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Purification of Laboratory Chemicals, 5th Edition, by Armarego, reads (p. 53):

Reagents such as water, ammonia, hydrochloric acid, nitric acid, perchloric acid, and sulfuric acid can be purified via distillation (preferably under reduced pressure and particularly with perchloric acid) using an all-glass still.

as well as (p. 479):

Sulfuric acid, and also 30% fuming $\ce{H2SO4}$, can be distilled in an all-Pyrex system, optionally from potassium persulfate. Also purified by fractional crystn of the monohydrate from the liquid. Dehydrates and attacks skin—wash immediately with $\ce{H2O}$.

I've never seen sulfuric acid fume (only spread with steam when heated), and Wikipedia states a boiling point of $\pu{337 ^\circ C}$, along the fact that it slowly decomposes to $\ce{SO3 + H2O}$ above $\pu{330 ^\circ C}$, so I'm wondering, mostly from a theoretical standpoint:

  1. How far lower can the boiling temperature effectively get under reasonably strong vacuum?

  2. Does the vacuum not also bring down the temperature level at which sulfuric acid degrades?

  3. What would be an ideal laboratory (not industrial!) setup and process for the safe distillation of sulfuric acid in fair enough amounts for laboratory use (say producing in a batch the equivalent of $\pu{5L}$ at 30% concentration)?

Nota Bene: I know it's a better idea to buy sulfuric acid at the desired level of purity, but that is not the question.

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Just on the purification of sulphuric acid, well the one he gets has a dark green tinge, and what is in the sulphuric acid is anticorrosive inhibitors, etc, and they are all organic in nature, and the simple heating up a beaker of sulphuric acid, and before it gets to hot, slowly add 100 ml of 3% grocery store bought food grade hydrogen peroxide, and stir whilst slowly heating, well before long the acid will be crystal clear, any fumes are the skeleton structures of the organic additives, and concentrating it back to 98% sulphuric acid is either density determined, temperature, to insure water has been removed hence both will get a much purer, crystal clear sulphuric acid. Anything organic will be consumed by the peroxide and what doesn't will turn into a gaseous product that evaporates of. The product described has served his associates and himself, for many years.

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    $\begingroup$ @Babs_Lawrenson: This information can be useful for some, but I'm not sure this really answers any of the central aspects of my question. $\endgroup$ – Hans Jul 11 at 10:05
  • $\begingroup$ This doesn't answer the question (which is about distillation and purification not the destruction of additives which still leaves a solution that has to be purified). And it is fucking dangerous even with weak peroxide. $\endgroup$ – matt_black Aug 10 at 13:45

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