Sciencemadness Discussion Board

Stability of strong perchloric acid?

chornedsnorkack - 8-1-2015 at 06:47

What is the range where perchloric acid is stable to detonation?

Dilute solutions of perchlorate in water are very inert to reduction. They are reduced to hydrogen by active metals (unlike nitric acid, which is reduced), and are only reduced by some transition metal cations, even that very slowly.

The azeotropic mixture of perchloric acid and water contains 71,6 % perchloric acid, and boils at 203 degrees without decomposition (compare again nitric acid, 68 %, 120 degrees and appreciably decomposes to NO2). While it can form explosive compositions with reducers under some circumstances, it is routinely used for quiet digestion of organics at temperatures 150...203 degrees.

Now, 100 % HClO4 is quite a different matter! It is sensitive, easily explodes, and also slowly decomposes at room temperatures, eventually exploding by itself.

But there are 28 % between 72 and 100 %! So, what are the properties of concentrated but not dry perchloric acid/water mixtures?

85 % perchloric acid forms a hydrate, oxonium perchlorate, which melts at 50 Celsius. It is also said to be viscous at melting point, and withstand heating to 110 degrees without decay. Naturally, being above azeotrope, its vapours are dangerous to condense.

Up to which concentrations would you feel safe to heat pure perchloric acid/water mixture to atmospheric boiling point? Heat the acid with reducers for digestion? Add reducers to cold acid? Store pure cold acid at room temperature?

AvBaeyer - 8-1-2015 at 18:58

Perhaps the attached will provide you with the information you seek. Much more available with a simple web search.

AvB

Attachment: Perchlorate Safety.pdf (166kB)
This file has been downloaded 1078 times

chornedsnorkack - 9-1-2015 at 14:59

Quote: Originally posted by AvBaeyer  
Perhaps the attached will provide you with the information you seek.

No, it does not! It gives the behaviour of perchloric acid between 50 and 72 % - but above 72 % basically all it says is that it´s dangerous. No specifics.
Quote: Originally posted by AvBaeyer  
Much more available with a simple web search.

There isn´t. I have searched. Everything available by a simple web search is simply more of the same: over 72 % is dangerous, no specifics. That mention of oxonium perchlorate not decomposing to 110 degrees is the only specific I could find, but it does not say what it does do on heating.

So has anyone found any specifics?

Bot0nist - 9-1-2015 at 22:10

K. Everett and F.A. Graf Jr. 
Handling Perchloric Acid and Perchlorates. 
In:— N.V. Steere, ed. 
Handbook of Laboratory Safety
Second Edition - 1971.

&

"The moral of that story is that concentrated perchlorates (along with chlorates, bromates, perbromates, iodates, periodates, permanganates, ferrates, plumbates, bismuthates, perxenates, and osmium tetroxide) and organic matter do not mix! That "employee" who mixed spilled concentrated HClO4 with sawdust to try to absorb it could hardly have made a better bomb; he should have used common silica sand instead, and should also have tried to neutralize the acid with a carbonate or bicarbonate solution." ~ JohnWW

You could peel through every mention of it in SciMad. I know I have read an account if it being distilled here at some point... drawing a blank though.

[Edited on 10-1-2015 by Bot0nist]

chornedsnorkack - 10-1-2015 at 02:44

Quote: Originally posted by Bot0nist  
K. Everett and F.A. Graf Jr. 
Handling Perchloric Acid and Perchlorates. 
In:— N.V. Steere, ed. 
Handbook of Laboratory Safety
Second Edition - 1971.

&

"The moral of that story is that concentrated perchlorates (along with chlorates, bromates, perbromates, iodates, periodates, permanganates, ferrates, plumbates, bismuthates, perxenates, and osmium tetroxide) and organic matter do not mix!


And yet, 60...70 % perchloric acid is routinely heated with organic matter! So where are the specifics of concentrated perchloric acid stability?

Thermodynamic approach to solving this

deltaH - 10-1-2015 at 02:49

Perhaps one can treat this the same way as with hydrogen peroxide solutions and calculate the thermodynamics of the decomposition reaction and see at what concentration the heat of reaction for forming all gases becomes negative...

There are two hypothetical decomposition reactions I could think of, one could write:

(1) 4HClO4 => 2H2O + 2Cl2 + 7O2
(2) HClO4 => HCl + 2O2

I found a paper that appears to have reliably modelled dH(f) for HClO4 in the gas phase [1].

∆H◦f,298[HClO4(g)] = −0.6±1 kcal/mol

Using additional NIST webbook thermodynamic data, per mole of gaseous perchloric acid, that give a heat of decomposition at 298K of -121kJ/mol for reaction (1) and -92kJ/mol for reaction (2)... if I haven't made some stupid error :mad:

That's a difference of only 29kJ/mol between the two which is pretty close. I have no idea if this difference will increase or decrease at elevated temperature, i.e. which will dominate at high temperature, reaction (2) produces slightly more volume of gases.

Now if we run with equation (1), we need experimental enthalpy of HClO4(l) solutions at various concentrations and then can calculated the heats for the following reaction:

(3) 4HClO4(dil.) + xH2O(l) => (2+x)H2O(g) + 2Cl2(g) + 7O2(g)

At some concentration the heat of reaction(3) will swing negative and I would say that that would be the upper limit which the pure solutions become really dangerous, though they'd still be very dangerous even below this point.

I have found and attached a paper that has these enthalpies at various concentrations [2] for your convenience ;)

I apologise, but I am too busy to crunch numbers :( , so it would be great if op would do some homework on this and do the calculation. The enthalpies of formation for water is readily available online. An EXCEL worksheet could be set up for this calculation. I happy to help if you get stuck.

References:

[1] Martin, J. M. L. (2006) Heats of formation of perchloric acid, HClO4, and perchloric anhydride, Cl2O7. Probing the limits of W1 and W2 theory. J. Mol. Struct. . 771(67), 19-26.
[2] Bidinosti, D. R. & Biermann, W. J. (2011). A Redetermination of the Relative Enthalpies of Aqueous Perchloric Acid Solutions from 1 to 24 Molal. Can. J. Chem. 34(11), 1591-1595.

Attachment: Perchloric acid solution heats.pdf (171kB)
This file has been downloaded 395 times

[Edited on 10-1-2015 by deltaH]

chornedsnorkack - 10-1-2015 at 04:02

Note that HCl is still a very soluble strong acid in water. You will always release some heat by reducing perchlorate to chloride.

70 % perchloric acid turns out not to be a particularly strong acid. One old post in this forum mentioned its Hammett acidity function in the region of -7...-8, comparable to 80 % sulphuric acid.