Insoluble impurities

B) A hot solution has to be filtered to remove traces of insoluble impurities, and kept hot meanwhile to prevent crystallisation of the main solute, which would otherwise choke up the filter. 

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

Students are familiar with the general process of recrystallisa-tion from their more elementary inorganic work. Friefly, it consists in first finding a solvent which will dissolve the crude material readily when hot, but only to a small extent when cold. The crude substance is then dissolved in a minimum of the boiling solvent, the solution filtered if necessary to remove any insoluble impurities, and then cooled, when the solute will crystallise out, leaving the greater part of the impurities in solution. The crop of crystals is then filtered off, and the process repeated until the crystals are pure, and all impurities remain in the mother-liquor. 

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. 

The crude material is therefore placed either in a round-bottomed bolt-head flask (Fig. 8) or in a conical flask, the solvent added (again in slight deficiency) and a reflux water-condenser fitted to the flask as shown. The mixture is boiled either on a water-bath or over a gauze, and then more solvent added cautiously down the condenser until a clear solution (apart from insoluble impurities) is again obtained. It is then filtered hot as described above. 

Animal charcoal has a further use. Occasionally, when recrystallising a crude product, it is found that the hot solutioi contains a very fine suspension of an insoluble impurity. This suspension may be so fine... 

For purification, transfer the acid to a 150 ml. flask containing 60 ml. of water, boil the mixture under reflux, and then add acetic acid in 5 ml. portions down the condenser until almost all the solid has dissolved avoid an excess of acetic acid by ensuring that the solvent action of each addition is complete before the next portion is added. A small suspension of insoluble impurity may remain. Add 2 g. of animal charcoal, boil the solution again for 10-15 minutes, and then filter it through a preheated Buchner funnel. Cool and stir the filtrate, which will deposit pale cream-coloured crystals of the acid. Collect as before and if necessary repeat the recrystallisation. Yield of pure acid, 9 g. m.p. 227-229°. 

The recrystallisation of diazoaminobenzene has to be performed with care, as the substance is freely soluble in most liquids and tends moreover to decompose if its solution is not rapidly cooled. Place 2 g. of the crude, freshly prepared, well-drained material in a boiling-tube, add about 15-20 ml. of ethanol and 1-2 drops of 10% aqueous sodium hydroxide solution, and then heat rapidly until boiling if the solution should contain insoluble impurities, filter through a small fluted paper, and at once cool the filtrate in ice-water. The diazoaminobenzene should rapidly crystallise out from the cold and stirred solution filter the crystals rapidly at the pump whilst the solution is still cold, as... 

If the crude substance contains an insoluble impurity, difficulty may be experienced at a later stage in estimating how much solute has crystallised from the cold solution. The hot solution should therefore be filtered into another tube through a very small fiuted filter paper contained in a small short-stemmed funnel. The solution must always be clear before cooling is attempted. 

The fused product contains about 60—85% barium sulfide, unreacted barium sulfate, and impurities present in barite and ash. The soluble barium sulfide is extracted from the mixture with water and separated from the insoluble impurities by filtration. 

Soluble impurities can be extracted by washing with deionized or distilled water foUowed by filtration (1,12,26). Powders prepared by wet chemical synthesis are often washed and filtered for purification prior to use. The dewatering (qv) process can be enhanced by pressure filtration. Organic solvents can be used to remove water-insoluble impurities and wash-water sensitive materials. 

Michler s ketone [4,4 -bis(dimethylamino)benzophenone] [90-94-8] M 268.4, m 179", pK 9.84. Dissolved in dilute HCl, filtered and ppted by adding ammonia (to remove water-insoluble impurities such as benzophenone). Then crystd from EtOH or pet ether. [Suppan J Ghent Soc, Faraday TransI 71 539 1975.] It was also purified by dissolving in benzene, then washed with water until the aqueous phase was colourless. The benzene was evaporated off and the residue recrystd three times from benzene and EtOH [Hoshino and Kogure J Phys Ghent 72 417 1988],... 

Sodium anthraquinone-l,5-disulfonate (HjO) [853-35-0] M 412.3. Separated from insoluble impurities by continuous extraction with water. Crystd twice from hot water and dried under vacuum. 

