Recrystallization: Introduction

Recrystallization is an experimental technique that utilizes solubility's temperature dependence to purify a solid.

The solubility curve for a typical solute/solvent combination is shown in the diagram at right. A point along the upward sloping line on the graph correspond to the maximum concentration possible at a specific temperature. At these points the solution is saturated. Supersaturated and unsaturated solutions are represented by regions above and below the maximum concentration line respectively. Notice that the maximum concentration increases as temperature increases; a behavior typical of many solutions.

In a typical recrystallization experiment, the solid to be recrystallized is dissolved in a minimum amount of hot solvent (rightmost point in above figure) . The mixture is then allowed to slowly cool. As the temperature drops, the maximum amount of solute capable of being dissolved in the solvent decreases. When the solution's concentration is equal to the maximum concentration given by the solubility curve, the solution is just saturated. At this point additional solid will not dissolve. At still lower temperatures, the maximum solubility is less than the original sample solution's concentration. Consequently, some of the dissolved solute must return to the solid state, that is recrystallize, in order for the solution's concentration to follow the solubility curve. Thus, the recrystallized solute is surrounded by a solution that is saturated for that specific temperature.

The reverse process, also illustrated in the figure above (movement to the right), occurs when the solid/solution mixture is heated. As the temperature increases, so does the maximum allowable solution concentration. Since higher maximum concentrations are now allowed, the solid begin redissolves. When all of the solid has completely dissolved no additional concentration increases are possible.