Sciencemadness Discussion Board - Max concentration of H2SO4 by heating alone?

Max concentration of H2SO4 by heating alone?

grndpndr - 8-6-2010 at 18:37

Ive been led to understand the azetrope of an H2SO4 /water mix is 93.3% H2so4 @338C @ abient pressure.Above this concentration requires other methods beyong heating.The nets full of the same info.Where am I wrong?Show me the science!And others are achieveing a guestimated? 96%-98% for instance starting with electrolyte?It would seem physically/chemically impossible.

i DID FIURTTHER SEARCHING AND FOUND A LARGE DISCREPANCY AMONG MODERN RELIABAL INFO.oNE CLAIMS 93.3@338C AZETROPE ANOTHER 98.3 @338C.AZETROPE oNLLY THE 93.3 CLAIMS THAT FIGURE AT AMBIENT PRESSURE.iDEAS?wHAT IS THE AMBIENT PRESSURE CLAIMED TO BE AND AT WHAT PRESSURE THE 98.3% AZZETROPE IS CLAIMED AND DIFFERENT METHODS OF TITRATION/s.g. iT SEEMS A WORTHWHILE QUESTION WITH CONFLICTING INFO OUT THERE AND iM NOT SPEAKING TOTSE/BOMBSHOCK HERE.a 5% DIFFERENCE IN h2so4 STRENGHTH IS SIGNIFICANT WE ALL AGREE I WOULD LIKE TO SEE AN EXPLANATION.aNYONE ELSE?

mULTIPLE EDITS NEW INFORMATION,TO LATE AND LAZY TO DOWNSIZE,SORRY VULTURE.

[Edited on 9-6-2010 by grndpndr]

Chainhit222 - 8-6-2010 at 19:14

funny enough I was going to post the same exact thing, citing your name and everything

. Maybe this is why my HNO3 was not 1.53 g/ml.

The WiZard is In - 9-6-2010 at 11:24

Quote: Originally posted by grndpndr

Ive been led to understand the azetrope of an H2SO4 /water mix is 93.3% H2so4 @338C @ abient pressure.Above this concentration requires other methods beyong heating.The nets full of the same info.Where am I wrong?Show me the science!And others are achieveing a guestimated? 96%-98% for instance starting with electrolyte?It would seem physically/chemically impossible.

------
bp varies over range of 315-338C due to loss of sulfur trioxide during heating to 300C or higher.

Hawyley's Condensed Chemical Dictionary
14th Edition
2001

grndpndr - 12-6-2010 at 14:37

BP again @ambient pressure says 290c.Also saw a chart with different pressures fron 1 bar down.@ .05Bar the azetrope was
approx 1.5% higher H2SO4 IIRC but the chart had 2 different
methods of comparing H2SO4 concentration which must be the answer as thier was about a 5% difference in azetropes.One of the methods was by % w/w H2SO4 the other was measured by molar wieght %.

hissingnoise - 13-6-2010 at 07:18

I think it's pure anhydrous H2SO4 that boils at ~290*C. . .

grndpndr - 13-6-2010 at 15:43

Whats that a trick question Hissingnoise?!100% anhydrous sulfuric acid decomposes @ boiling(yes 290c) losing S03/gaining water until the azetrope of 98.3% H2so4 which does boil @338c. Took me awhile to come to terms with this.F..g BackYard hobbyists!LOL
(Langes 10th Edition)

[Edited on 13-6-2010 by grndpndr]

[Edited on 14-6-2010 by grndpndr]

Hennig Brand - 14-9-2011 at 18:27

I know it has been a long while since this thread has been posted in, but I thought this chart might be of use.

Attachment: Sulfuric Acid Boiling Points.pdf (248kB)
This file has been downloaded 518 times

At no extra charge, a picture of the setup (in action) I was using to concentrate H2SO4 a few months ago. The temperature probe for the multimeter isn't the most accurate device on the planet, but getting the temperature up to 300C on it gets me acid that titrates at over 96wt%. I concentrated acid once that titrated at over 98wt%, but I boiled away almost half my acid in the process. I usually aim for between 96-97wt% and then not lose that much.

The thermocouple wire sensor (temperature probe) was run into a thin walled piece of small diameter glass tubing that had its end sealed with a propane torch. The probe wasn't left in the whole time, it was put in place periodically just long enough to get a temperature reading.

[Edited on 15-9-2011 by Hennig Brand]

AndersHoveland - 16-9-2011 at 13:22

Tell me if I understand this correctly:
Dilute sulfuric acid can theoretically be boiled down to a concentration of 98.3%, but this results in a loss of much of the sulfuric acid. Thus in practice, the highest concentration obtainable by boiling is 96%.

?

Hennig Brand - 17-9-2011 at 12:50

If you had a good thermometer and knew what you were doing, I don't think there is any reason why you couldn't get 98% H2SO4 with reasonably acceptable losses. I just find it simpler to stop just shy of the maximum concentration.

I think the main reason I boiled away so much of my sulfuric acid was because I was still getting used to my new temperature measuring device (multimeter with thermocouple wire temperature probe). I most likely had >98wt% acid for quite a while, before I stopped heating, and didn't know it. I do know that it seems like getting those last couple of percentage points of concentration is much more difficult than probably the 20% before (just an observation, I haven't tried to work it out or done a proper test really).

I am fairly sure that 98.3wt% is the azeotrope for sulfuric acid. This basically means that at this concentration it is impossible to raise the concentration furthur by simple distillation, because the composition of the vapor coming off is the same as the liquid acid it come from. You could boil and boil, and you would just waste acid. This it what I think I did that time I lost so much of the acid, which I referred to in my last post.

The freshly boiled down, concentrated H2SO4 is very hygroscopic so it absorbs moisture very quickly as it cools. I cover mine to prevent water absorption, right after it cools the first few degrees, with a glass bowl that almost perfectly seals the top of the glass coffee pot. When the acid is cool enough it goes straight into an airtight bottle for storage.

BTW, most of these coffee pots say not to even boil water in them, so be careful. I have gotten away with heating acid in a variety of coffee pots 50 times or more now probably, but I know there is a certain probability of breakage. The fumes are really bad too. I boil the acid down well away from people, and make sure that the wind is working in my favor.
I stay well away myself as much as possible.

Edit:
Did you notice in the picture above that there was no heat diffuser under the coffee pot? This is bad practice and I simply forgot to use one that one time. The truth is that the hotplate in the picture is so low powered that it provides fairly gentle heating, which may have saved me.

[Edited on 18-9-2011 by Hennig Brand]