Filtration of crystals

A) Filtration of crystals from the cold mother-liquor. 

The filtration of crystals is carried outusing a small conical Buchner funnel (C, Fig. 4, p. 10) or a funnel of similar design but having a sintered filtration plate. Alternatively an ordinary conical funnel in which is placed a circular perforated plate can be used. 

However, there are cases when the mother Uqnor has a relatively high viscosity dne to high concentration of dissolved solute, the filtration of crystals from the mother liqnor may require higher pressures as the driving force. This occurs quite often in the sngar indnstry where solntions have a high dynamic viscosity np to 100 MPa (cP). In snch cases, the pressure drop in the bed, as compared with water, is 100-fold. 

In the most cases it is not necessary to effectuate so quickly the filtration of crystalls, because the stabilition of thermodinamic equilibrium need a longer period. This is so even if the racemic compound of resolving agent is resolved using one of enantiomers of racemic... 

For the filtration of very small quantities of crystals, the simple apparatus shown in Fig. 46 is often used. It consists of a fine glass rod (sometimes termed a filtration nail ) which is flattened at one end, the flattened surface being preferably roughened. It fits as shown into a small funnel which replaces F (Fig. 45). A circular piece of filter-paper is cut e-g.y with a clean sharp cork-borer) so as to fit completely and snugly over the flat end. After draining, the nail is raised and the filter-paper and crystals are removed with forceps and dried. 

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. 

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 ... 

Recrystallise from methylated spirit, using animal charcoal for this purpose, use about twice the minimum quantity of methylated spirit required to obtain a clear solution, and filter through a funnel preheated by the filtration of some boiling solvent, as the tribromobenzene separates very rapidly as the solution cools. The 1,3,5-tribromobenzene is thus obtained as colourless crystals, m.p. 122° yield, 3 g. 

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. 

The apparatus depicted in Fig. 11,34, 1, intended for advanced students, may be used for the filtration of a small quantity of crystals suspended in a solvent either a Hirsch funnel or a glass funnel with Witt filter plate is employed. The mixture of crystals and mother liquor is filtered as usual through the funnel with suction. Rotation of the three-way tap wifi allow air to enter the filter cylinder, thus permitting the mother liquor to be drawn oflF by opening the lower tap. The mother liquor can then be applied for rinsing out the residual crystals in the vessel, and the mixture is again filtered into the cylinder. When all the crystals have been transferred to the funnel and thoroughly drained, the mother liquor may be transferred to another vessel the crystals may then be washed as already described (Section 11,32). 

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... 

---