The Buchner funnel has many small holes on the bottom of the cylinder with a flat shape. There is a long and narrow cylindrical outlet underneath. When in use, the filter paper disc is generally placed on the bottom of the cylinder, the funnel is inserted into the upper opening of the Buchner flask and the interface is sealed (for example, with a rubber ring).
The side port of the Brinell beaker is connected to a suction system. Then the solid and liquid mixture to be separated is poured into the upper part, the liquid component is drawn into the beaker under the action of negative pressure, and the solid remains on the upper part. The application in organic chemistry experiments to extract crystals is frequently used.
After the filtration, in this case, is completed, to wash away impurities on the crystal surface, a small amount of pure solvent can also be used. Both the Buchner funnel and the suction filter bottle are used to separate the precipitate and the solution. Use a water pump or a vacuum pump to reduce the pressure in the suction filter bottle to accelerate the filtration.
Use of filter paper in Buchner funnel
Operators will usually use the filter paper together with the filter funnels and Buchner funnels in experiments. Before the experiment, the operators will fold the filtration filter paper into a certain shape to make it suitable for the experience. The flower-like shape filter paper is frequently used. With the higher folding degree, the filter paper will provide a larger surface area and get a better filtering effect. But, please note that the folded filter paper should be not easy to break. It’s suggested to place a drained glass rod on the multi-layer filter paper evenly to prevent damage.
Before use, you need to prepare suction filtration equipment, such as a vacuum pump, suction filter bottle, Buchner funnel holder (sealing ring), vacuum tube (rubber tube), filter paper, Buchner funnel, etc. Wet the filter paper with water first, and pump it so that the filter paper is close to the bottom of the funnel to prevent the things to be filtered from leaking. Pour in the filtrate, turn on the machine to pump, and stir a little, to make only the filtered material left.
It is not possible to directly use an ordinary funnel, paste the filter paper, and connect to the suction pump for suction filtration (replace the Buchner funnel). Because the four walls of the funnel are close to the filter paper, negative pressure suction filtration does not work on it. The top corner of the funnel produces a pressure difference. Because the mouth is large and there is only one mouth, it is easy to break(The wet filter paper is not strong).
Summary of filter paper
The filter paper should not be cut too large, otherwise, it will leak easily, and it is better to just cover the hole. It needs to be wetted before suction filtration. If fast hot suction filtration is required, make sure that both the Buchner funnel and the suction filter bottle are heated. Turn on the suction pump first when filtering, pull out the pipe when it is finished, and turn off the power to restore normal pressure to prevent back suction. Choose the right size Buchner funnel and suction flask.
The specifications are important data for reference when using filter paper in the laboratory. Considering the applicability, some filter paper is processed by special production procedures.
For example, when testing nitrogen content in blood in the application of medicine inspection, nitrogen-free filter paper must be used. When referring to capacity, because solid particles which are collected during the process of filtration will block the micropore on filter paper, the more intensive the micropore is, the higher the capacity the filter paper has.
On the other hand, there are two key specifications of filter paper. One is filtration features; another is physical properties. Specifically, filtration features include air permeability, airlock, the maximum diameter of the micropore, and average micropore size. Physical properties include fixing quantify, thickness, deflection, depth of the corrugated, bursting strength, resin content, and so on.
In detail, fix quantify refers to the mass per square meter of filter paper. Thickness refers to the thickness of filter paper excluding the depth of the corrugated. Airlock refers to the resistance of filter paper to airflow. Air permeability refers to the air amount per unit of time under certain areas and certain pressure. Deflection refers to the non-deformability of filter paper.
Specification of the filter membrane
The specifications of Hawach filter paper for qualitative filter paper products can be found in the national standard (GB l 515). The aperture is about 80 μ 120 μ m, 30 μ m 50 μ m, and 1 μ 3 μ m, respectively. Hawach filter paper is committed to providing high filtration accuracy, which refers to the retention ability of filter paper to particles of a certain size, the size of a tiny sphere that can retain or filter out 50%, the resin content is usually from 10% to 30%, the depth of the grove to be pressed to enhance the stiffness of the filter paper in the longitudinal direction is usually 0-2 mm, and the maximum pressure that can be tolerated on the unit area of the filter paper is greatly enhanced by superior materials and delicate structure.
Steps for Operation
1. Fold the filter paper in the shape of the 90-degree round core. Then, divided it into three layers on one side, and open the other side of the filter paper to form a funnel shape. 2. Put into the funnel, pour some clear water into the funnel mouth, contact the soaked filter paper with the inner wall of the funnel, and pour out the rest of the clear water. 3. Place the funnel with filter paper on the funnel frame for filtration, lower the beaker in the neck of the funnel, and keep the tip of the funnel on the wall of the receiving container. 5. When the liquid needing to be filtered is injected into the funnel, the right hand holds the liquid beaker, the left-hand hold the glass rod, the lower end of the glass rod is close to the lower surface of the funnel, and the liquid cannot exceed the height of the filter paper in the funnel. 6. When the liquid passes through the filter paper and flows down the funnel neck, check if the liquid is running along the wall of the cup and pour it into the bottom of the cup.