Rotary evaporator
A rotary evaporator or Rotavapor is a laboratory device that is used in the chemical laboratory to concentrate (concentrate) solutions , i.e. to evaporate the solvent . It was developed by Lyman C. Craig and colleagues in the early 1950s . In 1957 the first commercial rotary evaporator was manufactured by the Swiss company BUCHI Labortechnik. Today the rotary evaporator is one of the standard devices in the chemical laboratory.
In addition, rotary evaporators serve as innovative devices for food preparation in molecular cuisine.
construction
A rotary evaporator consists of a heating bath, a steam pipe with standard ground joint , to the lower end of which the evaporation flask is attached, a condenser with a vacuum attachment and a receiving flask. The rotatable steam pipe is led through a shaft seal into the cooler at its upper end and can be set in rotation using a continuously adjustable motor . This whole structure is adjustable in height. In principle, it is a distillation apparatus whose evaporation flask can rotate around its longitudinal axis. The heat required for the distillation is supplied via a heating bath , into which the evaporating flask can be immersed by means of the height adjustment. The heating bath is usually filled with water , and more rarely with oil, in order to achieve higher working temperatures. There is also the possibility of evacuating the apparatus by means of a pump (generally separate and not belonging to the actual rotary evaporator) , i.e. H. lower the internal pressure. The rotary evaporator is usually used in conjunction with a vacuum pump.
Function and use
When the evaporation flask is heated in the heating bath, the substance to be distilled (mostly solvent ) evaporates and the steam flows through the steam pipe into the cooler. There it is deposited on the cooling surface, the condensate flows into the collecting flask. A separation of substances is achieved because their boiling points differ: At a given temperature certain substances evaporate and others do not (yet). Applying a vacuum to the apparatus lowers the boiling temperature so that higher-boiling solvents can evaporate at a lower temperature than would be the case at normal pressure (see also the table below). This temperature reduction is important if the substance contained in the solvent is temperature-sensitive. The reduced boiling temperature can prevent decomposition. The shaft seal in the device seals the rotating steam pipe against atmospheric pressure and maintains the vacuum inside the device.
The rotary evaporator is not applicable to every separation problem. If the boiling points of the components of the solution are too close to one another, they will not be separated. For example, it is not possible to cleanly separate mixtures of methanol and ethanol on a rotary evaporator . A fractional distillation is required for this . Even with solvents that boil too high, you have to resort to distillation at reduced pressure, as the achievable negative pressures and temperatures are limited depending on the apparatus: When using a diaphragm pump , the lowest pressure in practice is approx. 10 mbar and the achievable heating bath temperature is limited to 180 ° C when using an oil bath.
There are also fully automated rotary evaporators. In contrast to conventional devices, they can not only regulate input parameters such as pressure, temperature, speed. You can also monitor and control the entire process.
Working principle - why the rotation?
The main difference between a normal (vacuum) distillation apparatus and a rotary evaporator is to be found in the temperature distribution in the evaporation flask. While its content is almost the same temperature in a conventional distillation , this does not apply to the rotary evaporator.
At a moderate speed of rotation, there is no thorough mixing of the liquid, as the rotation does not result in a thorough swirling of the flask contents, in contrast to the use of a stirrer in a classic distillation. Rather, the liquid flows in a laminar manner along the piston wall. Adhesive forces act between the two , causing the liquid to follow the movement of the piston wall; at the same time, due to its viscosity , the liquid can only run off at a limited speed. As a result of the rotation, a thin, uniform film of liquid is drawn up on the warm inner wall of the piston. (This fact can be illustrated by turning a half-filled, inclined wine glass around its longitudinal axis). This also evaporates quickly there, as the heat required for evaporation is conducted through the flask wall into the interior of the flask and is available exactly there. Most of the heat supplied is therefore used up when the liquid film evaporates, so that the main part of the flask contents only a little heat from outside. The speed of rotation must be adapted to the viscosity of the solution. Low-viscosity solutions must rotate faster than higher-viscosity solutions.
Evaporation also takes place on the horizontal surface of the contents of the flask: The heat required for this is extracted from the liquid in the flask. This is therefore significantly cooler overall than the liquid film. However, the effective pressure inside the apparatus is determined by the vapor pressure of the boiling liquid, i.e. the liquid in the liquid film, the temperature of which is higher than that of the rest of the flask. Hence the latter cannot boil; see. also the pressure dependence of the boiling point .
The rotation therefore has two advantages of the rotary evaporator compared to classic distillation:
- Enlargement of the surface and therefore an increase in the rate of evaporation
- Prevents the solution from bubbling and billowing up, as evaporation mainly occurs from the liquid film on the surface of the flask.
