Just to pick up on this thread, with regard to continuous pH monitoring.
Take a look at any commercial swimming pool. We (they) use continuous pH -
Redox - Chlorine monitoring, all the time!

The only problem I find in my 'pool' is that because I use a special brew of
Chlorine and Bromine in the water the Glass pH probes only have a life of
some 10 months. But then 100 UK pounds to replace a probe against 400,000
swimmers per year isn't a bad ratio.

Philip Martin.


----- Original Message -----
From: Robert A. LaBudde <ral@LCFLTD.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, May 02, 2000 7:21 PM
Subject: Re: Looking for PH sensor


> At 12:55 PM 5/2/00 +1000, Gennette wrote:
> >Continuous pH is impossible.
>
> This is not quite true: it's done all the time in process control in
industry.
>
> >If the special composition glass bulb is left in the unknown pH liquid
for
> >an length of time it destroys the special oxides in the glass that pass
the
> >voltage differences.  To preserve the glass bulb you need to have the
same
> >pH liquid on both sides of it (usually KOH).
>
> You store pH electrode in pH 4 buffer for long periods. The problems arise
> with keeping the electrode in alkaline solutions (pH > 7). The salt used
in
> the interior is KCl, not KOH. Alkali dissolves glass slowly.
>
> However, nowadays there are solid state ISFET pH electrodes that don't
have
> the membrane or shelf life probes. Of course, that means they are also
> expensive!
>
> ================================================================
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
> Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
> 824 Timberlake Drive                            Tel: 757-467-0954
> Virginia Beach, VA 23464-3239                   Fax: 757-467-2947
>
> "Vere scire est per causas scire"
> ================================================================
>