Vacuum filtration

What is vacuum filtration?

Vacuum filtration (or suction filtration) is an indispensable technique for distillation, extraction, and purification. Compared with traditional, gravity-assisted filtration, adding a vacuum to your system greatly increases speed and efficiency in your laboratory.

Vacuum filtration is a separation method used to remove a solid from a liquid. Generally the solid is dissolved in a solution. The solution can be unsaturated, saturated, or supersaturated — the difference being the concentration of the solid in the liquid. A filter is used to separate the solid from the liquid, and a vacuum pump is used to force the liquid through the filter.

Vacuum filtration apparatus, adapted for laboratory work, is often used to isolate the product of synthesis of a reaction when the product is a solid in suspension. The product of synthesis is then recovered faster, and the solid is drier than in the case of a simple filtration. Other than isolating a solid, filtration is also a stage of purification: the soluble impurities in the solvent are eliminated in the filtrate (liquid).

This apparatus is often used to purify a liquid. When a synthesised product is filtered, the insolubles (catalysers, impurities, sub-products of the reaction, salts, …) remain in the filter. In this case, vacuum filtration is also more efficient that a simple filtration: there is more liquid recovered, and the yield is therefore better.

Vacuum filtration

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Structure of Vacuum Filter Setup


Suction filtration is a chemistry laboratory technique which allows for a greater rate of filtration. Whereas in normal filtration gravity provides the force which draws the liquid through the filter paper, in suction filtration a pressure gradient performs this function. This has the advantage of offering a variable rate depending on the strength of the pump being used to extract air from the Büchner flask. Care must be taken not to use such a strong vacuum that the filter paper rips (in which case all the solid will be lost back into the solvent) or in extreme cases the glass flask breaks. Suction filtration is used in recrystallisation experiments.


Working Principle of Vacuum Filtration

The working principle of vacuum filtration is to form negative pressure at the outlet of filtrate and use it as the driving force of filtration. The vacuum filtration equipment with intermittent operation is mainly introduced below. This equipment can filter suspensions of various concentrations.

Intermittent operation of vacuum filtration equipment has been developed because of its ability to achieve automated operation. The working principle of this equipment is as follows:

The containers are separated into upper and lower chambers by filter media, which constitutes a simple suction and filtration equipment. The suspension is added to the upper chamber. Under the action of pressure, the filter medium enters the lower chamber and becomes the filtered liquid. The solid particles are intercepted on the surface of the filter medium to form the filter residue.

During the filtration process, the vacuum filtration residue layer accumulated on the surface of the filter medium gradually thickens, and the resistance of liquid passing through the filter residue layer increases, and the filtration speed decreases. When the filter chamber is full of filter residue or the filtering speed is too small, stop filtering, remove the filter residue, and regenerate the filter medium to complete a filtering cycle.

After prefreezing the product, conditions must be established in which ice can be removed from the frozen product via sublimation, resulting in a dry, structurally intact product. This requires very careful control of the two parameters, temperature and pressure, involved in the freeze drying system. The rate of sublimation of ice from a frozen product depends upon the difference in vapor pressure of the product compared to the vapor pressure of the ice collector. Molecules migrate from the higher pressure sample to a lower pressure area. Since vapor pressure is related to temperature, it is necessary that the product temperature is warmer than the cold trap (ice collector) temperature. It is extremely important that the temperature at which a product is freeze dried is balanced between the temperature that maintains the frozen integrity of the product and the temperature that maximizes the vapor pressure of the product. This balance is key to optimum drying. The typical phase diagram shown in Figure 1 illustrates this point. Most products are frozen well below their eutectic or glass transition point (Point A), and then the temperature is raised to just below this critical temperature (Point B) and they are subjected to a reduced pressure. At this point the freeze drying process is started.

What is the vacuum filter used for?

In most cases, vacuum filtration is used to collect recrystallized solids. These are particles that were dissolved in water or some other solvent and then were recovered, or recrystallized, by heating. The liquid is evaporated in the process.

After most of the liquid has been evaporated, the contents of the beaker or flask are deposited onto a filter paper, which is placed within a Buchner funnel. The funnel is attached to a filter flask, which has a side opening through which air will be sucked out using a rubber tube attached to a vacuum.

The rubber tube is attached to the vacuum source first and tested. If there is suction at the other end of the tube, it is then attached to the side arm of the filter flask. Once secured, the top of the filter flask should have suction.

As the sample is poured onto the filter paper, the vacuum pulls the solvent through the paper and deposits it in the filter flask. The vacuum will remove as much solvent from the solid crystals as possible. The remaining solid on the filter paper is then collected and weighed.

Vacuum filtration is also used to separate solids from liquid solutions. Unsaturated solutions are solutions that can dissolve more solids. It is generally clear and has no suspended particles. Saturated solutions can’t dissolve any additional solids, but are generally clear as well. Supersaturated solutions are generally not clear and have suspended or sinking particles.

Solutions with suspended particles can be separated through vacuum filtration. Solutions in which the solid is dissolved, such as unsaturated solutions, generally require some form of chemical treatment or heating before vacuum filtration can be effective. Generally, saturated solutions can’t be separated with vacuum filtration, but a few particles may be collected if the filter paper has an adequate pore size.

The amount of solid collected through vacuum filtration generally depends on the pore size of the filter paper. A smaller pore size will yield more material, while a larger pore size will yield less material. Generally, the pore size of a filter paper is less than a micrometer in diameter.

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