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Re: Aquatic Plants Digest V3 #646

Hello Bob,

Your experience with hydrochloric acid surprises me, mainly because
hydrochloric acid is a "classical" strong acid and will completely
neutralize aqueous bases, including carbonates.

Although in its pure state hydrochloric acid is a gas, to my knowledge
it does not "gas off" from aqueous solution as, say, carbon dioxide
does. As a matter of fact, one can boil an aqueous HCl solution -- it
forms a constant boiling azeotropic solution of exact composition of
20.22% HCl and water -- without the fractionation required for "gassing
off". Evaporating a less concentrated solution results in *increase* of
the HCl concentration. Thus "gassing off" is not very likely with HCl.

At the concentrations we are talking about, 0.000 001 Normal H+ at pH 6
(that's 0.000 037 g/L HCl, if *only* HCl contributes to the acidity) any
reactions with the substrate will soon exhaust the available "excess"
acid. For the water to turn basic again, there must be a source of
alkalinity -- finding that source, the cause of pH rise, would likely
answer the question why the pH in your tank goes up with time. One just
can not raise the pH without supplying hydroxide ions from some source
(or removing the hydrogen ions).

While not questioning your experimental observations, I have a problem
with the proposed mechanism. In my *opinion* a more likely scenario for
reduction of acidity in a planted aquarium would be reaction with the
substrate (or with biota) of any HCl that remained *after* neutralizing
the various bases in solution. Such a neutralization reaction -- in
solution -- is practically instantaneous; liquid-solid reactions take a
while.

Best,

George


> George writes:
> 
> > I don't know about Australia, but in the US hydrochloric ("muriatic")
> >  acid is available in almost every hardware store for cleaning up
> >  concrete. It works about as well as sulfuric acid for purposes of
> >  acidification, because one uses so little of it.
> 
> I recently did extensive testing with muriatic acid.  It will reduce buffering
> agents such as carbonates to a degree, but for permanent pH changes, it is
> ineffective.  It manages to gas off very quickly as hydrogen chloride.
> 
> Bob Dixon.
> 
> ------------------------------