Powder addition funnel
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A powder addition funnel (PAF) is a type of lab glassware used to add powdered solids in a reaction flask, often under inert atmosphere. Its function is similar to that of dropping funnels, except it adds solids rather liquids.
A powder addition funnel consists of a conical or cylindrical container, which has a lid or female ground glass joint on the upper part. The lower part of the container has an opening which is attached to a glass tube with a PTFE screw conveyor (or auger), which may or may not be tilted, depending on the model. The plastic screw is engaged by rotating the screw head. The far end of the screw conveyor ends in a male ground glass joint, which points downwards. Some types of PAFs have a pressure equalizing tube, just like dropping funnels, though this is mainly found in PAFs that are meant to be used in closed systems.
How to use a powder addition funnel
- Make sure the screw works properly;
- Add the powdered solid in the container;
- Grease the joints and place the stopcock on the female joint and secure it with a keck clip;
- Place the greased male joint on the reaction flask and secure it with a keck clip;
- If your reaction needs inert atmosphere, use a vacuum pump to remove the air, then carefully inject the inert gas and repeat the procedure at least three times;
- Begin to slowly rotate the screw head to move the powder forwards in the flask
- Make sure your powder is not volatile, as it may sublime during the vacuum step;
- Likewise, don't open the vacuum/inert gas valve too quickly as it may blow your powder throughout your setup;
Powder addition funnels are sold by lab suppliers, though they're expensive. Sigma-Aldrich sells a 100 ml PAF for 300 euros.
DIY Powder addition funnel and alternatives
Since they're very expensive and aren't very often used, few chemists will need to work with PAFs. There are a few alternatives to them:
SM user Klute used a drying tube tilted downwards with a roughly 120° angle, with a rubber or plastic stopper with the same diameter as the interior of the tube, as a makeshift PAF. The tube is filled with powder then the stopper is placed in the tube. By pushing the stopper, the powder gets pushed in the reaction flask through a side neck. This setup, however, is not very effective, as the powder may be dumped all at once and some solid may remain on the interior of the tube. It is also very difficult to recharge the setup and cannot work on powders that will corrode the stopper or react with air/moisture and become a paste (antimony(III) chloride e.g.).
For adding powders that are soluble in solvents, most chemists will add the solid in a dropper funnel filled with inert gas, then connect it to a solvent reflux line under inert gas. Anhydrous solvent (in which the solid is soluble) is collected in the dropping funnel and the solid is dissolved in the dropping funnel. Often, a small stir bar is introduced in the funnel to aide in mixing the solution for faster dissolving. The dropping funnel is stoppered and placed back on the reaction flask. The solution is added to the reaction dropwise, just like any other liquid.
If the powder you need to add is not soluble in any solvents or has very limited solubility, there are two ways you can do:
- The wasteful way: increase the flow of the inert gas, remove the stopper from a side-arm and place a funnel in its place. The solid is added to the flask using a spatula, in small bits. This method is risky, as vapors or drops from the reaction will be blown out due to the stronger inert gas flow, and this is very dangerous, especially when working with potentially pyrophoric compounds. Likewise, if the reaction is slow, you will waste precious inert gas.
- The solid-first way: Add the solid material and a stir bar in the reaction flask first, connect the setup then apply vacuum/inert gas. After all is done, pour anhydrous solvent over the insoluble solvent using a dropping funnel, then add the liquid reagents in dropping funnels, and then slowly add them over the solid to react. This method is commonly used when preparing Grignard reagents using magnesium metal.
Handling and maintenance
Powder addition funnels are complex lab equipment and must be handled with care. Cleaning them is a complex operation.