The Production of Percholoric Acid
Creators: name: Adams, Horace Chamberlin
Publisher: name: California Institute of Technology
Year: 1925
Subjects: subject: Chemical Engineering
subject: FOS: Chemical engineering
subject: FOS: Chemical engineering

description: Due to the fact that perchloric acid resembles sulfuric acid in many respects without having many of the disadvantages of use possessed by the latter, the devising of a cheaper means of producing it would be well warranted by its increased use in both analytical and industrial work.  The present work was undertaken with the above idea in view and consisted in studying quantitatively the reaction taking place between nitric oxide (NO), nitric acid, and ammonium perchlorate.

Anhydrous perchloric acid is an unstable compound, decomposing spontaneously after a few weeks even when kept in the dark.  It is a violently reacting oxidizing agent for many organic substances but aqueous solutions of the acid are not affected by light and are not active oxidizing agents.

Perchloric acid solutions (60% strength being the standard commercial form of the acid) are strong, mono-basic solutions which are stable up to the 60% strength and have high boiling points.  The salts which the acid forms are nearly all soluble, the potassium, rubidium, caesium and thallium perchlorates being the only ones which are only slightly soluble.  This would recommend it over sulfuric acid in many cases where insoluble salts of the latter prevent its use or cause it to seriously interfere on this account in analytical determinations.  Also the difference in the solubilities of the potassium and sodium salts permits an easy separation of the perchlorates of these two metals.  The maximum boiling point of aqueous solutions at atmospheric pressure is 203° C., which corresponds to a constant boiling mixture having a perchloric acid content of 72.3%.  Below 160°C.  the distillate contains less than 1% of perchloric acid.  Most perchlorates are deliquescent except ammonium, potassium, lead and mercury salts.

There have been a number of processes devised for the production of perchloric acid, some of which are only of theoretical importance, but the following methods are the ones which have been used to some extent commercially:

(a) The mixing of dry NaClO4 or KClO4 with H2SO4 and distilling the mixture under diminished pressure.  The distillation process is a troublesome and expensive one and redistillation must take place to remove the H2SO4 and obtain a pure acid – the product containing 88-89% HClO4.  HCl can be substituted for H2SO4 being precipitated from the solution.  The HCl can then be distilled off from the filtrate and an approximately 95% HClO4 acid obtained.  However, there are small amounts of NaClO4 still remaining which require distillation under reduced pressure to effect their removal.

(b) The electrolytic oxidation of HCl in dilute solution is the method used in this country at the present time for commercial production.  However the resulting liquir contains HCl, chloric acids and chlorine in addition to the HClO4.  Also the acid obtained is quite dilute and must be concentrated for commercial use.

The above methods are expensive to operate due to the necessity of purification and concentration of the acid resulting from the reaction used, and the use of NH4ClO4 as a source of HClO4 has been advocated as the best means of overcoming the above objections.  NH4ClO4 can be obtained comparatively cheaply and when purified and the ammonium radical oxidized by oxides of nitrogen, there are no non-volatile impurities remaining in the product.  All gases and volatile liquids can be easily distilled off at atmospheric pressure and any further concentration of the product can be accomplished at the same time although no such amount of concentration will be required as in (b) cited above.

Investigation of the possibilities of the above method is the object of the present work which is a continuation of the work begun at the Institute in 1921 by R.W. Stenzel and continued in the following year by A.W. Knight and in 1924 by V.A. Kalichevsky.