Funnels, Buchner Hirsch

Vacuum filtration involves carrying out filtration under reduced pressure and provides a faster method of separating a precipitate from a filtrate than traditional gravity filtration methods. A Buchner, Hirsch or sintered glass funnel can be used during vacuum filtration. 

A ceramic funnel containing a flat perforated plate there are two types based on size and shape called Buchner funnels or Hirsch funnels. When you are filtering, the perforated plate is covered by a filter paper. 

The next laboratory period (4th day of lab experiment), collect the (S, R) diastereomer crystals (some of the (R, R) diastereomer is also present) by vacuum filtration using a Buchner funnel or Hirsch funnel. Rinse the solid in the funnel with about 1 mL of ice-cold methanol. Dry the solid on the Buchner funnel or Hirsch funnel, under vacuum, for several minutes. Pour the remaining filtrate into a bottle that... 

For the HCI salt Do exactly as above except use 6N Hydrochloric Acid. 6N HCI may be produced by diluting 60.4mL of "Muriatic Acid" to lOOmL with distilled water. Evaporate the bubbler solution to dryness then add 15ml of water, lOmL 10% NaOH soln. and heat gently to a boil with constant motion until dense white fumes appear. This will remove the Ammonium Chloride. Remove from heat while stirring as it cools down. Pulverize the dry residue, then reflux with absolute Ethanol for several minutes. Filter the refluxed soln. on a heated Buchner or Hirsch funnel, then distill the alcohol off the filtrate until crystals just begin to form. Allow the soln. to cool naturally to room temperature, then cool further in an ice bath. Filter the solution on a chilled Buchner funnel with suction. The yield of Meth iamine Hydrochloride should be around 55% of the theoretical. 

The reaction mixture is filtered through a Hirsch or Buchner funnel, and the bottle is rinsed with 50 ml. of benzene, which is also poured through the funnel. The filtrate is washed with two 50-ml. portions of 10% sodium chloride solution and three 2S-ml. portions of water (Note 4). The washings are extracted with three 10-ml. portions of benzene, and the combined benzene solutions are distilled under reduced pressure from a 250-ml. modified Claisen flask. The yield of ethyl -butylcyanoacetate, b.p. 108-109°/9 mm., is 79-81 g. (94-96%, based on the ethyl cyano-acetate used) (Note 5). 

Funnels, 49 Buchner, 51 dropping, 49 Hirsch, 51 hot water, 49 sintered glass, 50, 133... 

The Buchner and Hirsch funnels, and the filter flasks and tubes, are available with interchangeable ground glass joints (for example, Fig. 2.43(/)). The Buchner funnel is also available with a screw-thread side-arm for easy attachment via a plastic hose connector to the vacuum line (Bibby Science Products). 

For the suction filtration of small quantities of solid (less than 5g) contained in a small volume of liquid, a small conical Buchner funnel, known as a Hirsch funnel, is employed (see Fig. 2.43(h)) the filtrate is collected either in a small filter flask or in a test tube with side-arm, the arrangement being illustrated in Fig. 2.43(f). A small sintered funnel, or a slit-sieve funnel may also be employed. The procedure for filtration is similar to that already given for the Buchner funnel. 

The goal of vacuum filtration is usually to remove liquids from a solid to obtain a solid that is dry. An aspirator or a vacuum pump is used to provide suction though a rubber tube to a filter trap. The trap is attached to a filter flask (show to the right) by a second rubber tube. The filter flask is an Erlenmeyer flask with a thick wall and a hose barb for the vacuum tube. Filter flasks are used to filter material using a Buchner funnel (shown to the right) or a smaller Hirsch funnel. These porcelain or plastic funnels hold a circular piece of filter paper. A single-hole rubber stopper supports the funnel in the flask while maintaining suction. 

