drkcc - 11-10-2025 at 01:29
Good day to all you other users! This is my first post, so feel free to correct me if this post has been created under the wrong category and should
be removed or moved, I am fine with that.
I have recently been working on concentrating sulfuric acid by boiling off a lot of water using an old Heidolph MR2002 Magnetic Stirrer / Heater I had
lent out from a open source lab, which is rated to a Hotplate temperature of 350 C° according to its Manual.
Carefully turing up the temperature step by step as not to overheat my 1L Erlenmeyer beaker and checking the temperature of the glass with a Infrared
Temperature Thermometer Gun in lieu of a proper Contact Thermometer, I soon noticed that the temperature of the beaker remained far below the
temperature the Hotplate had been tuned to, even as it had clearly reached steady state temperature. For example, after tuning the plate to 250 C°,
the surface temperature of the glass near the bottom remained at 120 C°, and even when turning the heater all the way to 350 C° it never got past
170 C°. (Note: The beaker was insulated using aluminium foil as advised to keep more of the heat in) While I did expect the temp to be lower than the
hotplate itself, I hadn't expected the gap between medium and plate quite as large which I would imagine to be large issue had I needed consistent and
accurate temperatures for my process.
First I suspected that I was measuring the temperature incorrectly, but it appears my observation of the boiloff were consistent with my temp
measurements, as it shortly after slowed down to a crawl, then stopped entirely. Letting the acid cool and running a density check revealed it to be
70% concentrated. Whatever the issue was, reaching my target of at least 90% concentration was impossible.
Next I attemped to find a suspected fault in the electronics of the Heater, cleaning the contacts of the Hotplate, the Bridge plug and the outside of
the device for good measure. This did not help in increasing max temperature.
Now I am kind of stuck. Is it a common occurance for a Hotplate to degrade this bad, making reaching rated temp impossible? Or am I doing something
else wrong? Perhaps by beaker is too small for this Hotplate to get proper conduction? Hotplate diameter is 145 mm, while my Erlenmeyer beaker has a
diameter of 130 mm at is widest and a contact surface with 90 mm in diameter. Please lend my some advice with heaters if you have experience with
them, it would make my day.
Attachment: Heidolph BAL MR 2002 Manual (Ger) (1.8MB)
This file has been downloaded 67 times
esquizete_electrolysis - 11-10-2025 at 10:28
Lets approach this two ways.
First, based off of some approximations and quick calculations. The hotplate you provided outputs 630 Watts at full power, and after accounting for
the base diameter, you have a power input of about 2.43 W/cm^2, but lets call this 5 to assume that all heat is transferred to the surface and that it
is perfectly transferred to the liquid, as to give it a fighting chance. Lets say that your 1L flask has an outer surface area of about 1800 cm^2 and
that it has a thickness of about 3.5mm (once again, these are slight overestimates). Lets say that the internal temp is at the boiling point of 70%
sulfuric acid, which is 165C, and that the outside temp is 110C after the inclusion of the foil. After preforming the heat transfer equation, we find
that the flask loses about 716 W, which is more than the hotplate can output, even if we assume that all power is transferred to the solution with 0
resistance. Of course the actual outside temp is probably higher, but the amount of heat actually transferred to the flask is probably significantly
lower.
The math tells us that you need to either have greater power input (more powerful hotplate), or increase the insulation on the flask.
We didn't need math to tell us this however, as practical experience has taught me that boiling sulfuric acid is simply a bitch. Ignoring the dangers
that hot conc. sulfuric acid brings, its enough of a challenge to get it to that temperature in the first place. Thankfully, old tech is capable of
reaching these more extreme conditions. I would recommend getting an old pot that you don't care about that can fit you flask into it. Fill the bottom
with a cm or two of sand, place your flask within, and fill the rest with sand. Use a gas burner, a cheap hotplate or charcoal (you may get some ash
in your flask), and heat it. It will be slow, but you've provided the flask with a wonderful insulator, increased the surface for which heat can be
transferred, and it has the upside of protecting the bulk of the flask from possible thermal shock. The hotplate is the most attractive option, as
even Walmart sells a 1100W hotplate for about $12 USD.
