Agitation, mixing and grinding

What is agitation, mixing and grinding?

Most organic reactions require agitation, mixing and grinding due to various circumstances such as:

  • To help dissolve one of the components.
  • Proper mixing of the reagents when they are poured into the reaction.
  • Mezclas de disolventes inmiscibles.
  • Avoid the formation of lumps due to the appearance of precipitates.
  • To generate a homogeneous boiling.
  • Facilitate the course of heterogeneous phase reactions due to the presence of:
    • Solid-phase reagents.
    • Solid-phase catalysts.
    • Resin exchangers.

The most frequent methods in a laboratory are:

Magnetic stirring

It has become the most popular laboratory-scale stirring system. The vessel (Erlenmeyer flask, flask, beaker, etc.) is placed centered on top of the stirring plate. A magnetic bar (stir bar) is placed inside the vessel and coated with an inert material (polypropylene or Teflon), which is commercially available in different shapes depending on its application.

To avoid accidents caused by the vibration produced by the rotation, it is highly recommended to fix the flasks firmly with a clamp and to regulate the rotation speed appropriately. If the speed is too high, splashing or even a shock may occur, which could break the glass of the vessel, or if it is too slow, the agitation may be ineffective, especially in the case of reactions in heterogeneous or highly viscous phases.

magnetic stirring magnetic stir bar
Various models of magnetic bar (stir bar).

Mechanical stirring

It is used when large quantities (> 1000 ml) are handled or when reactions or very viscous mixtures where magnetic stirring is ineffective.

The assembly that is used consists of a flask, normally with several openings to be able to adapt another series of elements. A metallic or glass rod with a system of metallic, glass, ceramic, Teflon, etc. blades is introduced through the vertical mouth, which is moved by a motor with adjustable speed.

The flask is fitted with a stirring seal that allows the rod to rotate freely and prevents solvent vapors from escaping both hot and cold.

However, this type of device is rarely used in an experimental organic chemistry teaching laboratory.

Mixing and grinding

Another type of material used for mixing and crushing solids are mortars.

mixing grinding mortar pestle
Mortar and pestle.