Magnetic Stirrers

[BobUK]

Just mixed some sodium hydroxide in water using a magnetic stirrer.
Toward the end of the mixing I noticed gas bubbles coming from both ends of the magnetic stirring rod, and a black substance on each end.
There has to be an almost invisible crack on the seams of the plastic. The black stuff possibly coming from the magnet inside.

So I need to buy a couple of spare magnets. The thing is they are sold in all sorts of shapes.

Olive Cone, like a rugby ball.
Parallel, some with a rib around the centre.
Cross shaped.
A few peculiar ones looking similar to a Calthrop or concrete sea defence.

Are the various shapes used for mixing different forms of materials?
For example, powders, crystals, liquids, thick syrups etc..

 

[Saganich]

Well, most of the ones I have are octagonal with the pivot ring. The pivot ring helps keep the bar off the bottom and creates nice stable vortex but can cause spin out at high speeds. The octagon shape creates additional turbulence, which is good for high concentration stirring like 100g of sodium sulfite in a liter. Ball shaped are for round bottom flasks, not sure about the more odd shaped ones... never seen anyone using them. Cross would be good for taller flasks i suppose.

 

[grahamp]

You need a simple bar type if your container has a flat base. Most of the unusual shapes are for fluids with more viscosity, or where there is a lot of sediment, or the base of the container is curved.

I think the rule of thumb is that the length of the bar should be around 1/3 of the diameter of the vessel, so for a 250ml beaker something in the 20-30mm range.

For photographic purposes you just need to stir the liquid without causing a strong vortex.

Most of the companies selling these give an indication of the intended use. I always used cylinder or cylinder with center rim in the lab for mixing basic chemical solutions.

 

[xkaes]

The only ones I have are sealed in clear glass. Work great so long as you don't drop them.