Filtration with suction

The technique of the filtration of hot solutions has already been described in Section 11,28. The filtration of cold solutions will now be considered this operation is usually carried out when it is desired to separate a crystalline solid from the mother liquor in which it is suspended. When substantial quantities of a solid are to be handled, a Buchner funnel of convenient size is employed. The ordinary Buchner fimnel (Fig. 11,1, 7, a) consists of a cylindrical porcelain funnel carrying a fixed, flat, perforated porcelain plate. It is fitted by means of a rubber stopper or a good cork into the neck of a thick-walled filtering flask (also termed filter flask, Buchner flask or suction flask) (Fig. 11,1, 7, c), which is connected by means of thick-walled rubber tubing (rubber pressure tubing) to a similar flask or safety bottle, and the latter is attached by rubber pressure tubing to a filter pump the safety bottle or trap is essential since a sudden fall in water pressure may result in the water sucking back. The use of suction renders rapid filtration possihle 

If the filtrate is of value, it should be transferred to another vessel immediately the crystals have been drained. Frequently, the mother liquor may be con centra ted (suitable precautions being, of course, taken if it is inflammable), and a further crop of crystals obtained. Occasionally, yet another crop may be produced. The crops thus isolated are generally less pure than the first crystals which separate, and should be recrystaUised from fresh solvent the purity is checked by a melting point determination. 

When the volume of mother liquor is large and the amount of crystals small, the apparatus of Fig. II, 32, 1 may be used. The large pear-shaped receiver is supported on a metal ring attached to a stand. When the receiver is about two-thirds fuU, atmospheric pressure is restored by suitably rotating the three-way stopcock the filtrate may then be removed by opening the tap at the lower end. The apparatus is again exhausted and the filtration continued. 

After the main filtrate has been removed, the crystals should be washed in order to remove the mother liquor which, on drying, would contaminate the crystals. The wash liquid will normally be the same solvent as was used for recrystallisation, and must be used in the smallest possible 

If the solvent constituting the crystallisation medium has a compara tively high boiling point, it is advisable to wash the solid with a solvent of low boiling point in order that the ultimate crystalline product may be easily dried it need hardly be added that the crystals should be insoluble or only very sparingly soluble in the volatile solvent. The new solvent must be completely miscible with the first, and should not be applied until the crystals have been washed at least once with the original solvent. 

In order to dry the crystals, the Buchner funnel is inverted over two or three thicknesses of drying paper (i.c., coarse-grained, smoothsurfaced filter paper) resting upon a pad of newspaper, and the crystalline cake is remov with the aid of a clean spatula several sheets of drying paper are placed on top and the crystals are pressed firmly. If the sheets become too soiled by the mother liquor absorbed, the crystals should be transferred to esh paper. The disadvantage of this method of rapid drying is that the recrystaUised product is liable to become contaminated with the filter paper fibre. 

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Procedure 2. Follow Procedure 1 except that no solvent is employed. Pour the s3Tupy reaction mixture on to crushed ice, remove the resulting aryl sulphonyl chloride and/or sulphone, if a sohd, by filtration with suction and, if a hquid, by means of a small separatory funnel or dropper, and wash with water. 

An alternative method of purification consists in dissolving the crude sulphonyl chloride in the minimum volume of boiling chloroform, transferring rapidly to a warm separatory funnel, and separating the lower chloroform layer upon cooling the chloroform solution, the crystalline sulphonyl chloride separates, and is collected by filtration with suction. A further quantity is obtained by concentrating the mother liquor. 

If the atophan does not crystallise—this is rarely the case unless pyruvic acid which has been standing for some time is employed—pour the reaction mixture into a solution of 2a g. of potassium hydroxide in 1 litre of water, and extract the resulting solution two or three times with ether. Place the ether extracts in the ETHER RESIDUES bottle. Treat the aqueous layer with 70 ml. of glacial acetic acid with vigorous stirring. Allow to stand for several hours and collect the crude atophan by filtration with suction. 

A suspension of 40 g 3-acetylaminomethyl-5-amino-2,4,6-triodobenzoic acid in 180 ml acetic anhydride were mixed with 0.4 ml concentrated sulfuric acid. An exothermic reaction was thereby initiated. Acetylation was completed by heating to 80°C for three hours. The reaction mixture was then evaporated to dryness in a vacuum at a temperature not exceeding 50°C. The residue was treated with a mixture of 30 ml concentrated aqueous ammonium hydroxide and 40 ml water, whereby the solid material dissolved with spontaneous heating. Within a few minutes, the ammonium salt of the acetylated product started precipitating. The precipitate and residual liquid were cooled externally with ice after about 15 minutes. The salt was separated from the liquid by filtration with suction, and was washed with ice cold saturated ammonium chloride solution. 

Experiment.1—Benzil (5 g.) is heated for ten minutes to boiling on the wateT bath with 15 c.c. of alcohol and a solution of 5 g. of potassium hydroxide in 10 c.c. of water. After cooling, the suspension of potassium benzilate crystals is filtered as dry as possible by suction, and the salt, after being washed with a little alcohol, is dissolved in 20-30 c.c. of cold water. The solution is filtered and dilute sulphuric acid is added, at the boiling point, to the clear filtrate. The free acid is precipitated partly in the form of crystals. It is separated by filtration with suction while hot and washed with hot water. It can then be recrystallised at once from a large volume of hot water or, after drying, from benzene. Yield about 4 g. 

After some hours the blue-green compound of copper and acetylacetone is separated by filtration with suction, washed twice with water, transferred directly from the filter funnel to a separating funnel, and, after being covered with ether, decomposed by continuous shaking with 50 c.c. of 42V-sulphuric acid. The ethereal solution is separated and the acid layer is extracted with ether the extract is then combined with the ethereal solution, which is now dried over calcium chloride. After the ether has been removed by distillation the diketone is likewise distilled. The bulk of the material passes over at 125°-140° and, on repeating the distillation, at 135°-140°. The boiling point of the completely pure substance is 139°. Yield 15-20 g. 

Hot hexanes-ether (3 1, 27 mL) was then added, and the suspension was allowed to stir without additional heating for 10 min. After filtration with suction, the suspension was washed with cold hexanes-ether (3 1, 20mL) to give the desired ketone 2 as a white solid (3.02 g, 84%). 

Separation of Crystals.—This is generally effected by filtration with suction, vessels of size suitable to the quantities dealt with being selected. The crystals left on the funnel should be well pressed down and then washed a few times with small quantities of the pure solvent in order to remove the last traces of mother liquor. If the substance is easily soluble, too large quantities of solvent must not be employed for washing. When... 

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