What Is a Filter Rule?

Filtration is the most commonly used method of separating solutions from precipitation. When the mixture of solution and precipitate passes through a filter (such as filter paper), the precipitate remains on the filter, and the solution passes through the filter and flows into the receiving container. Filtration in the traditional sense refers to the use of a porous medium to trap solid particles in a suspension, thereby separating solid and liquid. In addition to centrifugal separation of bacteria, cells and their fragments, it can also be separated by conventional filtration methods ^[1] .

Filtration is essentially passing wastewater through a filter medium with fine pores. The pressure on the two sides of the filter medium is different. This pressure difference is the driving force of filtration. Wastewater passes through the fine pores under the driving force, while particulate matter and colloidal matter are blocked by the medium and cannot pass through. The particulate matter retained by the medium itself also functions as a filter medium, which is called a filter layer or filter cake. As the process progresses, the filter layer gradually thickens, and the resistance of the water flow will also increase, causing the water flow to decrease. At this time, the backwash method is needed to wash the filter medium with clean water to remove the trapped solid materials, and the filter cake should be removed in time. ^[2]

The temperature of the solution, the viscosity, the pressure during filtration, the pore size of the filter, and the state of the precipitate all affect the filtration speed. Hot solutions are easier to filter than cold solutions. The greater the viscosity of the solution, the slower the filtration. Decompression filtration is faster than atmospheric filtration. The pores of the filter should be properly selected. If it is too large, it will penetrate the sediment; if it is too small, it will be easily blocked by the sediment, making filtration difficult. If the precipitate is gelatinous, it must be heated for a while to destroy it, otherwise it must pass through the filter paper. In short, it is necessary to consider various factors to choose different filtering methods.

Atmospheric filtration

This method is the simplest and most commonly used, and it uses a glass funnel and filter paper for filtration. When filtering, fold the round or square filter paper into four layers, then tear off the corner of the filter paper, and open the filter paper into a cone, with a single layer on one side and three layers on the other side, and put them in the funnel. The cone angle of the funnel should be 60 so that the filter paper can be completely attached to the funnel wall. If the cone angle of the funnel is greater than 60, the folding angle of the filter paper should be appropriately changed to match the cone angle of the funnel. Then press the filter paper on the wall of the funnel with your index finger, and use a wash bottle to blow out a small amount of distilled water to wet the filter paper. The edge of the filter paper should be slightly lower than the edge of the funnel. If the filter paper is stuck on the wall of the funnel and there are still air bubbles between the two, you should press the filter paper gently with your fingers to drive out the air bubbles, and then add water to the funnel almost to the top edge of the filter paper. At this time, the inside of the funnel neck should be completely filled with water, and after all the water on the filter paper has been drained, the water column in the funnel neck can still be maintained. If it is unsuccessful, you can use your fingers to block the lower mouth of the funnel, lift the filter paper, and add water between the filter paper and the funnel wall until most of the funnel neck and cone are filled with water, and the air bubbles are completely discharged. At this time, press the paper edge down, and then open the lower outlet, and the water column can be formed at this time. During the entire filtration process, the funnel neck must always be filled with liquid, so that the filtration can be performed quickly.

When filtering, pay attention to: the funnel should be placed on the funnel rack, adjust the height of the funnel rack so that the outlet of the funnel rests on the inner wall of the receiver, so that the filtrate can flow down the wall without splashing. The solution is transferred first and then the precipitate is transferred. This will not slow down the filtration rate due to the clogging of the pores of the filter paper by precipitation. When transferring solutions and precipitation, use a stir bar. When transferring the solution, the solution should be dropped on the side of the three-layer filter paper to prevent the liquid droplets from breaking through the single-layer filter paper. The solution added to the funnel must not exceed 2/3 of the total volume of the cone filter paper. Adding too much will cause the solution to flow into the receiver from between the filter paper and the inner wall of the funnel and lose the filtering effect of the filter paper.

Wash the pellet if necessary. After the solution is transferred, add a small amount of detergent to the container containing the precipitate, stir it thoroughly, and wait for the solution to settle. Then pour the upper solution into the funnel. Repeat this operation 3 times, and finally transfer the precipitate to Filter paper. You can also transfer the entire pellet to filter paper, and then divide a small amount of detergent into several portions and add it to the funnel several times to wash the pellet.

