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Author: Subject: Shortcuts to perchloric acid?
hissingnoise
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[*] posted on 26-12-2002 at 15:48
Shortcuts to perchloric acid?


Does anyone know if hcl can be electrooxidised to hclo3 and hclo4 without using alkali metal salts.
I know from ex. that nh4cl cannot be oxidised electrolytically; the positive ammonium ion isn't a metal.
It shouldn't be tried because poisonous cloramines(vapour) are formed.
Thanks
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[*] posted on 27-12-2002 at 11:04


The main reason why one shouldn't electrolyse NH4Cl is because of the formation of NH4ClO3 and NCl3 both extremely sensitive explosives.

Electrolytic oxidation of HCl works, but faces a few practical limitations.
First of all you'll get higher electrode corrosion. Secondly, you won't be able to achieve a high concentration because HClO3 can only exist in solution up to 40%. If you go above that, you'll get explosive and poisonous ClO2 gas.




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FadeToBlackened
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[*] posted on 28-5-2003 at 09:10


Ph Z once posted about this one the E&W forum.

Ok here it is, complete and unabridged :p
Quote:

PHILOU Zrealone
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posted April 05, 2001 05:04 AM
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100% right firebreether!
BTW: here are processes to make HClO4 and NH4ClO4
For the Newbies: NH4ClO4 can't be made by electrolyse of NH4Cl because you go through the dangerous NH4ClO3(VOD=3300m/s, spontaneously, unpredictably, self detonating even in solution)!!!!
1)Use CaCl2 (or Ca(ClO)2) and electrolyse it the same way you would do to get KClO3 or NaClO3 from NaCl and KCl.Yes CaCl2 is not very soluble but it is good so believe me!
You will of course get some Ca(ClO4)2 aside with Ca(ClO3)2.
Concentrate by evaporation and add diluted H2SO4 - or what is the same add conc. H2SO4 to the diluted mother liquor. (DON'T ADD CONC H2SO4 TO A CONCENTRATE SOLUTION OF CHLORATE/PERCHLORATE!!!!! BECAUSE HCLO3 AND HCLO4 ARE EXPLOSIVE WHEN CONCENTRATE!!!!!).
White clouds of unsoluble CaSO4 (plaster) will form and precipitate.Add until no more precipitate form while taking some of the clear solution and testing it with H2SO4).

CaCl2+O+heat+electricity--> Ca(ClO)2+ Ca(ClO2)2 +Ca(ClO3)2 + Ca(ClO4)2
The more the time of reaction the more you go to Ca(ClO4)2!!!!
Ca(ClO4)2 + Ca(ClO3)2 + 2 H2SO4--> 2CaSO4 + 2HClO4 + 2HClO3
Filter the plaster through glass paper filter (used to cover walls before paint application to avoid irregularities-glass doesn't react with oxydising acid like perchloric and chloric what paper does!).

Now we have a diluted to medium concentration solution of HClO3 and HClO4 in water.
To get rid of the HClO3 for further process boil the solution by slowly increasing the heat in a wel ventilated area avoiding going over 150C.
By the magic of chemistry:
4HClO3(diluted)+heat--> 3HClO4 + HCl
(this reaction looks pretty the same as
4NaClO3 + heat (not too much)--> 3NaClO4 + NaCl
did you notice that?)
Anyway,upon slow boiling HCl is driven away (reason why it has to be done outside otherwise all metals and your lungs will be corroded); So does water.
You end with a solution stil concentrating in HClO4 by evaporation of the water.
Since HClO4 has an eutectic with 79%HClO4/21%H2O at approx 180C and that it becomes explosive when more concentrate, stop at 150C, no use to loose the valuable HClO4.

Now simply mix concentrate NH3 (in excess) solution with the HClO4 solution to get NH4ClO4. Evaporate in the open excess NH3 will go away. You get your NH4ClO4 from not to expensive starting materials.
BTW: DON'T USE a pan to boil the mix since METALS AND ACID ARE NOT GOOD FRIENDS always use pyrex glassware.Also to know HCLO4 is catalyticly decomposed by metals when very concentrate over 79%, it explodes aslo upon contact with alcools (and cellulose is a polyalcool) making the dangerous organic perchloric esters.

