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Re: Hydrochloric acid, alkalinity

> From: IDMiamiBob at aol_com
> Subject: Re: Aquatic Plants Digest V3 #647
> 
> While the HCl was interacting with the CaCO3 this was happening.  But
> alkalinity refused to go lower than 40ppm even after several doses.

	With the addition of enough HCl, the alkalinity will be driven
to zero.  40 ppm CaCO3 equivalent will require about 31 ppm of HCl to
do that.  If the HCl solution contains thirty-odd per cent HCl, then
0.1 mL of it per litre of water will do the job.  I would check the
alkalinity measuring system.

> No snail shells, no carbonate in the gravel.  Nothing at all to induce this
> reaction in a bare tank with nothing but water and a lid.  I can accept the
> CO2 loss thing, but pH went down and back up even after the reaction with
> carbonates stopped.  And at one time I drove it all the way to 5.0 and the
> next day it was back to 7.4.  CO2 can't account for that.

	I think it can.  A change of 2.4 in the pH implies a change in
CO2 concentration of a factor of 10^2.4, or about 250.  If you ended
up with the atmospheric equilibrium CO2 concentration of 0.5 ppm,
then starting with 125 ppm would do the job.  That much CO2 would be
generated by neutralising 125x100/(2*44) = 142 ppm CaCO3 equivalent.
(2 CO2 from each "CaCO3", because it is really the bicarbonate)

> > I never heard that HCl gas was produced from anything but the
> >  most concentrated hydrochloric acid solutions.

	The HCl is more strongly "bound" to the water than the water is
to itself, so the water leaves preferentially.  What is more, at the
sort of pH we are looking at here, there will be _very_ little actual
HCl anyway, the Cl- ions will be off on their own.
> 
> It does its own equilibrium thing, just like anything else.  I also observed
> that the pH would take 30% longer to rise all the way when I used a very tight
> fitting lid. on the tank.  Gas-off is the only explanation I have.  Anyone
> want to offer another?

	I think that it's CO2 that is leaving.  I'm sure it isn't HCl.


-- 
Paul Sears        Ottawa, Canada