Practical advice
As a consequence of the principles shown above, with the most common use of negative pressure, the pressure should first be slowly reduced without heating the evaporation flask in order to give the solution to be concentrated the opportunity to cool down as evaporation begins. If the flask is then immersed in the heating bath, only the solvent actually evaporates from the liquid film on the inside wall of the flask, and a delay in boiling or undesired sudden foaming of the solution is avoided.
Due to the very different temperatures in real operation of the rotary evaporator in the evaporation flask, the table below can only provide guidelines for the selection of working temperatures and negative pressures. If, for example, the bath temperature is set precisely in such a way that the vapor pressure of the liquid corresponds to the pressure inside the apparatus (generated by the vacuum pump), no effective evaporation of the liquid film can take place. In practice, the most advantageous working pressures to be used are always lower than would result from the vapor pressure table. If a vacuum is not applied during a distillation, as is the case with diethyl ether, for example, it should be ensured that the valve on the rotary evaporator is open, otherwise overpressure can easily arise if the cooling is insufficient.
The bath temperature is set to 60 ° C. in order to obtain optimum distillation of the solvent. There should be a temperature difference of 20 K between the individual stages - if the values from the table are used, the steam has a temperature of 40 ° C at the set pressure for the respective solvent, the water bath 60 ° C and the cooling liquid 20 ° C. 40 ° C / 20 ° C / 0 ° C would also be possible, but the pressure cannot then be taken from the table.
| solvent | Negative pressure (mbar) for boiling point at 40 ° C |
|---|---|
| acetone | 556 |
| Acetonitrile | 226 |
| 1-pentanol (amyl alcohol) | 11 |
| benzene | 236 |
| n -butanol | 25th |
| tert -butanol | 130 |
| Chlorobenzene | 36 |
| chloroform | 474 |
| Cyclohexane | 235 |
| 1,2-dichloroethane | 210 |
| cis - 1,2-dichloroethene | 479 |
| trans - 1,2-dichloroethene | 751 |
| Diisopropyl ether | 375 |
| 1,4-dioxane | 107 |
| Dimethylformamide (DMF) | 14th |
| 1,2-dimethoxyethane | 295 |
| Ethanol | 175 |
| acetic acid | 44 |
| Ethyl acetate (ethyl acetate) | 240 |
| Heptane | 120 |
| Hexane | 335 |
| Isopropanol | 137 |
| 3-methyl-1-butanol (isoamyl alcohol) | 14th |
| Butanone (ethyl methyl ketone) | 243 |
| Methanol | 337 |
| Dichloromethane (methylene chloride) | no negative pressure |
| n -pentane | no negative pressure |
| 1-propanol | 67 |
| Pentachloroethane | 13 |
| 1,1,2,2-tetrachloroethane | 35 |
| 1,1,1-trichloroethane | 300 |
| Tetrachlorethylene | 53 |
| Carbon tetrachloride | 271 |
| Tetrahydrofuran (THF) | 374 |
| toluene | 77 |
| Trichlorethylene | 183 |
| water | 72 |
| Xylene | 25th |
safety instructions
- When working with negative pressure, a rotary evaporator must either be in a fume hood or be surrounded by safety walls, e.g. B. be surrounded by acrylic glass so that as few splinters as possible are distributed in the event of an implosion.
- Low-boiling, highly flammable solvents such as diethyl ether and carbon disulphide should not be distilled off using a rotary evaporator because the risk of fire is then very high. The heat from a poorly lubricated lip seal can be enough to cause the fumes to ignite.
- Solutions of unstable products such as azides or peroxides must not be concentrated using a rotary evaporator, or only if special conditions are met. Under no circumstances should the solution be completely evaporated. In the case of ethers such as dioxane or tetrahydrofuran and many other ethers, ether peroxides can be formed which, in highly concentrated form, tend to explode.
Web links
- Vapor pressure table for solvents ( Memento from July 24, 2013 in the Internet Archive ) (PDF; 122 kB) BUCHI Laboratory Technology
Individual evidence
- ^ LC Craig, JD Gregory, W. Hausmann: Versatile Laboratory Concentration Device . In: Analytical Chemistry . tape 22 , no. 11 , 1950, pp. 1462-1462 , doi : 10.1021 / ac60047a601 .
- ^ Büchi Labortechnik GmbH .
- ^ Walter Wittenberger: Chemical laboratory technology . 7th edition. Springer-Verlag, Vienna / New York 1973, pp. 186–187, ISBN 3-211-81116-8 .
- ↑ Becker, HGO (Heinz GO), Beckert, R .: Organikum: organic-chemical basic internship . 23rd edition. Wiley-VCH, Weinheim 2009, ISBN 978-3-527-32292-3 ( OCLC 318670238 [accessed June 26, 2019]).
- ↑ Operating instructions for the rotary evaporator of Bielefeld University accessed on May 31, 2019.