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Re: Muriatic acid

George writes:

>  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.

In a planted aquarium, sure, maybe.  I don't claim to be a chemist.  But I
wasn't about to just dump something like muriatic acid into a tank foull of
living things.  My objective was to find a way to reduce alkalinity (aka
carbonate hardness) .  At Dave Gomberg's suggestion, I started with HCl.  I
got a prescription from a vet, so I could get a pharmacy to use their fancy
equipment and measure out a specific solution of HCl in distilled water.  I
think the original concentration was 2%.  I do remember the pH of the solution
was 1.7 or 1.8.  Figure out from there what the concentration was.  I measured
the of tsp. per gallon I used in a bare, clean, ten gallon tank with nothing
but tap water and a lid.  When I got the lid sealed tighter, the pH rise
slowed down.  Alkalinity dropped to 40ppm, and that was all I could get it to
drop.

That's what I did, that's my results.  Perhaps someone with a better knowledge
of chemistry could repeat the trials and make observations.  All I can say for
sure is that pH drops using HCl will not hold up long.  I suspected the same
would happen with Sulphuric acid, but pH stayed down after the initial battle
with the buffers in the water was won.

Bob Dixon