ASTM D1078 Test Method for Distillation Range of Volatile Organic Liquids
10. Factors Causing Superheating
10.1 In general, any condition whereby the temperature surrounding the vapor exceeds the temperature of the vapor in equilibrium with the liquid will cause superheating. Specific factors conducive to superheating are as follows, and should be avoided.
10.2 Flame in Contact with the Flask - The applied gas flame should be prevented from contacting more than the specified portion of the flask by the following procedures:
10.2.1 Maintain the correct overall dimensions and specified hole diameter of the asbestos cement board. The hole must be perfectly circular, with no irregularities.
10.2.2 Use a board that is free of cracks and checks.
10.2.3 Set the flask snugly in the hole in the upper insulating board.
10.3 Application of Heat - Attention should be given to burner placement, position, and character of flame, as follows:
10.3.1 Apply the source of heat directly beneath the flask. Any variation would result in heating a larger portion of surrounding air to a higher temperature than that of the flask.
10.3.2 The flame should not have a larger cross section than is necessary, and should be nonluminous.
10.3.3 Place the burner at a level such that the complete combustion area of a nonluminous flame is approximately 3/4 in. (20 mm) below the board.
10.4 Extraneous Heat Source - An extraneous source of heat such as sunlight falling directly on the flask can cause superheating.
10.5 Condition of Equipment - Observe caution in employing the apparatus for immediate reuse. For low-boiling materials, cool the heating unit to room temperature before starting the test.
10.6 Use of Electric Heaters - Electric heaters generally cause superheating. These should be used only after they have been proven to give results comparable to those obtained when using gas heat. The superheating effect obtained from electric heaters may be minimized, but not completely eliminated, by selecting a heater that, by its design, concentrates the heating elements to a minimum area, and contains a minimum amount of ceramic material in its overall construction. The fulfillment of these requirements will reduce, but not completely eliminate, the amount of extraneous heat radiating around the perimeter of the asbestos-cement board on which the distillation flask is placed. (See Appendix X1 for a more complete discussion of the problems encountered in the use of electric heaters.)
11. Calculations
11.1 Manual Distillation Calculations:
11.1.1 Thermometer Bore Correction - Apply the corrections for any variations in the bore of the thermometer as given by the calibration.
11.1.2 Thermometer Bulb Shrinkage Correction - Apply the correction for shrinkage of the mercury bulb of the thermometer as determined by any change in its ice or steam point where applicable. Other means can be employed, such as the use of a platinum-resistance thermometer or a National Bureau of Standards thermometer.
11.1.3 Barometer Pressure Correction - After applying the corrections for thermometer error, correct each reading for deviation of the barometric pressure from normal by adding algebraically the correction calculated as follows:
11.1.3.1 For values of K in degrees Celsius per millimetre of mercury
Correction = K(760 - P)
where:
K = rate of change of boiling point with pressure, in degrees Celsius per millimetre, as given in Table 3 (Note 7), and
P = barometric pressure in millimetres of mercury at standard temperature.
11.1.3.2 For values of K in degrees Celsius per millibar:
Correction = K(1013 - P)
where:
K = rate of change of boiling point with pressure, in degrees Celsius per millibar, as given in Table 3 (Note 7), and
P = barometric pressure in millibars at standard temperature and pressure.
NOTE 7 - For other pure compounds not listed in Table 3, the value K should be obtained from the literature. For narrow-boiling hydrocarbon materials, the value of K may be assumed as 0.00012 times the normal boiling point on the absolute temperature scale.
11.1.4 If the overall distillation range of the sample does not exceed 2°C, combined thermometer (bore irregularities and bulb shrinkage) and barometric corrections may be made on the basis of the difference between the observed 50 % boiling point and the true boiling point at 760 mm as given in Table 3.
11.2 Automatic Distillation Calculations - Barometric correction shall be done according to 11.1.3 and 11.1.4.
