Sciencemadness Discussion Board

Breaking Sulfuric and Nitric Azeotropes

Recessive - 5-1-2011 at 08:57


Sulfuric drain cleaner, which at best is at its azeotrope of 93.3%, is too weak for some purposes, and this post suggests how to exceed that azeotrope.

I believe metal sulfates that contain active metals are more hydrophilic than plain sulfuric acid. If an anhydrous or near-anhydrous active metal sulfate, maybe aluminum sulfate, were added to sulfuric acid, it would absorb water from the acid and then exist as a precipitate, allowing easy separation. That is to say, if one slowly stirred aluminum? granules into cold 92% sulfuric acid, the acid might approach 100%. The resulting aluminum sulfate hydrate could be separated and discarded.

US patent 5,012,019 is an analogous procedure in which magnesium nitrate trihydrate is used to concentrate 70% nitric acid to 98%, absorbing water to become pentahydrated and hexahydrated. Then it is heated to regenerate the trihydrate. Probably unhydrated magnesium nitrate would be even better.

Slowly adding aluminum to cold nitric might also work, turning the aluminum to hydrophilic nitrate. Some incidental aluminum oxide probably wouldn’t hurt and might help.

Too easy to be true? What am I missing?

This all seems so simple, I really should be experimenting, but I’m an asker, not a doer. Comments, please.

hissingnoise - 5-1-2011 at 09:32

AFAIK, phosphorus pentoxide is the only desiccant strong enough to remove water from H2SO4.
Carefully heating the acid to ~300*C until dense white fumes appear will concentrate it to 98% . . .
Adding metal nitrates will result in the formation of HNO3 and sulphate salt!

quicksilver - 5-1-2011 at 11:10

An enormous amount of the H2SO4 drain cleaner is actually waste from anodizing processes.

Retraction

Recessive - 5-1-2011 at 11:10

Well, as usual, I didn’t think far enough ahead. Adding metal to an acid will dilute the acid by removing it from solution to form the salt. The salt will then absorb some water, but probably not enough to compensate for the missing acid, or this would be a popular method of getting around the azeotrope.

However, producing the salt in one batch of acid and then transferring it to another batch of acid to dehydrate it could work.

For instance, a sulfuric / water solution that is 91.5% H2SO4 has about two molecules of H2SO4 for every H2O molecule. If all the H2SO4 were converted to a metal salt, there would be two molecules of salt for every water molecule, which can be more than dry enough to dehydrate acid.

However however, converting H2SO4 into a salt, without using water as a medium, might be tough.

Sorry about the previous stupid post, but it might be possible to salvage some method that would work along those lines.

Hissing Noise: Sulfuric cannot be concentrated beyond 93.5%, because the vapors at 93.5% contain the same percentage of acid as the flask, so nothing changes beyond 93.5%.

Phosphorus pentoxide would be fine, were it available. In the meantime I repeat: Salts made from active metals are more hygroscopic than the acid from which they are made, and these salts can be used to dehydrate the acid.

Rosco Bodine - 5-1-2011 at 11:20

http://www.generalchemical.com/sulfuric-acid-technical-data....

hissingnoise - 5-1-2011 at 12:38

- Recessive, 98% H2SO4 is commonly known as concentrated sulphuric acid!
It is so powerful a dehydrant that it will remove combined H2 and O2 from sucrose to leave a very pure amorphous carbon . . .

Calcs

Recessive - 5-1-2011 at 15:43

Rosco, thanks for the links. However, the data leans towards SO3, which I’ll never own.

Hissing Noise: I apologize – of course sulfuric can be concentrated beyond 93.5%. What I meant is that 93.5% is the limit for sulfuric obtained by simple distillation.

Below is some simple arithmetic that assumes US patent 5,012,019 is correct in stating that magnesium nitrate hexahydrate will remove water from 97% nitric acid.

Take 100g liquid comprised of 90g nitric and 10g water.

Add 2.0g magnesium and assume this is done right and it all turns to nitrate, which then weighs 12.3g. Then this nitrate becomes a hexahydrate which weighs 21.3g, with water comprising 9g.

