Adhering mother liquor

A few words should be added about the washing of crystalline precipitates with the object of freeing them from adherent mother liquor. The same solvent as was used for crystallisation must always be employed and, since its solvent power for the substance, even in the cold, leads to more or less appreciable loss, it must be used in the smallest possible amounts. Suction should not be applied while washing the precipitate is saturated with the solvent and then the pump is turned on. 

In preparing sodium monohydrated carbonate, the wet crystals may combine with the adherent mother liquor, forming the decahydrated salt, and this accounts for the high results usually obtained in determinations of the amouut of water in the crystals and it possibly explains how a number of other hydrated salts have been reported at different times. The evidence of the individuality of the crystals of sodium carbonate with 2, 2 , 3, 5, or 6H20 is quite inadequate. 

The precipitates commonly encountered in the analytical laboratory are practically insoluble and can be washed with unlimited quantities of liquid. Organic products, on the other hand, are usually more or less easily soluble in the solvent employed, and hence it is always necessary to use the smallest possible amount of wash liquid which will completely remove the adhering mother liquor. For this reason, gravity filtration is of litde value. Instead, filtration methods must be used which provide an appreciable pressure difference above and below the filter. This pressure difference is obtained by applying pressure above the filter, or suction below it. 

The feed enters near the center of the rotor. The liquid phase passes upward and overflows at the top of the rotor. As it does so, its solid content is sedimented out to the wall of the rotor by a centrifugal force of over 3000 g. The solids, sedimented against the rotor wall, are conveyed downward by the helical conveyor, carried up out of the liquid layer, and given an opportunity to drain on the smaller diameter beach section before discharging at the bottom to a receiver or conveyor. If displacement of the adhering mother liquor is desired, a rinse of pure solvent can be applied to the solids on the beach. 

Equilibrium incorporation of impurities can take place if they occupy spaces meant for other solute molecules (substitional defects), actual open spaces (vacancy defects), or smaller interstitial spaces. Nonequilibrium impurity uptake (in inclusions, dislocation planes, permanently adsorbed entities, and impurities left in adhering mother liquor) is common at the higher rates pushed in industrial processes, because the solute molecules attracted to the surface at a high rate can physically slow down reverse diffusion of the impurity. Additionally, any adsorptive attraction of the impurity to the crystal surface further exacerbates this effect. 

Let us suppose that the double salt is in equilibrium with the solution at a definite temperature, and that the composition of the solution is represented by the point e. The greater part of the solution is now separated from the solid phase, and the latter, together with the adhering mother liquor is analysed. The composition (expressed, as before, in gram-molecules of A and B to 100 gram-molecules of water) will be represented by a point e,g. f) on the line eS, where S represents the composition of the double salt. 

Another and neater way of separating the crystals from the adherent mother liquor is to pack two balanced 12-ml centrifuge tubes each half full with glass wool, distribute the crystal mush equally between the two tubes so that they remain balanced, and centrifuge for a few minutes (see Chapter III). This salt is used in preparing Nessler s solution, a sensitive reagent for ammonia, and is worth keeping. It may, however, be used for the preparation to follow. 

First suck the crystals as free as possible from adhering mother liquor. Then turn off the suction, pour over the crystals about 10 ml of concentrated nitric acid, and suck the acid through... 

Occurrence of Spontaneous Crystallization of an Unseeded Enantiomer. When we consider the purity drop on the phase diagram, it is the prerequisite that the seeds added to cause the preferential resolution are completely pure. However if the seeds are contaminated by the undesired enantiomer, the phenomena could be completely different. Since in general seeds are taken from a previous batch, they are likely to have been contaminated by the adhered mother liquor due to incomplete phase separation or by some non-equilibrium crystallization phenomena such as inclusion of mother liquor or agglomeration of crystalline particles. For the case of the former, drying of the crystals generates small particles of the undesired enantiomer which will then be introduced to a racemic solution as seeds. 

The PPE was obtained from the precipitator as a free flowing powder. Filtration and washing of the product need much attention because the adhering mother-liquor regains its solvent power upon depressurisation. If the filtration and washing are performed improperly, the precipitated particles glue together and form gel-like lumps... 

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