This step is often problematic and should NOT be carried out unless an unacceptable (use your judgement) amount of insoluble material is suspended in the solution. The difficulty here is that the compound tends to crystallize during the filtration so an excess of solvent (ca. 5%) should be added, and the apparatus used for the filtration should be preheated to about the boiling point of the solvent. Use a clean sintered funnel of porosity 2 or 3, or a Hirsch or Buchner funnel, and use the minimum suction needed to draw the solution rapidly through the funnel. If the solution is very dark and/or contains small amounts of tarry impurities, allow it to cool for a few moments, add ca. 2% by weight of decolorising charcoal, reflux for a few minutes, and filter off the charcoal. Charcoal is very finely divided so it is essential to put a... 

Swirl the mixture to be filtered and then slowly pour it into the Buchner or Hirsch funnel at such a rate so that the filtration is rapid. Note that the rate of filtration may slow as the cake of solid on the filter becomes thicker. 

Prelab Exercise Study the glassware diagrams and be prepared to identify the fractionating column, Claisen distilling hjead, ordinary distilling head, vacuum adapter, simple bent adapter, calcium chloride tube, Hirsch funnel, and Buchner funnel. 

Hirsch funnel fits into the filter fiask with no adapter, and is equipped with a polyethylene frit for removal of the crystals (Fig. 4). For larger quantities of material, porcelain or plastic Buchner funnels are used with pieces of filter paper that fit the bottom of the funnel. A filter adapter is used to form a vacuum tight seal between the flask and the funnel (Fig. 5). 

A) Filtration on the Buchner Funnel. The Biichner funnel is used for large-scale filtrations. If the quantity of material is small (<2 g), the Hirsch funnel can be used. Properly matched Buchner funnels, filter paper, and flasks are shown in Fig. 6. The Hirsch funnel shown in the figure is an old-style porcelain type. 

Once the crystals have been washed on the Hirsch funnel or the Buchner funnel, press them down with a clean cork or other flat object and allow air to pass through them until they are substantially dry. Final drying can be done under reduced pressure (Fig. 9). The crystals can then be turned out of the funnel and squeezed between sheets of filter paper to remove the last bit of solvent before final drying on a watch glass. 

Collecting and washing the crystals. Collect the crystals using the Pasteur pipette method or by vacuum filtration on a Hirsch funnel or a Buchner funnel. If the latter technique is employed, wet the filter paper with solvent, apply vacuum, break vacuum, add crystals and liquid, apply vacuum until solvent just disappears, break vacuum, add cold wash solvent, apply vacuum, and repeat until crystals are clean and filtrate comes through clear. 

To recrystallize the caffeine dissolve it in 5 mL of hot acetone, transfer it with a Pasteur pipette to a small Erlenmeyer flask, and, while it is hot, add ligroin to the solution until afaint cloudiness appears. Set the flask aside and allow it to cool slowly to room temperature. This mixed solvent method of recrystallization depends on the fact that caffeine is much more soluble in acetone than ligroin, so a combination of the two solvents can be found where the solution is saturated in caffeine (the cloud point). Cool the solution containing the crystals and remove them by vacuum filtration, employing the Hirsch funnel or a very small Buchner funnel. Use a few drops of ligroin to transfer the crystals and wash the crystals. If you wish to obtain a second crop of crystals, collect the filtrate in a test tube, concentrate it to the cloud point using the aspirator tube (Fig. 5 in Chapter 3), and repeat the crystallization process. 

After a recrystallization, you usually collect the new crystals by suction on a Buchner funnel (see Chapter 13, Recrystallization ). For microscale quantities you may have to use a Hirsch funnel—a tiny Buchner funnel with sloping sides and a flat porous plate (see Chapter 5, Other Interesting Equipment ). Or you might need the high-tech power of Craig tubes (see Chapter 14, RecrystalUzation Microscale ). Or you might be able to get away with a Pasteur pipet (Fig. 42). 

First, see the section on Pipet Filtering—Solids in Chapter 7 where you remove solvent with a pipet. I also mention using a Hirsch funnel. Just reread the section on the Buchner funnel filtration (Chapter 13, Recrystallization ), and where you see Buchner substitute Hirsch and paste copies of the Hirsch funnel drawing over the Buchner funnel. Just realize that the tiny disk of paper can fly up more easily. Think small. 

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