Sodium p-toluenesulfonate [657-84-1] M 194.2, pK -1.34 (for -SOs"). Dissolved in distilled water, filtered to remove insoluble impurities and evaporated to dryness. Then crystd from MeOH or EtOH, and dried at 110°. Its solubility in EtOH is not high (maximum 2.5%) so that Soxhlet extraction with EtOH may be preferable. Sodium p-toluenesulfonate has also been crystd from Et20 and dried under vacuum at 50°. 

Production of A1 metal involves two stages (a) the extraction, purification and dehydration of bauxite, and (b) the electrolysis of AI2O3 dissolved in molten cryolite Na3AlF6. Bauxite is now almost universally treated by the Bayer process this involves dissolution in aqueous NaOH, separation from insoluble impurities (red muds), partial precipitation of the trihydrate... 

Nitroso-pinene may be prepared for identification purposes as follows To a solution of 12 grams of sodium in 30 c.c. of 90 per cent, alcohol, 100 grams of pinene-nitrosochloride are added. The mixture is boiled on a water-bath, under a reflux condenser, until the reaction is complete. Water is added, the-clear solution filtered from insoluble impurities, and the filtrate poured into excess of acetic acid. The nitroso-pinene separates as an oil which solidifies to a yellowish mass in a few days. This is broken up, washed with water, and dried on a porous plate. It can be recrystallised from acetic ether, when it is obtained in the pure condition, and then melts at 132°. 

The mixture was stirred at ice bath temperature for 2 hours, 1 ml of methanol was added and the mixture was filtered to remove insoluble impurities. Two milliliters of water were added to the filtrate and the pH was adjusted momentarily to pH 1.5, to effect removal of theenamine block, and then to pH 4.5 with triethylamine. After stirring for an additional hour at ice bath temperature the reaction product,7-(D-0 -phenylglycylamido)-3-chloro-3-cephem-4-carboxylic acid (zwitterion) precipitated from the reaction mixture as a crystalline solid. 

After 30 minutes the solid sulfinic acid is separated on a fritted-glass filter. The sulfinic acid is dissolved from the filter by a mixture of 750 ml. of ether and 750 ml, of methylene chloride. The solution is dried over calcium chloride and evaporated to dryness under reduced pressure (bath temperature 25°) (Note 5). The residue is suspended in 50 ml. of water, and small portions of dilute ammonia are added to the well-stirred suspension until it has a pH of 9 (Note 6). Insoluble impurities are separated by filtration, and 2-nitrobenzenesulfinic acid is precipitated from the filtrate by adding 5-ml. portions of 6N hydrochloric acid with cooling the sulfinic acid precipitated by each portion of acid is separately collected on a Buchner funnel (Note 7). The acid, a pale yellow solid, is dried on a clay plate in a vacuum desiccator over potassium hydroxide pellets, m.p. 120-125° (dec.), weight 9.4-14.9 g. (50-80%). If the 2-nitrobenzenesulfinic acid is to be used for the hydrogenation of the next step high purity is required, and it is generally advisable to reprecipitate the acid once more in the same way (Note 8). 

The crude material is suitable for hydrolysis, and for other reactions, without further purification. The pure diacetate may be obtained by recrystallizing from 150 cc. of hot alcohol. The hot solution is filtered through a fluted filter to remove insoluble impurities. The yield of pure />-nitrobenzaldiacetate is 43-46 g. (47-50 per cent of the theoretical amount), m. p. 125-126°. 

Some precipitates dissolve when the temperature is changed. This strategy is used to purify precipitates. The mixture is heated to dissolve the solid and filtered to remove insoluble impurities. The solid is then allowed to re-form as the solution cools and is removed from the solution in a second filtration. Complex ion formation (Section 11.13) can also be used to dissolve metal ions. 

A compound whose solubility increases with temperature can be purified by recrystallization. The impure solid is dissolved in a minimum volume of hot water. The hot solution is filtered to remove insoluble impurities, and then the solution is cooled in an ice bath. The solubility of the compound decreases as the temperature drops, causing the substance to precipitate from solution. Soluble impurities usually remain in solution. Purification by recrystallization is not restricted to aqueous solutions. An organic solid can be purified by recrystallization from an appropriate organic solvent. 

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