Adding material to a flask

Solid reagents weighed in a weighing boat or folded filter paper/foil can be pinched slightly to slow the rate of addition into the reaction flask, allowing you to exert more control. Additionally, a spatula may be used to guide any solid reagent being added.

It may be nescessary to ensure that the mass transferred to the flask is known precisely, for example for a limiting reagent, or where accurately preparing solutions. There are several ways to acheive this:

• Reweighing the weighing vessel, and calculating the mass difference

• Washing any remaining material off the weighing vessel and into the flask using a solvent

Where washing residual material remaining from the weighing vessel, there are a few things to consider. The properties of the solvent and weighing vessel are important, as plastic weighing boats may be soluble in some solvents (especially chlorinated solvents such as dichloromethane), and therefore weighing by difference would be more appropriate. Where washing with solvent is carried out, this ensures that all of the weighed reagent is transferred into the vessel, ensuring the values recorded in the lab book for masses used are reflected in the solution or reaction mixture. It may also be desirable to rinse the neck of the reaction flask as well, to ensure that there is no material left in the neck, and all the solid is inside the flask.

Where the necks of reaction flasks are not being rinsed with solvent, it may be desirable to wipe the neck with tissue. This is especially important if other bits of glassware are to be fitted into the quickfit glass joint as any material caught here will hinder the joint connection and possibly damage the glassware. For instance some solids, especially metal powders or turnings can damage, score or even break glassware if material is caught in the joints. 

When transferring liquids, eg solvent, a glass funnel is often used to ensure that the material is transferred inside the flask. When removing the funnel, gently tapping it helps to ensure any residual amounts of liquid are transferred to the flask.