Re: Technical Grade Muriatic Acid

From: Etienne Garbaux <photographeur@softhome.net>
Subject: Re: Technical Grade Muriatic Acid
Date: Sat, 08 Oct 2005 02:29:34 -0400

> Michael wrote:
>
> > So what is the correct proportion to use to make Aqua Regia? Does
> > using a less concentrated form simply give a less concentrated
> > Aqua Regia mix (read: weaker)?

> The only time I saw aqua regia used, it was made with fuming HCl and
> fuming HNO3. I do not think that the small amount of extra water in
> the non-fuming varieties would have a material effect on the aqua
> regia -- that is, I think aqua regia can be made from the
> concentrated acids of commerce -- but I could be wrong. Perhaps
> Ryuji knows?

I certainly don't claim I am very familiar with aqua regia, or
muriatic acid, since I buy gold/platinum/rhodium in the hexachloro
acid form. But here's some stuff from my references.

Aqua regia is an alchemist term, and it was an empirical, indefinite
mixture. If you are interested in dissolving metals of platinum
family, reaction is:

8 HCl + 2 HNO3 + Pt --> H2PtCl6 + 4H2O + 2NOCl
(palladium would make H2PdCl4).

So the molar ratio of the acid is 1:4 or so would be good. The
reaction is fastest when heated to near boiling point of aqua regia,
and in 6 to 12M concentration of the acid.

What's the molar concentration of muriatic acid in tech grade? I
don't know, but 36.5% w/v would make 10M. If you really care,
determination would be a simple, straight-forward, freshman chem lab
assignment. I'd dilute the acid 1+39 and then titrate with 2.5N NaOH.

> An exact 1:3 proportion is not critical -- therefore, if one acid is
> slightly more dilute than the other, it shouldn't matter (assuming
> that acids slightly more diluted than maximum, but still
> concentrated, will work in the first place).

Agreed. Again, assuming that the application is dissolution of
platinum and palladium, as you see in the reaction above, nitric acid
is oxidizing the metal to ions, and HCl is quickly removing the free
ions to form hexachloroplatinic acid. Ratio of the two acids as well
as total acid concentrations are not very critical for this reaction.
Received on Sat Oct 8 03:19:50 2005