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A dropping funnel or addition funnel is a type of laboratory glassware used to transfer fluids, usually in a closed system.
There are two main types of dropping funnels:
- Simple dropping funnel: A dropping funnel consists of a (often) graduated cylindrical tube or in some models pear-shaped bulb, with a female ground glass joint on its top, where a stopper, either PP or glass is used. The lower part of the funnel has a stopcock, glass or PTFE which allows the flow to be controlled. he dripping tube is separate from the ground glass joint and extends a bit below the joint. Dropping funnels are similar in construction to separatory funnels, though the main difference is that separatory funnels don't have a male ground glass joint.
- Pressure-equalizing dropping funnel: this is the iconic dropping funnel, it consists of a graduated cylinder tube or pear-shaped bulb, which has an additional narrow-bore glass tube that begins from top of the bulb, usually has a small deformation upwards and continues to the base of the funnel, connecting with the lower tube, in the space below the stopcock and above the male ground glass joint. The purpose of this tube is to replace the liquid volume lost in the bulb with the equivalent gas volume from the flask into which the reagent is flowing. This allows the liquid from the funnel to drop smooth without having to remove the stopper, which is necessary when needing to add exact amounts of liquid from the dropping funnel and when handling air-sensitive reagents in a sealed, inert-gas environment. Without this tube, or some other means to equalize the pressure between a sealed receiving flask and the bulb of the funnel, the flow of fluid from the bulb will rapidly come to a halt.
Dropping funnels are useful for adding reagents slowly, i.e. drop-wise. This may be desirable when the quick addition of the reagent may result in side reactions, or if the reaction is too vigorous.
Most often than not, dropping funnels are made of borosilicate glass, while PP dropping funnels are rarely used.
How to use a dropping funnel
To use the dripping funnel, you must follow the next steps:
- Check the stopcock to see if it works
- Grease the ground glass joints
- Pour the liquid in the funnel through the upper female joint; make sure the liquid level stays below the upper opening of the pressure-equalizing lateral glass tube
- Close the funnel using the stopper and use a keck clip if required
- Place the dropping funnel on the flask in its respective socket
- Once in place, slowly open the stopcock to release the liquid inside the funnel
If you work in an inert atmosphere, you must do the following:
- If you're working under inert atmosphere, you will have to replace the air inside the funnel with vacuum followed by inert gas
- To do this, attach the funnel to the flask and perform the air removal with the funnel secured to the flask
- After the funnel is filled with inert gas, take a small flask or a female of female glass stopper in one hand and be prepared to remove the dropping funnel
- Immediately as you remove the funnel from its place add the female glass plug to the bottom of the funnel and close it, move quickly to limit the amount of inert gas that leaves the funnel
- Attach the dropping funnel to the inert gas solvent distillation unit by connecting the male joint of the said installation to the top female ground glass joint and secure it using a keck clip
- Make sure the pressure equalizing tube sits in an above position to the funnel but never below, as any dripping solvent that will fill the funnel may flow in the lateral tube, ruining your setup
- When you've collected enough solvent, close the flow stopcock and let the solvent cool to ambient temperature
- Remove the keck clip and immediately place the stopper on the funnel then close the male joint of the installation with its female plug
- Reattach the dropping funnel to the flask, by removing the stopper from the flask, then quickly remove the female stopper of the funnel and attach the dropping funnel to the flask and securing it using a keck clip
- If you collect a larger amount of solvent in the dropping funnel, a good idea would be to use a jack to slightly support the end of the funnel, to prevent it from snapping the male solvent dropping joint, as the funnel gets heavier and heavier
- When dropping liquid, it's best to avoid using the stopcock hole as a reference to the dropping rate, and instead simply open the stopcock very slowly and adjust the dropping rate by using the time period between the drops
Dropping funnels are sold by lab suppliers. They are a bit pricey compared to other glassware with ground glass, as their construction is more complex.
You can also purchase them online.