Solution crystallization, definition

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Recently, diamond synthesis has been successfully performed under high-temperature, high-pressure conditions in a system using kimberlite powder, various carbonates, sulphates or water as the solvent [13], [14]. Higher pressure and temperature conditions are required in a non-metallic solution than in a metallic solution, and the crystals obtained are mainly simple octahedral, differing from those observed in crystals grown from metallic solutions. Crystals synthesized in a non-metallic solution show the same characteristics as natural diamond Tracht. These observations indicate that the solvent components have a definitive effect upon surface reconstruction, and thus on the morphology of the crystals. 

E) Reduction of Nitrobenzene to Hydrazobenzene. Place in an eight-inch tube arranged for reflux 2 g of nitrobenzene, 3 ml of 6 N sodium hydroxide and 12 ml of ethanol. Heat the mixture in a water bath to about 75°. Remove the burner, and add zinc dust in 0.2 g portions until about 3 g have been added and the solution becomes pale yellow. The zinc dust is added by slightly raising the cork for a moment. The reaction is allowed to subside before any further amount of zinc dust is added. If it becomes violent remove the hot bath and cool the tube. When the reduction is complete, the mixture is poured into 40 ml of water, cooled, and then 10-12 ml of concentrated hydrochloric acid is added slowly, with stirring, until the solution is definitely acid. The crystals of hydrazobenzene which separate are filtered, and washed with cold water. 

In some circumstances, solutions can be prepared that contain more solute than needed for a saturated solution at a particular temperature. These solutions are said to be supersaturated. However, we must qualify this definition by noting that a supersaturated solution is unstable. Disturbances, such as jarring, stirring, scratching the walls of the container, or dropping in a seed crystal, cause the supersaturation to return to saturation, releasing heat. When a supersaturated solution is disturbed, the excess solute crystallizes out rapidly, returning the solution to a saturated state. 

The method of oxidation is essentially that of Russigd The product obtained is slightly but definitely better than that produced at a lower temperature, or by adding the sulfuric acid to the dichromate solution, or by using ferric clrloride as in A). These latter procedures give the same yield, but the product is less pure and contains a black, ether-soluble impurity which must be washed out carefully after crystallization from ether. 

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