Sciencemadness Discussion Board - Is there any trick to get stir bars to stay centered and spin on flat bottom flasks?

Is there any trick to get stir bars to stay centered and spin on flat bottom flasks?

RogueRose - 25-7-2020 at 01:44

I never realized how much a PITA this is to get the stir bar to stay centered and not wander off and start doing it's own dance to the side of the flask. I've been alright using 500ml & 1L flasks, I seem to be able to get the stir bar to spin, but when I used a 4L, it's impossible.

I drew concentric circles on my hot plate, each the size of a beaker or flask so I could center the glassware over the right spot, and that made things easier, especially with smaller flasks on a large hot plate (10" square).

My stir bars are the oval type with rounded bottom and it comes to a point on the ends and I have the oblong straight bars with a ridge in the middle. I found both are kind of a PITA, especially the oval types in the 4L flask.

I've tried just about everything form starting slow and increasing 1/20-1/40th of the max speed every minute or so and I can't seem to get anything past level 2 on the 4L - it just starts jumping around then stalls, but the motor spins. It works fine in many flasks, especially with smaller stir bars but when I use larger ones, it's just not working out.

Does anyone have any experience with this that could offer suggestions?

mackolol - 25-7-2020 at 01:46

If you're going to stir bigger volumes, you need bigger stir bar. If it's too small it will do the thing you described. Do you have the same issue with bigger ones?

[Edited on 25-7-2020 by mackolol]

Tsjerk - 25-7-2020 at 01:53

If the stir bar has too much friction, I will start jumping. So here your 4 liters of liquid is to heavy to get at high enough speed. Probably because the stir bar is too small.

A bigger stir bar could help, but only if the stirrer is heavy enough to maintain connection with the bigger bar. The magnet in the stirrer still has to be able to move around four liters of liquid.

CharlieA - 25-7-2020 at 15:46

I seem to have a problem in that flat bottomed beakers/flasks aren't really flat bottomed, but convex; the stir bars then that a tendency to slip to the edge of the container. The stir bars with a ring about their center seem to be less prone to slipping. Stirring speed and size of stir bar also seem to be factors.

RogueRose - 25-7-2020 at 16:47

Thanks for the replies. I think I'm going to order larger bars if I can find them. I think my largest is 35-40mm. I did realize there is something wrong with my speed knob and it is cutting in and out but it is very hard to tell when, so I think it is causing problems with keeping speed constant. I think it's just a potentiometer, so it should be easy to replace.

karlos³ - 25-7-2020 at 17:17

You know, my largest stirbar is 7-8cm long, I usually use it for things which do not only have lots of volume, but also lots of solids.
Like my bioreductions with celery gratings for example.
It is still the stirbar with which I have the most problems, I have to get it to stir with lots of care until it runs properly.
Usuall swirls the mixture by hand until I start the stirring too.
I think my stirrer is too weak for such things, as it can get easily out of control during these things.

Heptylene - 26-7-2020 at 03:48

Get large and strong stirbars, you won't regret the investment. Also, some stirbars have a ridge in the middle to stay centered. This is nice, and it also prevents wearing of the PTFE coating over time.

Pumukli - 27-7-2020 at 04:59

In my experince they make the most annoying clicking sound too. :-) (When stirring is relatively slow.)

CouchHatter - 27-7-2020 at 09:07

A cross-shaped stirbar will fix this, at least it did for me. I thought my hotplate stirrer magnets had deteriorated to nothing, but after using a cross-shaped stirbar I don't think there's anything wrong with the plate. My large erlenmeyers are also very convex on bottom, I think it helps to have a non-round stirbar. Make an octagonal one would work too but I haven't tried any of those.

Also they make neodymium stirbars that are green but I would only try those before replacing a hotplate stirrer. They are pricey.

Tsjerk - 27-7-2020 at 09:08

Quote: Originally posted by Pumukli

In my experince they make the most annoying clicking sound too. :-) (When stirring is relatively slow.)

That sound is terrible!

