Table 2

T-t.

N.

10.

20.

.30.

40.

50.

60.

70.

80.

90.

100.

110.

120.

130.

140.

150.

160.

170.

180.

190.

200.

10 . .

0.01

0 03

0.04

0.06

0.07

0.09

010

0.11

013

014

0.16

0.17

0.19

0.20

0.21

0.22

0.24

0.26

0.27

0.29

20 . .

0.02

0.06

0 09

0.11

014

017

0.20

0.22

0.26

0.29

0.31

0.34

0.37

0.40

0.43

0.46

0.49

0.51

0.54

0.57

30 . .

0.04

0.09

013

0.17

0.21

0.26

0.30

0.34

0.39

0.43

0.47

051

0.56

0.60

0.64

0.68

0.73

0.77

0.82

0 86

40 . .

0 05

0.11

017

0.23

0.28

034

0.40

0.47

0.52

0.57

0.63

0.69

0.74

0.80

0.86

0.91

0.97

1.03

1.09

114

50 . .

0.07

0.14

0.21

0.29

0.36

0.43

0.50

0.60

0.64

0.71

0.79

0.86

0.93

1.00

1.07

114

1.22

1.29

1.36

1.43

60 . .

0.08

0.17

0.25

0.35

0.43

0.51

0.60

0.70

0.77

0.86

0.94

1.03

112

1.20

1.29

1.37

1.46

1.54

1.63

1.72

70 . .

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

2.00

80 . .

0.11

0.23

0.34

0.45

0.57

0.68

0.80

0.91

1.03

114

1.26

1.37

1.49

1.60

1.72

1.83

1.94

2.05

217

2.29

90 . .

013

0.26

0.39

0.51

0.64

0.77

0.90

.03

1.16

1.30

1.42

1.54

1.66

1.80

1.93

2.05

2.17

2.31

2.45

2.54

100 . .

014

0.28

0.43

0.57

0.71

0.85

100

114

1.29

1.43

1.58

1.71

1.84

2.00

215

2.29

2.43

2.57

2.72

2.86

110 . .

0.16

0.31

0.47

0.63

0.79

0.94

1.10

1.26

1.42

1.58

1.73

1.89

2.04

2.20

2.36

2.51

2.67

2.83

2.99

3.15

120 . .

0.17

0.34

0.51

0.69

0.86

1.03

1.20

1.37

1.54

1.71

1.89

2.06

2.23

2.40

2.57

2.74

2.92

3.09

3.26

3.43

5. Superheating of Vapour. - When the amount of liquid in the still is very small, the vapour is liable to be superheated by the flame, and unless the bulb of the thermometer is thoroughly moistened with condensed liquid, too high a temperature will be registered. If a very little cotton wool, or, for temperatures above 230°, a little fibrous asbestos, be wrapped round the bulb of the thermometer, it remains, as a rule, thoroughly moist, and, with a pure liquid, heated by a naked flame, the thermometer registers a perfectly constant temperature until the last trace of liquid in the bulb has disappeared.

With a water or oil bath the danger of superheating is greater, and the cotton wool may become dry at the end of the distillation. In that case the temperature registered may be too high, though, as a rule, the error is not so great as it would be if the bulb were not protected.

If it is of special importance to determine the boiling point of a liquid with great accuracy during the course of a distillation, one of the two forms of still devised by Richards and Barry1 may be employed with advantage.

6. Correction of Boiling Point for Pressure. - The barometer must always be read and corrected to 0° (p. 229) and, in a long distillation or in unsettled weather, it may be necessary to read it frequently, for the boiling point of a liquid varies greatly with the pressure.

It is impossible to give any accurate and generally applicable formula for correcting the observed boiling point to that under normal pressure (760 mm.), but the following may be taken as approximately correct:0 = 0.00012(760 -p)(273 + t), where 0 is the correction in centigrade degrees to be added to the observed boiling point, t, and p is the barometric pressure.2

This correction is applicable, without much error, to the majority of liquids, but for water and the alcohols a better result is given by the formula,

0 =0.00010(760 - p)(273 +t).

Crafts 3 has collected together the data for a number of substances, from which the values of c in the formula 0 = c (760-p)) (273+ t) are easily obtained.4 Table 3 (p. 14) contains the boiling points on the absolute scale, T, the values of c and those of dp/dt for some of the substances referred to by Crafts and also for a considerable number of additional ones.5

1 "An Advantageous Form of Still for the Exact Measurement of Boiling Point during Fractional Distillation," J. Amer. Chem. Soc., 1914, 36, 1787.

2 Ramsay and Young, "Some Thermodynamical Relations," Phil. Mag., 1885, [V.], 20, 515.

3 Crafts, " On the Correction of the Boiling Point for Barometric Variations," Berl. Berichte, 1887, 20, 709.

4 There are a few misprints in the table given by Crafts, and since it was published many additional accurate determinations of boiling point and vapour pressure have been made.

5 Young, " Correction of the Boiling Points of Liquids from Observed to Normal Pressure," Trans. Chem. Soc, 1902, 81, 777.

The values marked with an asterisk have been determined indirectly, and are not to be regarded as so well established as the others.

When a boiling point is to be corrected, the constant c for the substance may usually be found by reference to Table 3. Either the constant for that substance in the table most closely related to the one under examination is to be used, or the constant may be altered in conformity with one or other of the following generalisations.