybe with a newer filter the change would be bigger.

Roger Miller