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Re: phosphate questions

> From:  (Dirk)
> Date: Fri, 16 Feb 1996 01:35:26 -0600
> Subject: Re: Aquatic Plants Digest V1 #257
> 
> I'm really sorry, folks, but would you mind translating the following?
> 
> >        Orthophosphate is _not_ a long chain phosphate, it is the
> usual phosphate, PO4---, HPO4--, H2PO4- or H3PO4, depending on the pH.
> At pH of interest to us, it will be almost entirely in the middle two
> forms.  Hexametaphosphate _is_ polymeric.
>
	I wrote this after reading in a previous post that orthophosphate
was a chain anion (i.e. lots of phosphates linked together).  It isn't,
it is just what is normally called "phosphate", and you do not want it
in solution in your tank!  Phosphoric acid (orthophosphoric acid) is
H3PO4, but the H's come off as protons (H+) as the pH goes up (i.e., as
you neutralise the acid).  At the sort of pH of interest to us, it will
be almost entirely HPO4-- and H2PO4-, i.e some of it will have 1 H left
and just about all the rest 2 H's left.  The mixture of these two ions can
be used as an effective buffer for pH in the range near 7, but of course
it is of little use unless you are planning to keep the tank in the dark!
 
> >Phosphate doesnt hold ferric iron in solution. It forms complex
> oxyhydroxide colloids which eventually precipitate into the substrate.
>
	I didn't write this, but it means that iron in the 3 oxidation
state won't be kept in solution by any phosphate there, it will tend
to precipitate out as small particles of what amounts to rust.

	This will depend quite strongly on pH - the more alkaline things
are, the more quickly iron in the 2 oxidation state will be oxidised to
the 3 state, and the more quickly it will then precipitate out.
(That's my two cent's worth.)

Paul Sears     Ottawa, Canada