Substance filtration

For the more rapid separation of insoluble substances, filtration under reduced pressure is performed (Fig. 19). A porcelain Buchner... 

The quantity of water used in the reaction must be sufficient to ensure that calcium chloride remains in the solution, whilst calcium hypochlorite is precipitated as a solid substance. Filtration is very difficult and is carried out by means of a special hydraulic press which operates under a pressure of 150 atm. sq. cm. Stainless steel gauze is used as a filter cloth. 

Isolation and purification of substances filtration, decantation, centrifugation, magnetic separation, crystallization, distillation. 

After the enzymatic treatment the juice can be clarified. Flocculation aids and fining aids (gelatine, silica sol, etc.) help to coagulate the cloudy substances and facilitate their separation by settling and filtration. Bentonites can be used to eliminate proteins and other cloudy substances. Filtration (Kieselgur precoat filtration with plate and frame filters, rotating vacuum filters, and sheet filters as police filters ) is used to produce a crystal clear juice. 

Large volumes of water often need to be processed to obtain sufficient quantities of aquatic humic substances filtration is the slowest step in this process. In a comparison of filter flow rates, Cranston and Buckley (1972) found filtering times for 47 mm (millimeter) diameter filters increased in this order glass-fiber filters < silver-membrane filters < organic-membrane filters (cellulose-acetate and cellulose-nitrate). They also report that substantial variation exists between different silver filters from the same manufacturer. This is caused by variation in permeability (number of pores per unit area), not pore size, and did not occur with the other filters studied. 

Sephadex A trade name for an insoluble hydrophilic substance prepared by cross-linking dextran, and used in gel filtration. It can also be linked to acidic or basic groups for ion exchange or to alkanes for the chromatography of lipophilic compounds. 

Filtration. Before discussing the practical details of the purification of solid substances by recrystallisation, it is convenient to describe here the general methods of filtration. The two principal occasions in organic chemistry when filtration is necessary are ... 

A) A solid substance has crystallised from a solution, and it is necessary to separate the crystals (i.e., the solute) from the cold mother-liquor by filtration. 

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. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

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... 

It is frequently necessary to concentrate a filtrate in order to obtain a further crop of crystals, or it may be necessary to concentrate a solution to a smaller volume. If the solvent is water and the substance is not volatile in steam, simple evaporation on a large dish on a steam or water... 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Owing to the great tendency of hydrazobenzene to undergo oxidation, all operations involving filtration should be carried out as rapidly as possible and air should not be drawn through it unnecessarily. The substance should be dried in a vacuum desiccator it can only be preserved in a colourless condition if it is kept in an atmosphere of carbon dioxide or nitrogen or in sealed vessels. 

Purification of anthracene. Dissolve 0-3 g. of crude anthracene (usually yellowish in colour) in 160-200 ml. of hexane, and pass the solution through a column of activated alumina (1 5-2 X 8-10 cm.). Develop the chromatogram with 100 ml. of hexane. Examine the column in the hght of an ultra-violet lamp. A narrow, deep blue fluorescent zone (due to carbazole, m.p. 238°) will be seen near the top of the column. Immediately below this there is a yellow, non-fluorescent zone, due to naphthacene (m.p. 337°). The anthracene forms a broad, blue-violet fluorescent zone in the lower part of the column. Continue the development with hexane until fluorescent material commences to pass into the filtrate. Reject the first runnings which contain soluble impurities and yield a paraffin-hke substance upon evaporation. Now elute the column with hexane-benzene (1 1) until the yellow zone reaches the bottom region of the column. Upon concentration of the filtrate, pure anthracene, m.p. 215-216°, which is fluorescent in dayhght, is obtained. The experiment may be repeated several times in order to obtain a moderate quantity of material. 

