[TheForge] RE: pickling solutions

[Bruce Freeman]

Oops.  Last sentence in first paragraph should read, " As the acid gets
used up the pH will
INCREASE to ~2 and STAY THERE until most of the acid is used up, at
which
point it will rise, approaching pH 7 (neutral).

Sorry 'bout that.
Bruce

>>> FREEMAB at pt.fdah.com 9/14/2004 2:47:18 PM >>>
The interesting thing about "weak" acids is that they maintain their
pH
moderately well as they are used up in the bath.  This is the buffer
effect.  In the case of bisulfate, the pKa is 1.92, or roughly 2. 
This
means that in a solution with exactly equal amounts of bisulfate ion
and
sulfate ion, the pH of the solution will be ~2.  [This, in turn, means
that the concentration of hydrogen (H+) ion in solution is ~0.01
molar.]
 The neat thing is that this pH changes only slowly as the ratio
changes.  Hence, if you start with bisulfate in water, the pH will be
less than 2 (i.e., more acidic).  As the acid gets used up the pH will
drop to ~2 and STAY THERE until most of the acid is used up, at which
point it will rise, approaching pH 7 (neutral).

In practical terms, this means that the only difference between a
"strong" solution of bisulfate and a "weak" one is how long it will
last
- how much metal it will pickle clean before it goes bad.  So my
suggestion is to make it "weak", then add more crystalline sodium
bisulfate if necessary.  That way you don't waste reagent.

In principal, one could add a colored indiator to the pickle, that
would change color when it went bad.  This might not work in practice
because of the color from, or from chemical interference from,
dissolved
metals.  Furthermore, these indicators may be more toxic than anything
else present.  They're fine to work with in dilution, but you wouldn't
want a bottle of the pure stuff around your shop.  So, lacking a pH
meter, the best measure might be how quickly the pickle works to clean
the metal.

BTW, "battery acid" (sulfuric) and muriatic (hydrochloric) acid are
stronger acids than sodium bisulfate and will undoubtedly work faster.

But they are more hazardous as well.  Phosphoric acid is about the
same
"strength" (in terms of pH) as bisulfate.  Acetic acid (vinegar) is a
good deal less "strong" but will clean metal overnight, in my limited
experience.

Bruce
NJ

>>> signs_of_life at hotmail.com 9/14/2004 12:53:23 PM >>>
What's a good concentration for Sodium Bisulfate for pickling? 
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