Filtration of Hot Solution

B) Filtration of hot solutions. The quickest method of removing traces of insoluble impurities from a hot solution is to... 

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... 

Filtration of Hot Solution (see also Filtration).—This operation is usually necessary in order to remove insoluble impurities, filter fibres, etc. When the substance does not separate rapidly from the hot solution, and the liquid filters quickly, the solution may be filtered through an ordinary funnel with a short stem, fitted with a folded filter paper (Pig. 1). Both funnel and paper should previously be warmed in a steam bath. Or, the solution may be filtered with suction, using suitable types of apparatus (Pigs. 27,28). The funnel and filtering medium should be previously warmed. When the filtrate is collected in a thick glass suction flask,... 

Filtration of a mixture after completion of a reaction will often be necessary either to isolate a solid product which has separated out or to remove insoluble impurities or reactants, in which case the desired product remains in solution. In this section the filtration of cold solutions is described the filtration of hot solutions is considered in Section 2.20. 

FIG. 2 Gravity filtration of hot solution through fluted filter paper. 

If crystallisation commences as soon as the solvent cools or if large quantities of hot solution are to be filtered, the funnel (and fluted filter paper) should be warmed externally during the filtration (hot water funnel). Three types of hot water funnel are illustrated in Fig. 11,1, 6 no flames should be present whilst inflammable solvents are being filtered through the funnel of Fig. 11, 1, 6, a. Alternatively, the funnel may be surrounded by an electric heating mantle (see Section 11,57) the heat input may be controlled by a variable transformer. When dealing with considerable volumes of aqueous or other solutions which do not deposit crystals rapidly on cooling, a Buchner funnel may be used for filtration (see detailed account in Section 11,1 and Fig. 11 1, 7, c). The filter paper... 

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... 

Bis(triphenylphosphine)iminium chloride [PPN]C1, which is easily synthesized3 or available commercially (Alfa Products, Danvers, Massachusetts the systematic IUPAC name for this salt is /u-nitrido-bis(triphenylphosphorus)(l+), is purified by dissolution in hot water, addition of activated charcoal, filtration of the solution, and crystallization from the cooling solution. The resulting crystalline [PPN] Cl is filtered and dried under high vacuum overnight. 

An example along the above lines may be the new solvent-spun fiber Tencel, by Courtaulds, which recently was introduced. It is claimed to have an environmentally clean process, with virtually total recycling of the solvent." The manufacturing process is said to involve the dissolving of pulp in hot amine oxide, filtration of the solution, and then spinning into a bath containing a dilute solution of the solvent. The bath removes the amine oxide from the fibers, which are washed and dried, and the removed solvent is reclaimed for further use. The final fiber is said to have a different molecular structure from that of normal rayon, and a smooth surface and a round cross section. The fiber is claimed to be stronger than cotton and normal rayon in both the dry and the wet states. 

One simple method of crystallizing organic compounds is to find a solvent where the material is soluble when hot but insoluble when cold. A classic example of this method is benzoic acid, which is soluble in hot, but not in cold, water. Insoluble impurities are removed by hot gravity filtration the hot solution is filtered through a fluted filter paper into a heated flask (Figure 5.1). The flask into which the material is filtered needs to contain some hot solvent, so that the funnel and the flask are kept warm, and crystals do not form prematurely. The filter paper is fluted to maximize its surface area and to minimize the contact with the cooler funnel. This technique works well when any impurities are insoluble in the hot solution or soluble in the solvent at any temperature. 

Sometimes the crude substance may contain an insoluble impurity, and on cooling the solution it may be difficult to judge how much of the solid matter is merely undissolved impurity and how much is solute which has subsequently crystallised from solution. To avoid this difficulty, the hot solution should be filtered, and should thus always be absolutely clear before cooling is attempted. Therefore filter the hot solution into a clean tube through a very small fluted filter-paper contained in a correspondingly small glass funnel, which should have had its stem cut off as that shown in Fig. 6, p. 12 (and for the same reason). Unless the upper part of the filter is cut awav to reduce its size to a minimum, a large proportion of the solution will remain held mechanically in the pores of the paper itself and only a few drops of clear filtrate will be obtained. 

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... 

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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