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Re: acid-treating gravel


Hoa Nguyen wrote:

> I recently traded in some plants for some fine-grained Cambria gravel that I
> wanted to add to my aquarium.  I tested it with Muriatic acid and it fizzed
> a lot though, so I know that it probably will raise my pH.  I am thinking of
> soaking it in diluted Muriatic acid to produce a "neutral" gravel.  My
> questions are:  
> 
> 1.  Will this work?

Maybe.  It could take a lot of acid before it stops fizzing.  It depends 
on how much calcite (calcium carbonate) there is in the gravel.  If there 
are any identifiable bits of shell then you will want to pick those out 
before treating things.
 
> 2.  Are there any bad, toxic by products (gas or otherwise)?

Not usually.  Just non-toxic dissolved salts that you will want to rinse out 
very thoroughly.

> 3.  What are the chemical reactions?  Where will the calcium go?

Muriatic acid is technical grade hydrochloric acid (HCl).  The acid contains
chloride ion (not reactive) and hydrogen ion (very reactive). The hydrogen 
ion drives this reaction to the right: 

	CaCO3 + 2H+ --> Ca++ + CO2 + H20

With slightly less acid you might instead get:

	CaCO3 + H+ --> Ca++ + HCO3-

The fizz is caused by the rapid generation of CO2.  Ca++ remains 
in the acid.  The first reaction slows down after a while and stops 
fizzing.  The second reaction takes over, but works more slowly.  The 
second reaction continues until the acid is nuetralized.

Probably you would want to stir the mix and change the acid out after it 
stops fizzing and continue adding fresh acid until fresh acid produces no 
bubbles.

Be very careful handling the acid - use gloves and eye protection.  HCl is
a gas at room temperature (the acid is a solution of HCl gas in water) and
the gas is always escaping from the acid.  So *don't* sniff the acid - the
fumes can combine with moisture in your nasal passages and give you a
nasty acid burn - and be careful how you store it.  HCl gas escapes from
most containers and is very corrosive. 

If I were to try this I'd probably use sulfuric acid.  Its dangerous, but
its more stable in storage and doesn't have the nasty corrosive gas
side-effects. 


Roger Miller