Re: Alkalinity

To: Aquatic-Plants at actwin_com
Subject: Re: Alkalinity
From: psears at emr_ca (Paul Sears)
Date: Tue, 3 Oct 1995 19:45:35 -0400 (EDT)
In-Reply-To: <199510031617.MAA03468 at looney_actwin.com> from "Aquatic-Plants-Owner at actwin_com" at Oct 3, 95 12:17:03 pm

> From: ac554 at freenet_carleton.ca (David Whittaker)
> 
> Paul Sears said....
> 
> >"alkalinity" is a term that is thrown around a lot without a
> >clear definition being presented.  As far as I can make out, it 
> >represents the amount of anions of weak acids present, i.e. those
> >that can be protonated in the pH range that interests us, say 5 to 9.
> 
> Isn't alkalinity just acid buffering capacity? It's measured by
> titrating to an endpoint. I believe this endpoint is pH 4.3 in the
> case of the Tetra kit and is equivalent to methyl orange alkalinity.
> So their kit measures carbonate and bicarbonate hardness, and
> phosphates as well. The phosphates buffer in the pH range encountered
> in our hobby. KH is a slight misnomer.
> 
> Dave 
> 
	Yes, the same thing - the anions of weak acids provide
the buffering capacity.  As you point out, the straight titration
method measures all of them, not just HCO3-.  I try to avoid
phosphates!  I'm not sure what you mean by "methyl orange alkalinity".
Its endpoint is pH 3.2 to 4.4 according to my reference book, which
agrees with your number for the Tetra kit.  The titration only
gives KH if there are no other weak acid anions around.


Paul Sears,     Ottawa, Canada.