Rotary evaporator

A rotary evaporator is a laboratory device, so one uses in a chemical laboratory for concentration ( concentration ) of solutions to evaporate the solvent. It was developed in the early 1950s. In 1957, the first commercial rotary evaporator by the Swiss company Büchi Labortechnik was prepared. Today, the rotary evaporator part of the standard equipment in the chemical laboratory.

In addition, be used in molecular gastronomy rotary evaporator as an innovative appliance for food preparation.

Construction

A rotary evaporator consists of a heating bath, a steam tube with standard ground joint, is attached to the lower end of the evaporator flask, a condenser with vacuum extension and a receiving flask. The rotatably mounted steam tube is guided at its upper end by a shaft seal in the condenser and can be displaced by means of a continuously adjustable motor to rotate. This whole assembly is height adjustable. In principle, it is nothing more than a distillation apparatus, except that the evaporator piston can rotate about its longitudinal axis. The necessary heat for distillation is supplied by a heating bath, into which the evaporator flask can be immersed by means of the height adjustment. The heating bath is most often, less often filled with water and with oil for the purpose of achieving higher operating temperatures. There is also the possibility to evacuate the apparatus by means of a (usually separate, not the actual rotary evaporator belonging to ) pump, i.e. lowering the internal pressure. (In most laboratories, the rotary evaporator is used in conjunction with a vacuum pump. )

Function and Applications

By heating the evaporator in the heating bath of the piston to be distilled material (usually solvent ) is vaporized, and the steam flows through the steam pipe to the radiator. There, it can condense on the cold surface and flow into the collecting flask. A separation of substances is achieved that differ their boiling points; so that at a given temperature, and other substances vaporise some (yet). By applying a vacuum to the boiling temperature of the apparatus can be reduced, whereby high-boiling solvent can be evaporated at a lower temperature than would be the case at atmospheric pressure (see the table below). This is important if the substance contained in the solvent to be freed from the solvent even in the process is sensitive to temperature. By working with a lower boiling point, decomposition can be prevented. The shaft seal seals off from the rotating steam pipe against atmospheric pressure, thus ensuring the maintenance of vacuum in the interior of the apparatus.

The rotary evaporator is however not applicable to any separation problem. If the boiling points of the components of the solution too close together, this apparatus is not sufficient for separation. As a practical example, it is noted that it is not possible to use mixtures of methanol and ethanol to separate cleanly. One then needs a fractional distillation. Also contributes to high-boiling solvents have to resort to a distillation under reduced pressure as achieved under the pressures and temperatures are restricted depending on the apparatus: With the use of a diaphragm pump of the lowest pressure in practice, at approximately a 10 mbar, and the achievable heating bath temperature is at The use of an oil bath to 180 ° C is limited.

There are now a rotary evaporator, which are fully automated. It can be manually or specify via a PC all experimental parameters (eg pressure, temperature, speed, duration of the experiment ).

Active principle - Why the rotation?

The principal difference between a normal (vacuum) distillation apparatus and a rotary evaporator is to be sought in the temperature distribution in the evaporator flask. While the contents thereof in a conventional distillation has approximately uniform temperature, this does not apply to the rotary evaporator.

At a moderate speed of rotation there is no strong mixing of the liquid, there is no thorough turbulence of the flask contents obtained by the rotation, in contrast to the use of a stirrer in a classic distillation. Rather, the fluid flows laminar to the bulb wall along. Between two work adhesion forces that cause the liquid of the movement of the piston wall follows; same time, the liquid to drain due to their viscosity at a limited speed. Therefore, a thin, uniform film of liquid is pulled up on the warm piston inner wall by the rotation. ( We will illustrate this fact by rotating a half-filled, inclined wine glass around its longitudinal axis ). This evaporates there immediately, as the required heat to evaporate is indeed passed through the piston wall into the interior of the piston and is right there available. Most of the supplied heat is thus consumed in the evaporation of the liquid film, so the body of the flask contents only little heat can be supplied.

Nevertheless finds the horizontal surface of the flask content an evaporation takes place and the heat required for this is withdrawn from the stationary liquid in the flask. This must therefore be much cooler than the liquid film. The effective pressure in the interior of the apparatus is, however, determined by the vapor pressure of the boiling point, so the liquid in the liquid film, the temperature of which is higher than that of the remainder of the flask contents. Therefore, the latter can not boil; see also the pressure dependence of the boiling point.

The rotation thus brings two advantages of the rotary evaporator compared to classical distillation:

Practical information

As a consequence of the principles shown above it follows that in the application of negative pressure, which is usually the case, first without heating the evaporator flask slowly, the pressure should be lowered to give the opportunity to the einzuengenden solution to cool by the onset of evaporation. If you dive then the piston is prevented from heating bath, so actually only evaporates the solvent from the liquid film on the inner wall of the piston, and a boiling retardation, or an undesirable sudden foaming of the solution.

Furthermore, it is clear that due to the cold temperatures that prevail in real operation of the rotary evaporator in the evaporator piston, the table below can only provide guidance for the selection of operating temperatures and suppress. If the bath temperature is for example set accurately so that the vapor pressure of the liquid corresponds to the ( generated by the vacuum pump ) pressure in the interior of the apparatus, so there can be no effective evaporation of the liquid film. In practice, the most advantageous to use work pressures are always lower than would result from the vapor pressure table. Is applied in a vacuum distillation, such as for example diethyl ether, it should be ensured that the valve is open, since it can easily lead to an excess pressure in a rotary evaporator at an insufficient cooling otherwise.

To obtain an optimum distillation of the solvent, the bath temperature is adjusted to 60 ° C. It should be between the individual stages, a temperature difference of 20 ° C. So that you can adjust the pressure according to this table. By this is meant, the steam amount was 40 ° C at the set pressure for the respective solvents, the water bath 60 ° C, and the cooling liquid should be 20 ° C. Optional would also 40 ° C-20 ° C-0 ° C, but the pressure can not be taken from the table, but must be determined self!