Boiling Point Determination

Table of contents

• Acknowledgments
• Introduction
• Procedures
  • distillation
  • mini scale
  • micro scale

Introduction

boiling point

temperature at which the vapor pressure of a liquid is equal to the external pressure

This definition suggests that there are two quantities that need to be determined:

• temperature
• external pressure

For the time being we assume that you know how to measure both of these (i.e., how to read a thermometer and a barometer).

Procedures

There are several methods used to determine the boiling point. The choice of method depends on the amount of material available. In order of decreasing amounts of sample required, we have:

1. distillation
2. mini scale
3. micro scale

distillation

This method is reasonably accurate (+/- 0.5^oC), requires a large amount of sample (> 5mL), and is time consuming. The advantage of distillation is that you will also automatically purify your sample.

The procedure just boils down to recording the temperature at which the distillation of the sample occurs. The distillation takes a reasonably amount of time so that reading the temperature can be done easily. The reason that a large amount of sample is required stems from the volume of the distillation flask and still head that have to be filled with vapor of the sample at external pressure before distillation occurs.

mini scale

This method is reasonably accurate (+/- 0.5^oC) and requires a reasonably small amount of sample (approx. 2mL).

a 2.1MB QuickTime movie

The procedure consists of bringing a small amount of sample in a mini test tube to a boil. Since the temperature of the vapor will not get appreciably higher than the boiling point as long as it is in the tube with more liquid, the boiling point can be read as the highest temperature achieved as the vapor condenses on the thermometer bulb:

• put a boiling stone in the mini test tube
• add about ¹/₂ inch of liquid
• clamp in stand in the fume hood
• insert thermometer into test tube with bottom of mercury bulb about ¹/₂ - 1 inch above the liquid
• carefully heat with mini bunsen burner till temperature remains constant
• read temperature

Note: Often the temperature will not become completely constant, but will fluctuate a little bit (about 0.3^oC) . In that case one reports the average temperature as the boiling point.

a 520KB QuickTime movie

Many liquid samples are flammable, and the vapor that escapes from the top of the test tube may catch on fire. This is more likely when larger bunsen burner is used, or when the sample "bumps" due to uneven heating. However, since the amount of flammable material is very small :

• don't panic
• move the bunsen burner away from the tube
• close the fume hood
• let the fire die out

micro scale

This method is more accurate than distillation or mini scale (+/- 0.2^oC) and requires a small amount of sample (approx. 0.2mL).

The procedure consists of determining the temperature at which the external pressure on the boiling liquid is large enough to overcome the vapor pressure in a capillary tube inserted closed side up in the liquid. When this happens, the vapor will condense and liquid will rise into the capillary.
Usually a melting point apparatus is used to heat the sample.

Instructions on how Bell capillary tubes can be made from Pasteur pipets are given elsewhere.

• using a micro pipet fill about ¹/₂ inch of a melting point tube with sample
• insert a Bell capillary tube, open end down
• push the Bell capillary down into the liquid with a section of a long capillary tube. The long capillary tube resting on top will keep the Bell capillary submersed. Make sure that the sample does not fill the Bell capillary.

a 2.3MB QuickTime movie

• insert the sample-loaded tube into the melting point apparatus
• slowly heat the sample (about 2 ^oC per minute); bubbles will start to come out of the inverted Bell capillary (expansion of gas)
• keep heating till a steady stream of bubbles comes "out of" the Bell capillary (really the nucleation point for formation of vapor is at the capillary rim; the inside is nearly filled with pure sample vapor by now).
• turn the heater off
• carefully note the temperature at which the liquid starts to rise into the inverted Bell capillary (as soon as the temperature drops below the boiling point under these conditions, the vapor condenses to liquid and liquid fills
• record temperature