The Buchner Funnel and Filter Flask

The Buchner funnel (Fig. 46) is used primarily for separating crystals of product from the liquid above them. If you have been boiling your recrystallization solvents dry, you should be horsewhipped and forced to reread these sections on recrystallization  

Get a piece of filter paper large enough to cover all the holes in the bottom plate, yet not curl up the sides of the funnel. It is placed flat on the plate (Fig. 46). 

Clamp a filter flask to a ringstand. This filter flask, often called a suction flask, is a very heavy-walled flask with a sidearm on the neck. A piece of heavy-walled tubing connects this flask to the water trap (see Fig. 48). 

Now use a rubber stopper or filter adapter to stick the Buchner funnel into the top of the filter flask. The Buchner funnel makes the setup top-heavy and prone to be prone—and broken. Clamp the flask first, or go get a new Buchner funnel to replace the one you ll otherwise break. 

The water trap is in turn connected to a source of vacuum, most likely, a water aspirator (Fig. 47). 

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Collect the second crop of solid using the Buchner funnel and filter flask, washing it with ice-cold hexane (3x5 mL). Dry the combined crops in vacuo (0.1-0.5 mmHg) for several hours, yielding 28.1-30.0 g (90-5%) of white solid, m.p. 77-9°C. 

The Buchner Funnel and Filter Flask Just a Note... 

The filtration is best effected by the use of reduced pressure employing a preheated Buchner funnel and filter flask. 

A prerequisite for any filtration tests is the Buchner funnel with filter flask and the filter leaf. In addition a good vacuum source is required. This should preferably be better than the simple water tap system. 

For solids which melt above 100° and are stable at this temperature, drying may be carried out in a steam oven. The crystals from the Buchner funnel should then be placed on a clock glass or in an open dish. The substance may sometimes be dried in the Buchner funnel itself by utilising the device illustrated in Fig. 77,33,1. An ordinary Pyrex funnel is inverted over the Buchner funnel and the neck of the funnel heated by means of a broad flame (alternatively, the funnel may be heated by a closely-fitting electric heating mantle) if gentle suction is applied to the filter flask, hot (or warm) air will pass over the crystalline solid. 

Filtration by means of suction is generally employed where possible in the operations of organic chemistry, since more rapid and more complete separation of the mother liquor is in this way effected. For this purpose several types of apparatus are in use. For large quantities of material the Buchner funnel and flask (Fig. 27) are used, the filter paper being cut... 

Filtration of large quantities of solution. For the filtration of larger quantities of hot solutions it is advisable to use a funnel heated by a hot water jacket, shown diagrammatically in Figure 9. If the solvent is inflammable, the side arm is heated with a free flame until the water boils, and the flame is shut off before the filtration is started. Where steam is available it may be used to advantage as the source of heat. If the amount of solution to be filtered is fairly large, a Buchner funnel and suction flask, shown... 

When crystallisation is complete, the mixture of crystals and crude mother-liquor is filtered at the pump, again using a Buchner funnel and flask as described on p. 10, and the crystals remaining in the funnel are then pressed well down with a spatula whilst continual suction of the pump is applied, in order to drain the mother-liquor from the crystals as effectively as possible. If it has been found in the preliminary tests that the crystalline material is almost insoluble in the cold solvent, the crystals in the... 

Add 5 g. of potassium hydrogen tartrate and 5 g. of antimony trioxide (each being finely powdered) to 30 ml. of water contained in a small flask, and boil the mixture under a reflux water-condenser for 15 minutes. Then filter hot, using a Buchner funnel and flask which have been preheated by the filtration of some boiling distilled water. Pour the clear filtrate into a beaker and allow to cool. Potassium antimonyl tartrate separates as colourless crystals. Filter, drain and dry. Yield, 5 g. The product can be recrystallised from hot water, but this is usually not necessary. 

Dissolve 5 g. of aniline hydrochloride in 120 ml. of hot water contained in a 200 ml. conical flask and then add 4 g. of potassium cyanate. Heat the solution on a water-bath for 30 minutes, adding about 1-2 g. of animal charcoal towards the end of the heating if a slight turbidity has developed. Now bring the solution quickly to the boil over a gauze, and filter it at the pump, using a Buchner funnel and flask which have been preheated by the filtration of some boiling distilled water. The clear... 

When the ij hours boiling is complete, preheat a Buchner funnel and flask by pouring some boiling water through the funnel with the filter-paper already in position, and then quickly filter the boiling solution. Transfer the filtrate to a beaker to cool, and then wash the insoluble residue of diphenylurea on the filter twice with hot water, and drain thoroughly. Cool the filtrate in ice-water the monophenylurea separates as colourless needles. Filter at the pump and drain well. Recrystallise the crude product from boiling water, as in the previous preparation. Yield of monophenylurea, 2 5-3 g. m.p. 147°. 

The insoluble residue of diphenylurea from the original filtration is chemically almost pure. It may be recrystallised from hot rectified spirit or ethanol, a process which will be necessary if the material contains fragments of porcelain. When using either of these solvents, however, the hot solution should be filtered at the pump using a small Buchner funnel and flask which again have been preheated by the filtration of some of the hot solvent, as the solution when cooled rapidly deposits the diphenylurea. iSym-Diphenylurea (or carbanilide) is thus obtained as fine colourless crystals, m.p. 237° yield, 1-1 5 S ... 

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