RogueRose - 29-7-2020 at 01:52

I've experienced the clicking too, what a PITA! I was just using one of my stir bars in a clear solution and I noticed something that I'm not sure if it is normal or not. The stir bar is oval with pointed ends, and basically 4 sided with flat sides that come to a point on either end.

IDK if it is supposed to spin on the curved part or the flat sides. The bar is not balanced correctly (which I'm just noticing now) and I can't get it to sit on the rounded side at all w/o it flipping to the flat side - almost like there's a magnet pulling it to to sit on one side, but it was on a wooden table w/ no metal around it.

I'm not thinking that there is a reason many of these stir bars are so cheap - they are probably supposed to be balanced. I notice that I haven't seen the word "balanced" in any of the cheap stir bar packs..

This is what the one I am using looks like.

Dr.Bob - 29-7-2020 at 10:59

The oval stirbars are designed to work in round bottom flasks, not flat ones, They will never work well in flat bottomed flasks. Simple hexagonal or cross ones work much better in erlenmeyer flasks and beakers. They also make circular type ones that work well in flat bottom vessels.

RogueRose - 29-7-2020 at 22:22

Quote: Originally posted by Dr.Bob

The oval stirbars are designed to work in round bottom flasks, not flat ones, They will never work well in flat bottomed flasks. Simple hexagonal or cross ones work much better in erlenmeyer flasks and beakers. They also make circular type ones that work well in flat bottom vessels.

Thank you for pointing that out, I hadn't thought that these were meant for RBF's only, though I did know that at one point, lol.

I did a search for the circular ones and didn't seem to have much success finding any, or at least what I was picturing. I did find some very interesting and unique ones that I never knew existed.
https://www.rosesci.com/Products/products.php?search_go=Spin...

Eddie Current - 30-7-2020 at 16:43

It's a trade off between the strength of the stirrer/bar magnetic properties, how the torgue imparted by the revolutions impacts, and the viscosity of the solution in question. There's no simple answer.

Then you look at stir bar design:

https://www.belart.com/corporate/Resources/Stirbar%20Shape%2...

This is a very clever design:

https://pubs.rsc.org/en/content/articlelanding/2013/ob/c3ob4...

Also, if you're contemplating very touchy or temperamental reactions (e.g. Grignards), then you might want to read this:

https://www.chemistryworld.com/news/warnings-that-dirty-stir...

Dr.Bob - 31-7-2020 at 17:19

I hope that I am not being too commercial, but if anyone wants to try any of these, I still have a few or more of almost every one of the types made, other than the rare earth ones, and I am out of most of the ovals for RBFs, other than very small and very large, and out of the simple hexagonal ones excepr for 2" and larger, mostly new. I have both new and used in many of those, like angled vane, fleas from 3-15 mm, crosses, circles, etc. The problem is that I have a few of about every one of them (not kiding, I bet I have at least 50 types/sizes), not a 100 of each, so it is hard to create a listing for each on ebay. So if people want to try an assortment of these, for about 10-20% of the list price, let me know. Many are new, some are used once or twice, and a few are very used. But I can create any mix that you want, of the ones that I have.

zed - 13-8-2020 at 20:07

Stir-bar problems? Brown at one time, was using flasks and bottles, with concave bottoms.

Not easily achieved these days, with the high cost of labor, and the disappearance of glassblowers.

Wandering stir bars, are not a problem for me, at room temperature. I use relatively small cylindrical Neodymium magnets shrouded inside of HDPE tubing. Powerful magnetic coupling allows me to rage.

Get to warmer temperatures, and the magic departs. Neodymium magnets lose their mojo. Gotta somehow provide a high degree of coupling, possibly via Samarium Cobalt. Saw some on eBay, that weren't offensively priced, but they were squarish rather than cylindrical. Another problem.

Could modify the stir plate itself, to pack stronger magnets. But, it starts to become a complicated project.

It would be so much easier if those Samarium magnets were cheaper. Ummm. Perhaps they are actually cheap, if they will do the job? Sometimes you just have to bite the bullet.

Maybe Dr. Bob, has something that can help you.