MadSci Network: Chemistry

Re: Can a dilute solution of an acid be made more concentrated by boiling it?

Date: Wed May 20 02:20:18 1998
Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Area of science: Chemistry
ID: 895626273.Ch

Hi Thomas,

	Generally no. Strong mineral acids such as HCl, HBr, HNO3, HClO4 all have
significant vapor pressures in aqueous solution and will evaporate away
when boiled. Sulfuric acid that is free of water is made by hydrolysis of
pyrosulfuric acid which is the product of sulfuric acid and SO3. Pure
sulfuric acid boils at much higher temperature than water but the various
hydrates of sulfuric acid boil at lower temperatures and would boil away
without leaving behind any unhydrated sulfuric acid. 

	Weaker acids such as acetic and trifluoroacetic can be purified by
fractional distillation. Still other acids like tosyl acid (a stronger acid
than HCl) and benzoic acid can be recrystallized because they are solids. 

	Acids, as a class, are not prone to explode when heated. A strong acid may
explode when water is added to it because the heat released upon hydration
of the acid causes the water to vaporize rapidly. This is why dilution of
acid must always be done by slow addition of the acid to water (not the
reverse). HOWEVER, heating of strong acids will release dangerous fumes
into the air!!! Particularly HClO4 vapor will deposit explosive residues
wherever it goes. Another way to cause an "explosion" with a strong acid is
to mix it with a strong base. Drano, and other similar products, are
composed of strong bases in concentrated aqueous solution. One can also buy
something at hardware stores called "concentrated drain opener" which is
essential pure sulfuric acid. If one happened to own both products and
tried to use them together to unclog the drain a disaster would result!! 

	Strong acids should only be used in a well equipped laboratory and all
industrial and academic labs have ready access to concentrated acids at
cheap prices through commercial vendors. Thus, I would strongly discourage
anyone from attempting to make a concentrated acid from a dilute acid, even
in a lab.    


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