Method 1. Dissolve 76 g. of thiourea in 200 ml. of warm water in a 750 ml. or 1 litre round-bottomed flask. Dilute the solution with 135 ml. of rectified spirit and add 126-5 g. of benzyl chloride. Heat the mixture under reflux on a water bath until the benzyl chloride dissolves (about 15 minutes) and for a further 30 minutes taking care that the mixture is well shaken from time to time. Cool the mixture in ice there is a tendency to supersaturation so that it is advisable to stir (or shake) the cold solution vigorously, when the substance crystallises suddenly. Filter off the sohd at the pump. Evaporate the filtrate to about half bulk in order to recover a further small quantity of product. Dry the compound upon filter paper in the air. The yield of hydrochloric acid filter off the sohd which separates on cooling. Concentrate the filtrate to recover a further small quantity. The yield of recrystalhsed salt, m.p. 175° is 185 g. some of the dimorphic form, m.p. 150°, may also separate. 

The following alternative procedure is recommended and it possesses the advantage that the same tube may be used for many sodium fusions. Support a Pyrex test tube (150 X 12 mm.) vertically in a clamp lined with asbestos cloth or with sheet cork. Place a cube (ca. 4 mm. side = 0 04 g.) of freshly cut sodium in the tube and heat the latter imtil the sodium vapour rises 4 5 cm. in the test-tube. Drop a small amount (about 0-05 g.) of the substance, preferably portionwise, directly into the sodium vapour CAUTION there may be a slight explosion) then heat the tube to redness for about 1 minute. Allow the test tube to cool, add 3-4 ml. of methyl alcohol to decompose any unreacted sodium, then halffill the tube with distilled water and boil gently for a few minutes. Filter and use the clear, colourless filtrate for the various tests detailed below. Keep the test-tube for sodium fusions it will usually become discoloured and should be cleaned from time to time with a little scouring powder. 

B. With 70 per cent, sulphuric acid. Reflux 1 g. of the substance (e.g., benzanilide) with 10-15 ml. of 70 per cent, sulphuric acid (4 3 by volume) for 30 minutes. Allow to cool and wash down any acid which has sublimed into the condenser with hot water. Filter off the acid, wash it with water, and examine for solubility, etc. Render the filtrate alkaline... 

Incipient hazes may not be removed at all by simple filtration. An array of fining procedures have been developed to achieve stable clarity in such cases. Fining agents ate substances that ate or become insoluble in wines, and, as they precipitate, adsorb or coptecipitation incipient sources of cloudiness. Ptopedy used, the fining agents themselves ate not retained in the wines and thein effect is subtractive rather than additive. 

Enclosed agitated filters are useful when volatile solvents are in use or when the solvent gives off toxic vapor or fume. Another significant advantage is that their operation does not require any manual labor. Control can be manual or automatic, usually by timers or by specific measurements of the product. Most filters are made of mild steel, with the exposed surfaces protected by lead, tile, mbber lining, or by coating or spraying with other substances as necessary. Filtration areas up to 10 m are available and the maximum cake thickness is 1 m. Apphcations are mainly in the chemical industry for the recovery of solvents. 

Bonded Whiskey. Bonded whiskey is whiskey stored at least four years in wooden containers where the spirits have been in contact with the wood surface. It is unaltered from the original character by the addition or subtraction of any substance other than by filtration or chill proofing, is reduced in proof by the addition of water to 100° proof (50 vol %) and botded at 100° proof, and is produced at the same distillery in the same season (January through June or July through December). 

The ferrous ions that dissolve from the anode combine with the hydroxide ions produced at the cathode to give an iron hydroxide precipitate. The active surface of ferrous hydroxide can absorb a number of organic compounds as well as heavy metals from the wastewater passing through the cell. The iron hydroxide and adsorbed substances are then removed by flocculation and filtration. The separation process was enhanced by the addition of a small quantity of an anionic polymer. 

D. Methionine.—A suspension of 21.5 g. (0.063 mole) of this tricarboxylic acid in 350 cc. of hot water is heated on the steam bath and 40 cc. of concentrated hydrochloric acid (sp. gr. 1.19) is added. Carbon dioxide is immediately evolved and the substance goes into solution. After heating for one and a half hours, 200 cc. more of concentrated hydrochloric acid is added and heating is continued for forty-five minutes longer. The solution, on cooling, deposits phthalic acid this is filtered off and washed with two 50-cc. portions of water (Note 3). The combined filtrate and washings are evaporated to dryness on the steam bath under reduced pressure, and the dry residue is dissolved in... 

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