Sciencemadness Discussion Board

Help With Magnetic Stirring Bars

zoombafu - 30-12-2011 at 11:21

Hi, I have recently upgraded myself from a flat-bottom-flask-with- rubber-stoppers-and-all-sorts-of-connecting-tubes distillation setup, and now I have a RBF ground glass joint setup. The question that I have is what size magnetic stirring bars do i need for a 2000 mL, 1000mL, 500mL, and 100mL RBF. Also do you need a stirring bar in a 50mL(or smaller) flask? Any help would be greatly appreciated.

Magpie - 30-12-2011 at 16:31

I have 5 sizes of stir bars:

3"
2"
1"
1/2"
1/2" (lightweight)

I have used all of then at one time or another. My largest flask size is 1000mL. The 1/2" and 1" probably get used the most.

peach - 31-12-2011 at 03:10

Have a look on the Bel-art site here. When you click the bar, the description will give you a flask size recommendation.

There are stir bars for 50ml flasks yes. There even tinier bars for 5ml flasks. And tinier again for stirring vials. A topic of interest recently has been stirring sub 1ml solutions.

Stirring bigger flasks, like 1 and 2l can be difficult nay even a little dangerous, as the bars are prone to decoupling from the magnet underneath and go whizzing round the flask. They're heavy enough, and can be moving quickly enough, to potentially pop the walls of the flask.

Whether or not it needs stirring is another matter. Quite a lot of the reactions I do in tiny flasks, I will stir. Particularly those under vacuum. With the big 2l flask, I tend not to even use it. If I am using it, it's probably to distill a lot of solvent and doesn't need stirring anyway.

If you can run something in a beaker (it doesn't need condensing or an inert atmosphere) that is by far the best way to stir things; with a big fat ass flat stir bar. Some of the things I've done in a beaker would be impossible to stir in an RBF with a magnetic bar. They are good for big neutralisations, especially if you have things going from solids to solutions, where they will need a lot of stirring and time to completely react and homogenise.

There are neodymium stir bars for reactions up to 150C. The neodymium magnets are much stronger and less prone to decoupling, but they loose their magnetism at higher temperatures.

Various stirring thingies.
<a href="http://img687.imageshack.us/i/img2120ja.jpg/" target="_blank"><img src="http://img687.imageshack.us/img687/4116/img2120ja.jpg" alt="Free Image Hosting at www.ImageShack.us" border="0"/></a><br>

The one in that 50ml flask is about 6-8mm (quart inch odd), but the flask is full of benzene and other filthy shit which I don't want on my hands at 9am, so it's not on display at present.
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A 10ml RBF
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The strangely shaped flea bars will stir things like this 50ml pear shaped distillation glass. These bars can also be cut into different shapes for weird tasks. Despite owning much more expensive glass, this piece was remarkably cheap and I end up using it on a regular basis. Unlike the vacuum jacketed glass, I can warm the sides of this up with a blowtorch if something tries solidifying on the way out.
<a href="http://img580.imageshack.us/i/img2118y.jpg/" target="_blank"><img src="http://img580.imageshack.us/img580/1266/img2118y.jpg" alt="Free Image Hosting at www.ImageShack.us" border="0"/></a><br>

[Edited on 31-12-2011 by peach]

Arthur Dent - 31-12-2011 at 07:33

Ha hah, Peach, we seem to shop at the same place!

Anyway, in the image above, you see between the condenser and the vacuum spout one of these tiny "grain of rice" teflon magnets (I bought a bunch of these) and they are quite useful in a pear-shaped bulb such as this since they tend not to rattle at all when they reach a specific speed. I also use them extensively in test tubes. They come in lengths from 4mm to 1.5cm.

I've used this mini distilling kit only once and I had set-up a cylindrical sandbath on my hotplate using a flat-bottomed soup can that fit the bulb quite well. But this is one of those kits that I use quite seldom.

My biggest bar is 5 cm long. I use it with my 1 liter erlenmeyer. It stirs up quite a storm in a jiffy. I wish I had a few of these football-shaped bars though, heven't seen any at my glassware shop.

Robert

zoombafu - 31-12-2011 at 11:52

Thank you very much peach and arthur!

Ive been looking for a site that has those flask recommendations, so thank you for that bel-art link.

I remember that once I was mixing a nutrient broth solution solution in a beaker, and the magnet decoupled, bounced once or twice and the beaker busted. The broth got all over my hot plate and started burning, the smell was horrible :( .

Dr.Bob - 3-1-2012 at 13:58

I have a bunch of oval shaped stir bars available still. I only had a few a while back, but have found an entire bag of them. Most are used, some look new. Most sell for $10 or more on Bel-Art or VWR, I sell most around $3, and if you want an assortment of oval ones, I can provide a assorted mix of 5 sizes for ~$10. The only size I have very few left of is the 3/4" x 3/8" oval (which work well in 14/20 glassware). I have lots of the micro sized ones which are about 3/8 x 1/8", and many of the larger sized ones.