When washing the pellet, you should also follow the principle of a small number of times to improve efficiency. Finally, after checking the impurities in the filtrate, it can be judged whether the precipitate is washed.

Decompression filtration (suction filtration)

To speed up filtering. Commonly used under reduced pressure filtration. There are many porcelain holes on the Buchner funnel (porcelain funnel), and a rubber stopper is installed on the lower neck to connect with the suction filter bottle. The suction filter bottle is used to receive the filtrate, and a branch pipe is connected to the suction system.

The pump that reduces pressure is the pump. The rapid water flow in the pump continuously takes the air away, thereby reducing the pressure in the suction filter bottle, creating a pressure difference between the liquid level in the Buchner funnel and the filter bottle, which can increase the filtration rate. Install a safety bottle between the rubber hose connected to the pump and the suction filter bottle to prevent the tap water from being sucked back due to the closing of the water gate or the change in the flow rate of the pump to avoid contamination or dilution of the filtrate. If the filtrate does not need to be retained, a safety bottle may not be used.

In the decompression filtration operation, the preparation before filtration must be done first. Inspection device: the long tube of the safety bottle is connected to the water pump, and the short tube is connected to the suction filter bottle; the neck of the Buchner funnel is opposite to the branch pipe of the suction filter bottle, which is favorable for suction filtration. Paste the filter paper: The size of the filter paper should be slightly smaller than the inner diameter of the Buchner funnel. It is better to cover all the small holes on the porcelain plate. First, a small amount of distilled water is blown out of the washing bottle to wet the filter paper, and then the water pump is turned on, so that the filter paper is closely attached to the porcelain plate in the funnel before filtering.

When filtering, pour the clear solution into the funnel along the glass rod, and then transfer the precipitate to the middle part of the filter paper after filtering. The filtrate surface in the suction filter bottle should not reach the horizontal position of the branch pipe, otherwise the filtrate will be pumped out by the water pump. When the filtrate quickly rises to the branch tube of the suction filter bottle, the rubber tube on the suction filter bottle should be removed, the funnel should be removed, and the filtrate should be poured out from the upper mouth of the suction filter bottle before continuing the suction filtration. It should be noted that when the filtrate is decanted from the upper mouth of the suction filter bottle, the branch pipe must be upward. During the suction filtration process, the water pump must not be turned off suddenly to prevent back suction. After filtering, unplug the rubber tube first, and then turn off the water pump. Use your finger or glass rod to gently lift the edge of the filter paper to remove the filter paper and sediment. The filtrate in the suction filter bottle was finally poured out from its upper mouth. The method of washing the precipitate is the same as when using a glass funnel, but do not filter the washing solution too fast. You can properly open the water valve a little smaller or stop suction filtration to prevent the precipitate from washing out.

If the filtered solution has strong acidity, strong alkalinity or strong oxidizing property, the solution will destroy the filter paper. In this case, asbestos fiber should be used instead of filter paper to spread on the Buchner funnel. The other operations are exactly the same as the vacuum filtration operation. This method is more suitable for retaining the filtrate and discarding the precipitate. If sedimentation needs to be retained, replace the Buchner funnel containing asbestos fibers with a glass sand funnel. It should be noted that the glass sand funnel is suitable for filtering strongly oxidizing and strongly acidic substances, and is not suitable for filtering alkaline substances, because alkali easily interacts with glass.

In addition, vacuum filtration is not suitable for the filtration of colloidal precipitates, because colloidal precipitates can pass through filter paper during rapid filtration. Precipitation with very fine particles will form a dense precipitation film on the filter paper due to reduced pressure suction, making the solution difficult to permeate, and the acceleration cannot be achieved, so it is not suitable to use the reduced pressure filtration method.

Hot filtration

If the solutes in the solution will precipitate after cooling down, and we do not want these solutes to precipitate during the filtration process and remain on the filter paper, we need to filter while hot.

When filtering, the glass funnel can be placed in a copper hot funnel, and the hot funnel interlayer is filled with hot water to maintain the temperature of the solution. Alternatively, the glass funnel can be heated on a water bath with steam before filtration, and then used. This method is relatively simple and easy to implement. In addition, when hot filtering, the neck of the glass funnel should be as short as possible, so as to avoid the solution staying in the funnel neck for too long during filtering, which will cause blockage due to cooling and precipitation of crystals ^[3] .