Heidolph LABOROTA 20 eco - Manual (Page 65)

65
E
DISTILLATION SETTING
20. General
Using rotational evaporators for thermal breakdown of substances needs some basic
considerations about optimizing the distillation process. Parameters stipulated below are
to be considered for making optimal settings.

Rotational speed, evaporator flask
Increasing rotational speed will increase rate of distillation. Shorter distillation time
preserves thermal stability of your substances to be distilled.

Difference between heating bath and vapor temperatures:
This temperature difference should not be less than 20° C to maintain proper rate of
distillation. As long as your substances in distillation allow, you may even chose a higher
temperature difference.
Rule of thumb: doubling actual temperature difference doubles rate of distillation.
 Temperature difference between vapor temperature and coolant:
To maintain adequate condensation rates, we recommend a 20° C temperature
difference between the solvent vapor temperature and coolant. A low temperature
difference will affect solvent recovery. For substances with high evaporation heat, you
should increase temperature difference.
21. Solvent data

Examples for using table and chart:

The table displays essential substance data, the chart makes reference to the
relationship between pressure and boiling point of some solvents. In this case, 1/T is
plotted as a function of log p in accordance with the Clausius-Clapeyron equation.
a) Distillation of a solvent contained in the table:

On the vacuum controller that can be supplied as an accessory, the value given in the
column "Vacuum for Spd at 40°C" is set as the required value for Set Vacuum.
b)
Distillation of a solvent contained in the chart, with a boiling point other than
40°C:

Mark the boiling point desired on the temperature axis of the chart

Draw a line to the right to get an intersection with the solvent

Vacuum pressure required for this one situation is found by drawing a vertical line from
this intersection down to the horizontal axis.
c) Distillation of a solvent not contained in the table or chart:

The following notes may be helpful to determine the appropriate vacuum pressure:

The line gradient is a function of enthalpy of evaporation. It is similar for chemically
related substances with boiling points close to each other. Hence, the plotted lines may
be used as a guide for substances with slightly different boiling points.

Using a water-jet or diaphragm-type pump, you may lower boiling point by approx. 100°C.

Rule of thumb: decrease pressure to a half will lower boiling point by approx. 15°C.