Re: CP digest 445

Liane Cochran-Stafira (
Thu, 24 Aug 1995 12:25:38 -0500

>On Wed, 23 Aug 1995, Ellen York wrote:

>How does pH change with RO?

I haven't done extensive testing of pH in RO water, but the three or four
systems I have worked with give water that tends to be somewhat acid -
around 6.0. The tap water going into the system is slightly alkaline -
between 7 and 8, usually closer to 8. These were all very large systems
designed for laboratory water supplies. We have used RO water to grow some
Sarracenias and Utrics, and I've used it extensively for all sorts of
culture work on algae, protozoa, bacteria, and other aquatic critters with
few problems. Over time I have sometimes noticed a slight mineral buildup
if water is allowed to evaporate and the same container is used over and
over without cleaning. I know that the tissue culture and molecular folks
prefer to run the RO water through a final ion exchange or milli-Q system
before using it because of trace amounts of some minerals/metals.

It seems like the only times I would notice any real problems was towards
the end of the membrane's "lifetime" or if it was due for maintenance. The
usual indicators, for me at least, were problems associated with
preparation of various buffer solutions. I'd start getting precipitation
which wouldn't happen if I used the tissue culture quality water or RO
water from a clean system.

As Wayne Forrester mentioned, it is important to make sure the pipes are
safe, i.e., they don't contain any metals that might leach out. I believe
all the pipes for our systems are plastic.

Hope this helps,

Liane Cochran-Stafira
Dept. of Ecology and Evolution
The University of Chicago
1101 East 57th Street
Chicago, Illinois 60637-5415
phone: 312-702-1930