If this hexahydrate is removed, the nitric acid will be at 98% concentration or more. The weight of the nitric will drop from 90g to 79.5g, to produce the Mg nitrate.

Note that this scheme will produce hydrogen gas, and although this won’t burn your lungs under normal conditions, it can if it catches fire inside them.

I have limited interest in concentrating nitric acid this way, but do note that it does not require a condenser, which is a plus, particularly for people who lack equipment.

I am interested in an analogous arrangement for sulfuric acid. Obviously nitrates can’t be used with sulfuric, but if magnesium were added to sulfuric, I have to believe that dehydration would occur, calculated as above.

In the first post I used the term “active metal salt” but as I think it over, that is too broad a term. Lithium and beryllium would be almost as good as P2O5, but they are expensive and hard to get. Sodium is also exotic, though the salts are easy to obtain. Dried sodium nitrate could have some value in dehydrating nitric. Potassium and calcium are not that active. Aluminum would be ideal, but if it didn’t work, magnesium would. Finely powdered magnesium is a taboo, but the coarser stuff might be available, and magnalium, a mixture of aluminum and magnesium, is less taboo. I use magnesium as an example because the patent provides figures for the hydrates.

Rosco Bodine - 5-1-2011 at 15:57

Quote: Originally posted by Recessive  
Rosco, thanks for the links. However, the data leans towards SO3, which I’ll never own.

Hissing Noise: I apologize – of course sulfuric can be concentrated beyond 93.5%. What I meant is that 93.5% is the limit for sulfuric obtained by simple distillation.


Where are you getting the information or idea that 93.5% is the limit for concentration by distillation ? What I am seeing is more like 98.3% for the azeotrope. 97% is probably a practical limit for concentration by boiling. A pyrosulfate
could possibly be added to kick it the extra 1.3% but it
would get there anyway to 98.3% during a distillation.

http://www.akersolutions.com/Documents/PandC/Mining%20and%20...

There's a lot of data on the page that has nothing to do with SO3 .


[Edited on 6-1-2011 by Rosco Bodine]

Over and Out

Recessive - 6-1-2011 at 16:41

Rosco – My statement that H2SO4 has an azeotrope at 93.3% is from this web page:

http://www.qvf.com/en/ProcessSystems_3/Mineral%20Acids/Conce...

The web site owner produces acids, and has great graphs for nitric, sulfuric and hydrochloric. I should probably confirm the 93.3% elsewhere, but I’m also noting that drain cleaner is in that region. 98% sulfuric is common, but I don’t think it’s the azeotrope.

Again, thanks for the links and I value your comments, but that data isn’t relevant to my quest. I’d like to know boiling points for varying mixtures of sulfuric, nitric and water, and maybe NIST would have them, but that could be a time-consuming trip and a big “maybe,” though no doubt interesting. (Note that finding boiling points 1*C apart for a 100*C range for three fluids generates one million pieces of data.)

To go back and to repeat myself, the basic paradigm here is that active metal salts are more hygroscopic than the acids that produce them, and that means dehydration for the acid. Would anyone doubt that this is true in the cases of lithium and beryllium? Where does this phenomenon stop, on the atomic table, and for which acids? Nitric is a particular problem, because it can oxidize or nitrate, but I think there’s only one reaction with sulfuric.

At any rate, looks like I gotta do it myself. I was afraid of that. I’ll be back, but don’t hold your breath.

entropy51 - 6-1-2011 at 18:02

The azeotrope is 98%.

It is not unheard of to find Rooto drain cleaner of 97 to 98% concentration. It depends on the batch. I titrate every new bottle I buy and if memory serves me correctly Magpie has found comparable results.

grndpndr - 6-1-2011 at 19:44

This seems like deja vu but ill stick m y kneck out again. Ive seen 2 figures for azetrope of H2SO4 93.3 and 98.3. IIRC 93.3% was the maximum achieved by boiling @338c.The 98.3 was the result of boiling
100% H2SO4 @338c.The 100% apparently losing S03 gaining water?

The liquid fire drain cleaner MSDS claimed a specific gravity of
1.84,97%-98%?