I will caution people that you need to consider the size of the joint, if you have a 14/20 joint, you can only fit in the smaller ones, if you have 24/40 joints, you still cannot fit an unmodified 3/4 dia. oval in it. (We often trim them with a razor to fit in side a 24/40 joint flask if we want good stirring in a big flask.)

Bob

Panache - 6-1-2012 at 04:12

Peach are you an aristocrat (with an endowed income) with a castle or stately home within which you sit and muse by windowside, hounds attentively splayed at your feet.

I think you are and its great, please don't dispel this myth if its not true as i find its possibility enjoyable, although i do hate anything other than strickly egalitarian virtues.

I am Australian after all, the home of egalitarianism and its dark underbelly the tall poppy syndrome.

That is to say keep posting as such, good show.

Magpie - 6-1-2012 at 11:28

Quote: Originally posted by Panache  
...muse by windowside, hounds attentively splayed at your feet.


Good one Panache...I've wondered about this myself.

But more likely Persian cats winding around his feet.
Quote: Originally posted by Panache  

I am Australian after all, the home of egalitarianism and its dark underbelly the tall poppy syndrome.


Please explain this syndrome.

starman - 6-1-2012 at 19:04

Quote: Originally posted by Panache  
Peach are you an aristocrat (with an endowed income) with a castle or stately home within which you sit and muse by windowside, hounds attentively splayed at your feet.

I think you are and its great, please don't dispel this myth if its not true as i find its possibility enjoyable, although i do hate anything other than strickly egalitarian virtues.

I am Australian after all, the home of egalitarianism and its dark underbelly the tall poppy syndrome.
That is to say keep posting as such, good show.

Got to agree with Panache about Peach.He has an incredible array of equipment (drool) for a young guy.

Bot0nist - 6-1-2012 at 19:40

The tallest poppy get snipped first.

http://en.wikipedia.org/wiki/Tall_poppy_syndrome


[Edited on 7-1-2012 by Bot0nist]

Arthur Dent - 7-1-2012 at 04:19

From what has been described above, here's how I imagine our friend Peach:



;)

Robert

zoombafu - 7-1-2012 at 10:39

It is possible that this is his lab...

Peach's Lab

Hexavalent - 8-1-2012 at 08:52

Back onto the topic of stir bars, I've heard of people making their own by dipping ordinary magnets into wax. What are your thoughts?

zoombafu - 8-1-2012 at 11:37

Quote: Originally posted by Hexavalent  
Back onto the topic of stir bars, I've heard of people making their own by dipping ordinary magnets into wax. What are your thoughts?


While that does sound like a possibly good idea, they probably wouldn't be able to be used at high speeds and stirring hot solutions.

peach - 9-1-2012 at 01:13

Ha ha!

I was just thinking of Monty Burns laying in his clean room, {muffled}"Release the hounds!"{muffled}.

Magpie is right though, I have kitties, which sit 20ft up on the roof in the rain meowing at me through the windows because they can see me doing something below.

I have seen that guys lab, I think he lives in Ireland. I'm not sure. But it's amazing if that's his house. No lab is complete without a pipe organ to play evil overlord music on whilst waiting for something to finish.

Dipping magnets in wax I doubt is such a great idea. The chances of the wax being dissolved, attacked, contaminating the reaction, taking part in the reaction or making the results a serious mess to clean up are high enough for it to be a downer.

Because they are spinning, the wax will gradually wear away or could be knocked off in places as it collides with things in the flask and rubs against the bottom. Warm it up at all, or run a reaction that gets warm on it's own, and the wax will be off. It'll also be more prone to shattering if you cool it down.

Any defect in the wax will allow the reactants contact with the magnet. Given how harsh some reaction conditions can be, the magnet it's self will certainly react with a lot of them.

Compare the cost of your reactants, solvents and substrates, and your own wasted time and effort, with spending a few dollars on a PTFE bar, which you can continually pound with just about anything you like and it won't do anything.

Stir bars used to be glass encapsulated before isostatic PTFE molding came along. That's a better option and something you can easily achieve at home with a blow torch. But it still has issues when compared with PTFE; glass cracks rather easily, and you will likely have a pocket of air inside it, which will try expanding as things warm up.

Another option is to buy a length of PFA or FEP tubing from eBay. Those are both sister polymers of PTFE but, quite unlike PTFE, they can be manipulated more akin to normal plastics, by hot forming (they can be injection molded). You could drop a magnet into a length of that and then hot crimp the ends shut. By the time you've bought some, paid the postage and done it, you'd still be better off getting an actual stir bar.

Platinum is often used for engagement and wedding rings. PTFE is as incredible in terms of it's resistance. It is even more incredible that it is so cheap and widespread. Do not ever attempt to use that reasoning to explain a teflon wedding ring.

[Edited on 9-1-2012 by peach]