Solvent Residue

[Ovidroid]

Hello All,

I recently acquired some n-heptane in the form of a brand well-known and once discontinued. It is now sold by a different company than it once was, but the MSDS says it contains 100% n-heptane.

I did perform an evap test on it some days ago, and I thought it turned out fine at the time.

Today, however, I looked into the can and noticed some loose particulate matter at the bottom of the can. I am color blind so I can't tell you what color it is, but it almost looks like dirt or small rusty metal flakes or something.

I decided to perform another evap test given that I have used more of the solvent since the first test and that means I have been drawing ever closer to that sediment.

I put about 20mL into a clear pyrex baking dish and let it dry. Without looking particularly closely it looked fine (i.e. if some random person were to look at it they'd probably say it was "clean" -edit, the forum wants to put in a satisfied emoticon here when I am anything but satisfied). But when held up to a light I noticed some sort of faint hazy residue which pretty much traced out the progression of evaporation. I thought maybe it was because I evapped in the garage.

So I cleaned the dish and put a few more milliliters into it and let it evap in the house. There was less residue, but there was still something there.

I feel a bit desperate here. When evap testing your solvent is there ANYTHING left at all, or is the dish squeaky clean?

I just finished the extraction using this solvent and I will not be able to perform another extraction for who knows how long. I really don't want to throw out my spice.

I am also thoroughly upset that this well-known solvent, typically used for re-X, is NOT apparently evapping clean, and I have no idea why.

Do any of you have any suggestions? I know I shouldn't take a particular brand for granted (many of you are aware of the relatively recent chemical changes in a popular brand of naphtha), but it's frustrating that something advertised as 100% n-heptane is leaving ANYTHING at all in my dish. And I have no idea if that residue is in my spice.

 

[Ovidroid]

Follow-up:

I put a few drops on a different glass surface and this time blew on the solvent until it evaporated.

There doesn't seem to be residue in the center of where the solvent was, but only where the edges of the solvent pools were.

Each dried-up solvent pool looks like a water spot (you know, after washing dishes and letting them dry) except that the insides seem clean and the outsides have some sort of residue.

The residue is fairly resilient. There is resistance when I run my finger over it, versus the clean glass, and when I wipe over it with a dry paper towel it smears somewhat.

The glass was clean before. Could it be soap residue from when it was previously cleaned?

I may be grasping at straws, but I don't want to lose my spice.

I am very, very, very irritated about this.

Any thoughts or suggestions would be appreciated.

 

[downwardsfromzero]

The only true answer would be to distil your solvent if you're not wanting to buy new, laboratory grade material.

Then use the purified solvent to recrystallise with.

Distillation of flammable solvents requires a modicum of expertise, especially with regards to safety.

There is a possibility you're looking at soap residue on the glass. This would be removed with a hydrochloric acid rinse followed with acetone, but then the same purity questions would apply for those liquids as well.

Your spice can also be cleaned up with a mini A/B. You would have to ensure your solvent doesn't contaminate the product and there's maybe a couple of ways to do this without having to distil:

*Put your solvent in the freezer before use and then filter it. Of course, it needs to warm up before using in the extraction!

*Treat the solvent with activated charcoal, then filter it. Celite or bentonite will help with the filtration here.

If you've chosen an aliphatic hydrocarbon as the extraction solvent, i.e. naphtha or heptane, only collect the crystals by freeze precipitation, not complete evaporation.

 

[Ovidroid]

Thanks for your response, downwardsfromzero.

Distilling is not an option for me, for a number of reasons, not the least of which is my lack of any experience or equipment suited for the task.

I am done with this particular can of solvent, it's dead to me. I'd punch it in the face if it had one, and I'm not a violent person.

I considered ordering lab grade heptane, but the turnaround time was not suitable for me at this point, plus I am a *little* paranoid to order something like that, even though heptane is not a watched chemical as far as I am aware. The go-to brand, however, is mostly legitimate to naive onlookers and everyone else's experiences with it sound like flowers, rainbows, and unicorns.

So, I ordered two more cans of this go-to brand, hoping that at least one of them will NOT have this sediment at the bottom and will evap clean. If they are suitable, then I will recrystallize with the clean solvent, since the impurity is clearly soluble in heptane, and hopefully greatly reduce the presence of this offensive substance. Perhaps a couple recrystallizations to be sure.

Is your suggested mini A/B intended to convert the freebase to salt to shake loose any of the contams from the offending solvent? I have never performed an A/B procedure, but what I read about it suggests that it would take care of impurities and oils from the plant. Would the same principle apply to solvent impurities?

 

[downwardsfromzero]

Is your suggested mini A/B intended to convert the freebase to salt to shake loose any of the contams from the offending solvent?

Exactly. Keep all reagents and solvents to the minimum required and only use freeze precipitation for recovery of the target material. Keeping the cooling process as slow as possible will help to keep contamination from solvent impurities to a minimum.

Re-using the solvent will help to minimise losses in the long run.