[Edited on 7-1-2011 by grndpndr]

Blasty - 7-1-2011 at 06:13

According to Wagner, "Manual of chemical technology", page 281:

"The ordinary sulphur acid of commerce contains 93 to 96 per cent, of the so-called monohydrate, H2S04. Exceptionally, a stronger acid of 97 or, at the utmost, 98 per cent, is obtained by further evaporation in glass or platinum vessels; stronger acid cannot be obtained in this manner, as the monohydrate is dissociated even at a moderate temperature, leaving acid of 98 to 98.5 per cent."


According to Geoffrey Martin, "Industrial and manufacturing chemistry: a practical treatise", Volume 1, page 238:

"Sulphuric acid can be concentrated by evaporation up to 98.3 per cent. H2S04, and with this concentration it possesses its highest boiling point of 338° C., and can be distilled without decomposition."

According to Büchel, Moretto & Woditsch, "Industrial inorganic chemistry", page 114:

"Temperatures of 320°C are required at atmospheric pressure to concentrate up to 96% sulfuric acid. The highest concentration attainable by evaporation is 98.3% (azeotropic composition)."



[Edited on 7-1-2011 by Blasty]

quicksilver - 7-1-2011 at 07:54

H2SO$ from drain opener IS going to vary because it is a surplus waste item; resold to the drain opener companies from industry. It certainly will vary and sometimes it may even be rather clean. It all depends on where it came from. H2SO4 is perhaps the largest single chemical manufactured in the world.

entropy51 - 7-1-2011 at 08:27

Quote: Originally posted by quicksilver  
H2SO$ from drain opener IS going to vary because it is a surplus waste item; resold to the drain opener companies from industry.
This is the source of certain brands, but not Rooto.
Quote: Originally posted by quicksilver  
It certainly will vary and sometimes it may even be rather clean.
Or even indistinguishable from reagent grade, in the case of certain batches of Rooto.

Recessive - 7-1-2011 at 14:40

This is finally getting interesting. Good citations, Blasty.

As regards the true azeotrope of sulfuric acid, the graph at http://www.qvf.com/en/ProcessSystems_3/Mineral%20Acids/Conce... plots the relationship between acid concentration in the boiling acid vs. concentration in the vapor, and it is indistinguishable from a vertical line, starting at about 93% sulfuric up to about 98%. Perhaps trivial factors become determining ones for the azeotrope.

Note that if 98% is possible, there will still be a very large loss of acid distilling along this vertical line.

Also, “sulfuric monohydrate” is 84%.

I think Rooto is in the business of selling sulfuric acid, and if they can get it cheap, they do, and if they can’t, they may give you some nice stuff. I believe they’ve taken sulfuric out of Home Depot and Lowes, but maybe it’s around elsewhere.

True Azeotrope Value

Recessive - 7-1-2011 at 17:02

Well, one final word on the value of the sulfuric azeotrope: Everybody who doubted the 93.3% azeotrope was correct and my assertion of 93.3%, quoting a web site in the acid business, was wrong. The site’s text says “93.3%”, but its graph shows 98%. http://www.qvf.com/en/ProcessSystems_3/Mineral%20Acids/Conce...

Starting distillation at, say, 93%, I think over 90% will distill away before the remainder reaches 98% but it’s nice to know it will get there eventually. But there’s got to be a better way.

Thanks for the help from all you doubters.

Rosco Bodine - 7-1-2011 at 17:05

The figure of 93.3% for the azeotrope is very probably a misprint for 98.3% and the stated boiling point of 338C does correspond to 98.3% H2SO4 d 1.84 which is the constant boiling azeotrope of H2SO4. With regards to the acid loss due to the increasing amount of acid in the distillate as the concentration nears the 98.3% it would be good process economy to divert and save the fraction coming over from perhaps 90% to 98.3% for recycling, adding it to the next batch being concentrated.

The boiling point for the monohydrate d 1.788 is 290C
and the b.p. for the dihydrate ( d 1.650 @0C ) is 167C
These are concentration waypoints worth noting

figures are from my CRC and textbooks

Gee do you think there could be a nexus between the disappearance of many interesting raw materials and excessive online discussion of off label uses for same