2)Even less expensive:
Mix under stiring sodium chlorate with concentrate HCl in excess (you can only reach max 37% HCl)and heat gently until it dissolves. cool down NaCl cristallises out (solubility of NaCl is 30g/100g H2O); filter out and keep the 37% HClO3 solution.
NaClO3 + conc HCl < == > HClO3 + NaCl
Boil the HClO3 slowly in an open space following the same reaction as in the first process:
4HClO3 + heat --> 3HClO4 + HCl
HCl is more volatile and fky away.
Go to 150C.
THen when cool add some excess NH3 solution.
HClO4 + NH4OH --> NH4ClO4 + H2O

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[Edited on 28-5-2003 by FadeToBlackened]
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Marvin
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[*] posted on 29-5-2003 at 12:14


Ph Z wont thank you for digging up that post, its very bad.

You cant make chloric or perchloric acid by electrolysis of HCl. This will only get you chlorine gas.
Electrolysis of NH4Cl will mainly liberate nitrogen, oxidising the ammonium part through side reactions. Some NCl3 will be generated, but ammonium chlorate isnt a threat, particulally if the solution is acid, which it will be, and while there is free chloride, which there will be while NH4+ ions are present.
CaCl2 is very soluable, a weird mistake, rather more soluable than sodium or potassium chloride and this is important becuase it alows you to make calcium chlorate from sodium chlorate. Making calcium perchlorate the same way probably wont work though becuase of the low solubility of the group 1 perchlorates. This is annoying, as electrolysis of CaCl2 doesnt work.

If you use CaCl2 in a chlorate cell, everything starts fine, but the very low solubility of calcium hydroxide causes the cathode to be rapidy covered in a thin layer of it that causes the cell resistance to skyrocket. Current drops and production rate is very small. Adding HCl is the logical solution, but make the solution acid enough to prevent this and it turns the cell instead into a chlorine gas factory. Reaction of chlorine gas with the alkali hydroxide produced at the other side of the cell is an important reaction anyway in the normal chlorate cell, and conversion isnt going to be useful without it.

Using the CaSO4 ppt reaction as a driving force is useful theoretically, but practically difficult and annoying, its voluminous, very difficult to filter, so you can only make dilute solutions with it. Anyone sane would crystalise out chlorate from this before trying to make the acid as Ph Z has suggested, side product of boiling the mixed acid would be chlorine gas, not HCl, it hasnt been thought through as the equations have been laid out to show a comparason of chloric and perchloric acid chemistry with the salts. One that doesnt actually exist in these examples becuase strongly acid chloride gets oxidised to chlorine rather easily.

Adding HCl to NaClO3 is highly unlikley to work. My information is the chlorate will simply oxidise the HCl to chlorine gas until it ceases to be acid. I havnt tried this myself, so I'm not certain.

It seems likly little if anything specific on the post has actually been tried.
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[*] posted on 20-11-2003 at 18:38


Speaking of preparation of HClO4 electrochemically from hydrochloric acid, I recently checked out a book entitled "Perchloric Acid and Perchlorates" by Alfred A. Schilt and it gives some information on it, doesn't sound too practical but figured it would be good to post it for sake of completeness.
Quote:

"Electrolytic oxidation of hydrochloric acid to perchloric acid, as described by Walker in 1918 and in greater detail by Goodwin and Walker in 1921, proved relatively inefficient for commercial purposes. Production of chlorine rather then perchloric acid is favored at higher concentrations of hydrochloric acid, so it is necessary to employ very dilute [0.1-1.5 N] solutions and thus extensive evaporation to concentrate the product. Some improvements were forthcoming. Rakov and coworkers found that [1] perchloric acid formation begins at 2.4 V and reaches a maximum value at 2.8-2.9 V, [2] lowering the temperature to -20 significantly accelerates the process, and [3] concentration changes of the chloride ion from 0.5 to 1.8 N and of perchlorate ion from 3 to 8 N are without effect except for decreased current efficiency at the highest concentration of hydrochloric and perchloric acid. In another study, Rakov et al. found that platinum anodes afford higher yields of perchloric acid then did Pt-Ti or Pt-Ta bianodes. Rakov and Shimonis, in a patent granted in 1976, claim increased current efficiency is attained in the electrosynthesis of perchloric acid from 1 to 4 N hydrochloric acid by using an iridium anode at a potential of 2.9-3.3 V and a temperature of -5 to -30."

Like I said, not the most tempting of propositions to perchloric acid but interesting to know that it can be done by electrolysis of HCl (aq).




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