RogueRose - 25-6-2018 at 22:05
I'm going to order a set of 8 magnetic stir bars from china ($4 so it's worth a try even if they aren't good)
I'm basically looking to use these in beakers, Erlenmeyer flasks, RBF's and flat bottom round flasks with standard solutions, nothing really thick
like CaSO4, BaSO4 but I need to try to dissolve some Ca(OH)2 in both Na/K carbonate solutions to try to make their hydroxides.
$4 shipped
Diameter Length
1 * 4mm 8mm
1 * 5mm 10mm
1 * 7mm 15mm
1 * 8mm 20mm
1 * 7mm 25mm
1 * 8mm 28mm
1 * 8mm 30mm
1 * 9mm 35mm
& $6.50 for these
4x8mm
6x10mm
7x15mm
8x20mm
9x25mm
10x30mm
11x35mm
15x40mm
I've seen some others but they are a little more pricey. like $6-10 each for some of these.
Sulaiman - 26-6-2018 at 02:55
IMO the first type are best - for flat-bottomed vessels,
as there is is a lot of magnet compared to ptfe.
I have the second type as the shape is more suitable for RBFs
I have not tried the other shapes but they look suitable for flat-bottomed vessels only.
Herr Haber - 26-6-2018 at 05:42
I have a couple of star shaped ones that were given to me. They're for test tubes.
The first kind and second kind I use most often. I also have triangular shaped ones to stir stuff that'll create a lot of solids *while* rotating.
Obviously they're very ineffective to dissolve a solid but are perfect to keep it suspended.
Finally, for extra thick slurries I have some dumbbell shaped. These I truly loved. They're also perfect when you're agitating something that might
not like friction too much...
alking - 27-6-2018 at 11:59
If I could only have 1 set I'd go with the first, but I use both #1 and #2 depending on the application. I almost, but not entirely, only use flat
bottom flasks. Those dumbbells sound interesting, I might have to pick a few of those up.
walruslover69 - 27-6-2018 at 12:19
This shows a pretty good guide to most shapes https://www.belart.com/corporate/Resources/Spinbar%20Shapes%...
Sigmatropic - 28-6-2018 at 11:27
It took me 5+ years in chemistry to encounter a Circulus stir bar. A colleague of mine, who's almost retired after a lifetime in chemistry, had never
seen one. Seemed to be especially helpfull to reduce the formation of molecular seive dust. I would simply remove that dust by decantantion or use
diffusion instead of stirring (that is, waiting 24h+) but for every pot there's not only a lid but also a stirbar 
.
Edit: I did notice that the stir bars of this almost retired colleague had taken on the shape of a stirring egg. Just imagining how many cycles
that must've took makes my head hurt.
[Edited on 28-6-2018 by Sigmatropic]
weilawei - 29-6-2018 at 14:27
Anyone happen to have one of the dumbell shaped ones for sale? I've had a real run lately of stirbars getting stopped up, and that seems like it might
do the trick. Preferably sized for the 500mL to 1000mL RBF range? eBay has them in 5 packs for around 65$, and Amazon will sell me one for around 21$.
nimgoldman - 1-7-2018 at 12:03
For beakers, I like to use the "bars" which are round or triangular (make stronger vortex and more easily dissolve solids).
The bars with ring have smaller friction.
For round bottoms, I like to use the oval ones, here is the reference table from my local lab supplies provider:
20x10 mm: 10 - 100 ml
25x12 mm: 100 - 200 ml
30x15 mm: 250 - 500 ml
40-50 x 20 mm: 2 000 - 3 000 ml
Finally I like using the star one for reducing bumbing during distillation but the oval and straight ones work just as fine.