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Author Topic: Unknown isopropanol contaminant  (Read 471 times)

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BROe

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Unknown isopropanol contaminant
« on: January 03, 2017, 01:15:44 PM »

I recently bought some Isopropanol in the form of Iso-Heet to be used as a cleaning and reaction solvent. According to the MSDS sheet put out by the manufacturer, the product contains 99% IPA and 1% "proprietary additive". In order to separate the alcohol from the additive, a simple distillation has always been sufficient as there remains in the boiling flask a high-boiling syrupy amber liquid.

However, when I was cleaning the recently-bought Iso-Heet, it was distilling over at 45C rather than at its usual boiling point of 83C. Checking the distillation periodically I did not notice any of the "lines" normally observed when two liquids of different densities mix (I have heard them called Schlieren but I have no idea if this is an accurate term). Despite this one sticking point the liquid I distilled had a density within 0.01 of pure IPA, it was oxidized by acidic permanganate, it has the characteristic odor of IPA, and I observed no change in the solution density when I mixed a small amount of this liquid with some of my old stock that I am certain is (or rather was) pure Isopropanol.

I suspect that this impurity must have a density very similar to that of pure Isopropanol such that when the two are mixed there is little to no observable change, and that I have an azeotropic mixture of the two as when doing a second distillation I only collected one fraction at 43C. The other option is that the liquid in question simply isn't Isopropanol. Does anybody know of any further qualitative tests or separation methods I could try, maybe there is something I'm missing? I am tempted to see if I can't have a sample sent to a lab to have an IR and NMR done but that would likely be rather costly, so I view it as a last resort.

MSDS:
http://www.servicechamp.com/images/28202msds.pdf
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Borek

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Re: Unknown isopropanol contaminant
« Reply #1 on: January 03, 2017, 09:01:49 PM »

the product contains 99% IPA and 1% "proprietary additive"

Quote
it was distilling over at 45C rather than at its usual boiling point of 83C

Strange. Intuition tells me if the additive has a low boiling and is present in very small quantities it should be lost very fast. Also, if there is an azeotropic mixture and it contains below 1% of the other component (as would be in your case) I would expect its boiling point to be close to the BP of IPA. If the whole sample boils at so much lower temperature for a long period of time something is IMHO seriously off.
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Babcock_Hall

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Re: Unknown isopropanol contaminant
« Reply #2 on: January 04, 2017, 04:59:43 AM »

Are you confident that the thermometer was low enough in the distillation apparatus?  If it sits too high, it gives a reading that is too low.
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BROe

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Re: Unknown isopropanol contaminant
« Reply #3 on: January 04, 2017, 06:31:55 AM »

The thermometer I use is jointed so it always sits in the same place in the stillhead, the last distillation I ran using it was of azeotropic nitric acid and I was getting appropriate readings there. A faulty thermometer was one of the first things I tested for after the first distillation, I checked it using boiling water as a standard and it did fine. Also part way through the distillation I swapped my analog thermometer for the digital thermocouple and was getting the same weird temperature readings.

*The picture is from the second distillation I'm currently running
 

*EDIT: In case the picture isn't showing up (as I can't see the image), the thermometer descends about two inches past the joint into the stillhead, the bottom being even with the midpoint of the arm that connects to the condenser
« Last Edit: January 04, 2017, 07:43:47 AM by Borek »
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P

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Re: Unknown isopropanol contaminant
« Reply #4 on: January 05, 2017, 03:58:58 AM »

Getting the obvious out of the way....  the pressure is the same yea? Reduced pressure is pretty good for distillation, maybe it has vac'd down too low?   Probably not, as you would know this, but just checking.  I used to reduce pressure regularly when trying to purify something by distillation.


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Babcock_Hall

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Re: Unknown isopropanol contaminant
« Reply #5 on: January 05, 2017, 09:44:55 AM »

I am by far not the best organic chemists here, but I would have put the thermometer lower, if that is possible with your apparatus.  I like to have the top of the bulb of liquid about even with the bottom of the elbow of glass.  On the other hand, I don't have an explanation for why some liquids would boil at the correct temperature but not others.
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BROe

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Re: Unknown isopropanol contaminant
« Reply #6 on: January 06, 2017, 04:57:37 AM »

Somewhat of a minor breakthrough, I was using that same isopropanol to clean some of my glassware and after a few hours of letting it air dry  I came back to see a sort of oily film coating all the flasks I had rinsed with the IPA. The film had a very heavy, sweet, crude oil type of smell and on rinsing with tap water formed a milky emulsion that was quite difficult to remove with water. I ultimately ended up washing this out with a few small rinses of methanol.

On the topic of thermometers, I just ran a distillation of methanol (also Heet brand, my backup solvent I suppose it could be called) and I was getting temperature readings that were pretty spot on, my digital thermocouple probe that sits about half an inch higher in the still head than my analog thermometer was getting a reading of 65.5, within acceptable levels of uncertainty for the thermocouple. Could it be that the greater volatility of methanol over IPA is negating the effects of a thermometer placed higher in the still head? Perhaps running tests with liquids of a low vapor pressure and higher boiling point would be more definitive.

Getting the obvious out of the way....  the pressure is the same yea? Reduced pressure is pretty good for distillation, maybe it has vac'd down too low?   Probably not, as you would know this, but just checking.  I used to reduce pressure regularly when trying to purify something by distillation.

No I wasn't pulling a vacuum, though that is something I would like to try in the future.
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Re: Unknown isopropanol contaminant
« Reply #7 on: January 11, 2017, 12:00:33 AM »


No I wasn't pulling a vacuum, though that is something I would like to try in the future.

Makes distillation much easier....  and you can distil heat sensitive chemicals due to the greatly reduced temperatures you work at. You can get pressure/temperature curves which are east to read off with a ruler that will direct you as to your target temps and pressures for your system.

Babcock hall might have a point with that thermometer placement - I didn't see it. Worth a try if possible to set it lower?
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