Filtration is the most commonly used experimental operation for separating liquids and solids. The current junior high school chemistry textbook only introduces the operation points of filtration in a rough way, but in the actual operation process, many detailed problems are often encountered. Talk about your own solutions to the problems often encountered in the filtering operation, for reference only.
First, how to choose the funnel and filter paper
The size of the funnel depends mainly on the amount of precipitate to be filtered or the amount of precipitated solids, rather than the volume of the liquid. If the amount of sediment or solids is large, the funnel selected will be large, and vice versa. The cone angle of the funnel should be It is 60°. The diameter of the pipe diameter is suitable. It is too thick to hold the water column. The flow rate of the water is too slow. The time required for filtration is too long. The end of the pipe diameter should be slightly inclined.
The selection of filter paper is based on the experiment. The filter paper is divided into qualitative filter paper and quantitative filter paper. Qualitative filter paper is mainly used to study the physical properties and chemical properties of substances in the filtration operation; quantitative filter paper is mainly used for quantitative analysis of substances. In the filtration experiment, it is often used in qualitative experiments, so most of the qualitative filter paper is used. After the selected filter paper is placed in the funnel, the edge of the paper is about 5 mm lower than the edge of the funnel.
Second, how to assemble the filter
First, fold the selected filter paper twice, and the second fold should not completely coincide with the fold of the first fold. When such filter paper is placed in the funnel (top angle 60°), the sharp corner and the funnel wall There is a certain gap, but the upper part can be perfectly attached to the funnel wall. The filter installed in this way is faster than the filter on which all the surfaces are attached to the funnel. When folding, do not fold the top corner of the filter paper. The seam is flattened so as not to weaken the strength of the tip. Then cut off the two corners of the three layers of paper so that the upper part of the filter paper can be closely attached to the funnel wall after wetting.
Next, put the stacked filter paper into a suitable funnel, wet the filter paper with the water of the bottle, and gently press the upper third of the filter paper against the funnel wall with your fingers. When the water is injected into the funnel, the funnel neck should be filled. Water, or use your fingers to block the end of the funnel neck and fill it with water to the top of the funnel. The funnel neck maintains a continuous water column, which will produce downward gravitation and accelerate the filtration process.
Third, how to filter correctly
When filtering, the glass rod is opposite to the mouth of the beaker containing the filter; the end of the glass rod is close to the three layers of the filter paper in the funnel, but the filter paper cannot be touched; it should be kept vertical (I think the glass rod is inclined to cause filtration) Liquid overflow); the tip of the neck of the funnel abuts against the inner wall of the mouth of the beaker of the filtrate. The liquid transferred each time must not exceed two-thirds of the height of the filter paper, preventing the filtrate from flowing out of the wall without passing through the filter paper. For residual in the beaker The liquid and solid materials should be flushed with solvent or distilled water in a small number of times, and the washing liquid is transferred to the funnel for filtration.
Fourth, how to properly wash the sediment
If it is necessary to wash the sediment, it should be washed immediately, otherwise the sediment will be cracked or agglomerated after being placed on the filter paper for a long time. It is not easy to rinse. It can be rinsed with the original solvent, distilled water or other suitable detergent. For a clean Empty beakers to replace the beakers that originally received the filtrate, so as to avoid re-filtering large volumes of liquid due to sedimentation of the filter paper. It is advisable to wash the collected sediments in each wash. Use a small amount of lotion when washing. Carefully rinse from top to bottom along the circumference, and flush the sediment to the bottom of the funnel. Do not make the liquid flow rate too strong, otherwise the sediment will be washed out of the filter. The glass rod should not be used to stir the material in the funnel to avoid scratching the filter paper. Generally wash 2 or 3 times, can be basically washed.
5. How to check if the sediment is washed?
According to the type of impurities that may be detected on the sediment, a suitable reagent is added to the last eluate to check the degree of washing. If the mixture of Na2SO4 and BaCl2 is completely reacted, it is checked whether the precipitate is washed or not. Net, should choose AgNO3 solution. If the AgNO3 solution is added to the last eluate without precipitation (AgCl), the precipitate has been washed.
6. When filtering, if there is too much filtrate and it is beyond the edge of the filter paper or the filter paper is scratched, what should I do?
The funnel and filter paper can be washed 2 to 3 times with a small amount of raw solvent, and the original filtrate is filtered again with the washing solution.
7. Is it necessary to use a filtration operation to separate the sediment and the liquid?
When the amount of separated precipitate is small, it can be contained in a centrifuge tube and centrifuged at room temperature for precipitation. The supernatant of the precipitate can be aspirated by a pipette. It can be washed and centrifuged as needed. Only when the amount of sediment is large It is suitable to be separated by filtration.
Eight, there are other ways to filter operations
In order to make the filtration speed fast and convenient to wash, the Buchner funnel can be used for vacuum filtration, which makes filtration and washing less time-consuming and easy to wash; when the filtration needs to be carried out at a certain temperature, the insulation funnel should be used for filtration.
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