Keep in mind that scientific equipment is often ludicrously expensive for what it is, and what you pay for is usually the accuracy that it can
provide. Stuff like boiling sulfuric acid does not need to be accurate, it just needs to be good enough. Best of luck!
macckone - 11-10-2025 at 15:13
Get a cheap walmart hotplate. You may need to bypass the thermostat, depending on model. Then use pipe insulation to wrap the flask instead of
aluminum foil. You need to get the hotplate to 100C above the boiling point of your liquid to get it to boil properly.
macckone - 13-10-2025 at 17:28
Ps when I say pipe insulation I mean fiberglass stuff. $7 at home hardware store.
drkcc - 4-1-2026 at 15:11
Sorry for taking so long to respond, but I have finally returned!
The math might not have been necessary but I appreciate it nevertheless! The strat of using a pot, sand and a very powerful plate does sound quite
good, but knowing how furiously the stuff starts to bump at only 70% I didn't want to dare damaging or breaking my glassware as I would have no good
means of stirring that way. So I went the way of adding better insulation instead.
I bought a cheap fire extinguishing blanket at a local hardware store, created a sewing pattern in the conical flask shape and comissioned a
seamstress to make me two of these makeshift thermal blankets. These wrap around the beaker approx. two and a half times and are held on by the two
velcro straps.
I immediately got to testing. The insulation proved highly effective.
Even though conditions were much worse with freezing temperatures outside, the outer shell of the insulation did not exceed 92° C in temperature at
very worst. Getting a little too greedy I initially attempted filling the beaker a little too far and had some acid soak into the fabric, damaging the
upper seam and washing out burnt fabric into the glass fibre staining it black.
(Mental TODO: Acquire bump trap for this mess ASAP)
After some testing my process would look like this: Fill flask to 600 mL with 15 m% conc. Acid. Initially set hotplate temp to 150°, then increase by
50° every half hour to keep thermal stress low. At 250° C and 300° C I would let the plate run for an hour each to reduce the level in the flask
before turning up the heat further. Finally I would put the temp all the way to 350° C and run it at that level for 3 hours and 15 minutes. I'd let
the beaker cool for half an hour before removing the insulation, then another half and hour before I'd fill it off. Including setup that was approx. 7
and a half hours of processing for ~50 mL 90 m% (guess only, I didn't want to waste any by measuring) at a yield of ~80%.
I ran this process three times in total and also concentrated the rest from my earlier escapades, accruing a total of less than half a litre of 90+%
acid that I have yet to test just how high I managed to concentrate. I could likely have gone 'all the way' but didn't want to waste my precious
resource by blasting it into the atmosphere.
After running through several tens of hours of testing, the mantle is still holding up quite well, just the upper seam has been almost completely
destroyed from the acid splashes. The very innermost edge of the bottom seam also finally charred from the intense thermal load.
Interestingly, the velcro plastic has weathered the acid far better than the seams.
Its very nice that I got some production going, but it would be nice if could do it somewhat faster or somehow reduce the acid I lose by evaporation.
I have considered adding a vigreux column onto the beaker and running with less heat, but this would have only take even longer and I would have ran
out of the short winter daytime.
I'll likely attempt vacuum distillation next as this could possibly save a lot of time waiting for heating and then waiting to cool down again.
I will also post the templates for the thermal blankets along with some instructions on how to make them once I have improved on their design a
little. I've been playing with the thought of replacing the fabric seams with PTFE Coated Fiberglass Teflon Tape in the near future for extra wear
resistance and will be replacing the velcro with metal snap buttons in an improved version.
Here some images of one thermal blanket before and after testing:
Attachment: 0hrs_testrun.heic (135kB)
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Attachment: 20hrs_testrun.heic (179kB)
This file has been downloaded 33 times
Dr.Bob - 8-1-2026 at 05:32
Yes, the thermal capacity of sulfuric acid is large, and the amount of heat it takes to boil a ml of water is huge, compared to most other chemicals.
So even a good lab hotplate will be challenges by this. And many cheap lab hotplates only provide 300-500 watts of heat, as they are really designed
for small amounts.
It sounds like you are making progress, slowly, but steadily. Best of luck.