At one point, I was doing an ETN synthesis, and didn't have anything that was both strong enough to stir the mixture with, and wouldn't also react
with the concentrated nitrating mixture. I usually have a bunch of bamboo skewers on hand when doing any sort of chemistry on the cheap. They make
great disposable stir rods for reactions that aren't very sensitive, and the larger ones are quite strong. I wasn't interested in turning them into
nitrocellulose though, so I tried to find something to use as a cover. Glass tubing would be too much work and tends to crack easily. I had a bunch
of used polyethylene disposable pipettes though; cutting the tip off of one allowed the skewer to fit inside easily. However, it wasn't very sturdy
as-is, and was hard to use to break up clumps. So I took some plaster of Paris, filled the pipette like halfway up with the powder, then added water,
and used the skewer to mix it all up. Eventually, the skewer got stuck in the pipette, which is, of course, what I wanted. It worked better than
anything I'd ever used for stirring an ETN nitration mixture as well; the bulb on the end worked great for breaking up clumps against the side of the
beaker. Even though I used a bamboo skewer, there's no reason you couldn't use a metal rod instead, to make it even sturdier. The only downside is
that the thermal resistance only goes up to about 100C, but for plenty of reactions (especially nitrations) that isn't an issue.ficolas - 17-12-2016 at 13:56
I once used a metal bar covered in teflon wrap for piping, it was surprisingly good, as liquid didnt reach the metal, and the teflon didnt unwrap.
However what I was stirring wasnt really thick, so I dont know how it would work for a thick mixture.
Edit: I misinterpreted the question, I used this for a magnetic stirrer, covering with teflon wrap a whole big rod may not be that practical.
[Edited on 17-12-2016 by ficolas]j_sum1 - 17-12-2016 at 14:20
I use glass stirrers exclusuvely. About a third of mine are broken thermometers that I have melted the sharp edges off.Melgar - 17-12-2016 at 18:43
I've used glass rods before too, but they're nowhere near as strong as the one I made out of a bamboo skewer and a plaster-filled pipette. After all,
if glass stir rods were very strong, you wouldn't have so many broken ones. :-p
Seriously though, glass thermometers pieces and rods in general work fairly well, but having that bulb on the end of the stirrer made it a lot more
effective at both stirring the mixture and breaking up clumps. I believe this was a 7mL pipette, so pretty good size. NeonPulse - 18-12-2016 at 04:42
When I first started out chemistry I also did an ETN synthesis and my stirring rod was 2 lengths of a solid fibreglass rod with steel wire and epoxy
bracing the middle. It can stir the thickest mixtures with no problems and was quite resistant. It stood up to several nitrations before I got some
glass rods. It only changed colour slightly.I still use it occasionally.careysub - 18-12-2016 at 18:28
Interesting fact:
The key breakthrough that allowed large blanks of optical glass to be cast, making large telescope lenses possible, around 1800, made by Pierre Louis
Guinard was the adoption of fire clay stirring rods for stirring the glass melt, a technique that remained in use until about 1945 when it was
replaced by a platinum paddle (poor amateurs doing optical glass casting today still use fire clay rods).
In large commercial melts a solid fire clay rod was replaced by a fire clay tube in which a water cooled pipe was inserted for strength and cooling.
Along that same line of thought - glass tubes could be reinforced with metal rods, though for this to be effective, you would probably need to heat
the glass so that it bonds to the rod, and in that case you would need a good match for the coefficient of thermal expansion.
Titanium and 410 and 430 stainless steel closely match soda lime glass. Borosilicate has very little thermal expansion so Invar or Kovar would
probably work (that is why Kovar was developed).
[Edited on 19-12-2016 by careysub]Elemental Phosphorus - 5-1-2017 at 05:54
Could a metal rod with a passive oxide layer be used? Seems strong and cheap enough, although passivation with a metal like steel might have to be
done on a case by case basis.MrHomeScientist - 5-1-2017 at 06:55
What's wrong with glass rods? Readily available online. If you don't want to go that route, they sell them as fancy drink stirrers all over the place.Melgar - 5-1-2017 at 10:55
Well, the stir rods I described were made from junk lying around, and thus were free. Also, they're considerably stronger than glass stir rods.
They're also too soft to scratch or chip the sides of the vessel, which could be really important when dealing with more sensitive compounds.
Could a metal rod with a passive oxide layer be used? Seems strong and cheap enough, although passivation with a metal like steel might have to be
done on a case by case basis.
Anodized aluminum might be usable. After all, nitric acid can be stored in aluminum vessels. The sulfuric acid might complicate things, since
aluminum can reduce it to sulfur dioxide or hydrogen sulfide, but seeing as it's an oxidizing acid itself, that may not be an issue. The only problem
I can foresee is that many energetic materials become sensitized in the presence of aluminum, tannerite being the most obvious and well-known example.
