5.4B: Predicting the Boiling Temperature

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The boiling point of a liquid or solution drops when the pressure is reduced in a distillation apparatus. It is helpful to be able to predict the altered boiling point depending on the pressure inside the apparatus.

The lowest pressure attainable inside the apparatus depends largely on the vacuum source and the integrity of the seal on the joints. Lower pressures are attainable when using a portable vacuum pump\(^{14}\) than when using a water aspirator or the building's house vacuum (Figure 5.49). Due to the very low pressures possible with oil pumps in portable vacuums, these vacuum distillations should be conducted in the fume hood behind a blast shield.

Figure 5.49: Vacuum sources: a) Portable vacuum pump, b) Water aspirator, c) House vacuum.

Water aspirators are the most common vacuum source in teaching labs because they are inexpensive. When a water aspirator is used, the vacuum pressure is always limited by the intrinsic vapor pressure of water, which is often between \(17.5 \: \text{mm} \: \ce{Hg}\) \(\left( 20^\text{o} \text{C} \right)\) and \(23.8 \: \text{mm} \: \ce{Hg}\) \(\left( 25^\text{o} \text{C} \right)\).\(^{15}\) the vacuum pressure is also very dependent on water flow, which can vary greatly. If an entire lab section uses the water lines at the same time, the water flow can be significantly compromised, leading to a much higher pressure than \(25 \: \text{mm} \: \ce{Hg}\) inside an apparatus. The number of students using aspirators at one time should be limited as much as possible.

If a manometer is available, the distillation apparatus should be set up and evacuated without heating to measure the pressure. The expected boiling point of a compound can then be roughly estimated using a nomograph (found in a CRC or online) or through the general guidelines in Table 5.9. If a manometer is not available and a water aspirator is to be used, the expected boiling point can be estimated using an approximate pressure of \(20 \: \text{mm} \: \ce{Hg}\), although the pressure will likely be higher than this.

Figure 5.9: Approximate boiling point of compounds at reduced pressure (all in ºC).\(^{16}\)
Boiling point at 760 mmHG	150	170	200	220	250	270	300
Boiling point at 20 mmHG	62	78	101	117	141	157	181
Boiling point at 18 mmHG	60	76	99	115	139	154	178
Boiling point at 16 mmHG	58	73	97	112	136	151	174
Boiling point at 14 mmHG	56	71	94	108	133	148	171
Boiling point at 12 mmHG	52	68	90	104	129	144	167

\(^{14}\)A Kugelrohr apparatus can obtain pressures as low as \(0.05 \: \text{mm} \: \ce{Hg}\), as reported by the Sigma-Aldrich operating instructions.

\(^{15}\)J. A. Dean, Lange's Handbook of Chemistry, 15\(^\text{th}\) ed., McGraw-Hill, 199, Sect 5.28.

\(^{16}\)Selected values from: A. J. Gordon and R. J. Ford, The Chemist's Companion. A Handbook of Practical Data, Techniques and References, Wiley & Sons, 1972, p 32-33.