Re: P and M alkalinities
---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Subject: Re: P and M alkalinities
To: Aquatic-Plants at ActWin_com
Date: Fri, 17 Jan 1997 09:55:17 -0500 (EST)
> From: ac554 at freenet_carleton.ca (David Whittaker)
> Subject: Re: P and M alkalinities
> Alkalinity measures bicarbonate, carbonate and hydrate ions primarily.
> Phenolphthalein alkalinity is assumed to equal all of the hydrate and
> one half of the carbonate alkalinity. Methyl orange alkalinity is equi-
> valent to total alkalinity. The endpoints of titration for the P and M
> alkalinities approximate pH 8.3 and pH 4.3 respectively. In natural
> waters phenolphthalein alkalinity is usually absent which is why the
> Hach kit registers zero.
At the phenolphthalein endpoint, the OH- concentration is
10^-5.7 molar (pretty low) so any significant quantity of OH- in the
solution to begin with has been neutralized. About 99% of the CO3--
ions have been converted to HCO3-, so the titration shows all of the
OH- alkalinity, and half of the CO3-- alkalinity, because a full
measure of the CO3-- alkalinity requires neutralization of the HCO3-
produced in that first step.
That is the origin of the "half" in Dave's posting, which
I forgot in mine on the same topic. Thanks, Dave.
Paul Sears Ottawa, Canada
Finger ap626 at freenet_carleton.ca for PGP public key.