I have never performed an A/B procedure, but what I read about it suggests that it would take care of impurities and oils from the plant. Would the same principle apply to solvent impurities?

Yes, to a certain extent although you'd need to be using a clean solvent to be sure of this of course. Being as you'll be pulling back into solvent at some stage. Once you've got a solution of freebase in non-polar solvent there are several options available. Naphtha and heptane arguably are favoured to a large extent on aesthetic grounds. The often impressive crystals that can result from freeze precipitation certainly have their appeal.

Sodium carbonate and isopropanol would give a potential workaround from your situation without having to worry about the contaminated heptane.

 

[GordoTEK]

I'm curious about this subject, when people say there should be no residue, is this an exaggeration? Sure, there should be nothing waxy or powdery, but if you evaporate virtually any liquid including water, on a super clean piece of glass (like a microscope slide) then get a bright flashlight and angle the glass just right so the light is hitting from a side angle, aren't you pretty much always going to see some type of "watermark like smudge"?

I was reading another thread where a guy was doing evap tests with 3 different, supposedly ultra-pure, lab grade solvents, and he reported that all 3 left this very minor "watermark" type residue after evaporation.

I ask this because every evap I've done has left behind some minor mark, and I'm wondering if this perfect evap concept is urban ledgend? If you look straight on, without super critical examination and angled lighting, it looks like nothing was left behind -- is this what people really mean by an evap with "no residue"?

 

[concombres]

I'm curious about this subject, when people say there should be no residue, is this an exaggeration? Sure, there should be nothing waxy or powdery, but if you evaporate virtually any liquid including water, on a super clean piece of glass (like a microscope slide) then get a bright flashlight and angle the glass just right so the light is hitting from a side angle, aren't you pretty much always going to see some type of "watermark like smudge"?

I was reading another thread where a guy was doing evap tests with 3 different, supposedly ultra-pure, lab grade solvents, and he reported that all 3 left this very minor "watermark" type residue after evaporation.

I ask this because every evap I've done has left behind some minor mark, and I'm wondering if this perfect evap concept is urban ledgend? If you look straight on, without super critical examination and angled lighting, it looks like nothing was left behind -- is this what people really mean by an evap with "no residue"?

I have been wondering this same thing forever. I have been fine using hardware store naphtha that left the faint puddle outlines. Im not sure this is healthy but i am still here to talk about it today.

I dont mean to suggest that this is safe in any way but then i dont think anyone can really answer that for sure without knowing what the puddle mark residue is composed of.

I have thought about having individual containers of solvent analyzed before use but im not sure how one would go about that without drawing suspision & spending a couple hundred dollars.

 

[downwardsfromzero]

I have thought about having individual containers of solvent analyzed before use but im not sure how one would go about that without drawing suspision & spending a couple hundred dollars.

You'd be better off spending that money on some decent distillation apparatus and be sure of your solvent with every batch.

 

[Ovidroid]

Thanks, downwardsfromzero.

I just received the new heptane. They both also have the strange sediment. What on earth could these sediments be? Each of three cans of this stuff has it. It doesn't float, it seems to be very very light (just from looking at its behavior in the can) and red or brown or something like that. It wouldn't be something added to dry it out since heptane isn't miscible with water, yet it's present in three different cans of this stuff, which suggests either sloppy production/packaging with something that is shedding particles into the product or this stuff is purposely added.

I evapped a few mL from each of these cans as well and still noticed the watermark effect.

For something that is advertised as 100% heptane, I just don't understand what could be left behind when this is evaporated.

I will start by doing a simple recrystallization.

Then I think I might look into your sodium carbonate/isopropanol idea. Of course, finding highly-pure isopropanol isn't just a matter of walking into the local drugstore, they tend to carry max 70%. I guess I need to do more research on that aspect. I had never considered that approach.

 

[Ovidroid]

GordoTEK and concombres, that is interesting that you guys have watermarking from your solvents, too. I wonder how widespread this really is for those of us who use off-the-shelf solvents.

I can't afford, and have no experience with, distillation equipment as suggested by downwardsfromzero. I would love to learn how to do that, but it isn't an option for me for a number of reasons, so I'm stuck with off-the-shelf options. I may need to look into activated charcoal as downwardsfromzero also suggested.

I have noticed as the solvent evaporates that it starts to look like it has dirt or dust trapped in it (though it definitely isn't dirt or dust) or like bubbles form. I have watched one of these "bubbles" dry and it left a little deposit of the watermark residue. The solvent is perfectly clear and uniform, though, until it is distributed in a thin layer and left to dry.

I sure wish I were a chemist right about now, with all kinds of fancy lab equipment, to figure this out. But I'm not. So I look at this and scratch my head and wonder what the heck this stuff is and why an MSDS says 100% heptane when there is clearly other stuff here.

 

[downwardsfromzero]

I may need to look into activated charcoal as downwardsfromzero also suggested.

To be honest, you'd struggle without suction and some kind of glassware.

If you're not treading that path you'll have to find better solvent or an alternative method entirely.