Melting-Point Methods and Apparatus

2.7 Melting-Point Methods and Apparatus

Overview of Melting Point Determination

The process of determining the melting point of a compound involves heating a small amount of solid material and identifying the temperature at which it transitions from solid to liquid. Various types of heating devices can be utilized; however, capillary tubes are predominantly used to conserve sample amount.

Capillary Tubes and Sample Preparation

1. Preparation of the Sample

To begin determining a melting point, the solid needs to be carefully transferred into a melting-point capillary tube. These tubes come with one sealed end and are readily available commercially. The procedure for loading the sample includes:

  • Placing a small amount of the solid (approximately 2–3 mm in height) onto a clean watch glass and inserting the open end of the capillary tube to press and fill the sample into it. It is important to avoid using filter paper for this step as it may introduce fibers into the tube, complicating the results.

  • Following this, a piece of 6- to 8-mm tubing (around 1 m long) should be used. The capillary tube must be dropped several times through this larger tubing, sealed end down, to pack the solid sample firmly.

2. Melting-Point Determination

The melting point of the crystalline solid is established by heating the packed capillary tube. The preferred method for accurate measurement involves heating at a rate of approximately 1–2 °C/min. Such a gradual increase in temperature facilitates thermal equilibrium between the sample and the thermometer containing mercury, ensuring reliable results.

  • Many organic compounds may experience a structural change before melting, often resulting in a softer, “wet” appearance, which should not be mistaken as the onset of melting. The start of melting should be defined as the temperature where the first minute droplet of liquid forms.

  • Melting typically occurs over a range of temperatures; typically, the lower end of this range is marked by the appearance of the first liquid droplet, while the upper end is when the last solid has completely melted.

Melting-Point Apparatus

1. Thiele Tube

A basic melting-point apparatus is the Thiele tube, which features a design that enables even distribution of heat applied via a heating liquid in the sidearm due to convection currents; hence, stirring is not necessary. The characteristic control over temperature is achieved by adjusting the flame provided by a microburner.

  • To attach the capillary tube to the thermometer, either a rubber band or a small segment of rubber tubing should be used, ensuring it's positioned close to the upper end of the capillary tube.

  • Ensure that the thermometer and capillary apparatus do not come in contact with the Thiele tube's glass to maintain accuracy. Also, the height of the heating fluid should be appropriately monitored to avoid any spillage resulting from thermal expansion.

2. Electric Melting-Point Devices

Modern laboratories have improved upon traditional methods by using electric melting-point devices. For example, the Thomas–Hoover unit allows the measurement of melting points of up to five samples concurrently, equipped with a motor-driven stirrer in the silicone oil bath.

  • Adjustments to the voltage through a knob control the heating rate of the oil. Additionally, some models include a movable magnifying lens, allowing better visibility of both the thermometer readings and the sample within the capillary tube.

  • Other practical electric melting-point units utilize a heated metal block—providing a uniform heat transfer to the capillary tube, with temperature readings given by an inserted thermometer.

3.3 Physical Constants: Melting Points

Definition and Importance of Physical Constants

Physical constants of compounds are precise numerical values that are associated with measurable properties, such as melting points. These constants play a crucial role in the identification and characterization of substances in a laboratory environment, provided that the measurements are taken under conditions that ensure accuracy, including specific temperature and pressure.

Commonly Measured Physical Properties

Some common physical constants include:

  • Melting Point (mp)

  • Boiling Point (bp)

  • Index of Refraction (n)

  • Density (d)

  • Specific Rotation ([α]₀)

  • Solubility
    Melting point, boiling point, and solubility are frequently encountered properties, with specific rotation applicable only to optically active molecules.

  • Regardless of whether the sample is known or unknown, physical constants along with other relevant properties, including color, odor, and crystal form, should be recorded in a lab notebook to ensure effective tracking.

Measuring Melting Point of Pure Substances

The melting point of a pure substance is defined as the temperature at which both the solid and liquid phases exist in a state of equilibrium without any temperature change. In an ideal scenario, heating a mixture of solid and liquid phases at the melting point will not cause a rise in temperature until complete melting occurs. Conversely, if heat is removed, the temperature will not decrease until all the liquid solidifies.

  • It should be noted that the melting and freezing points are identical for pure substances, and the melting point is often communicated as a range—the lower temperature indicating the onset of melting and the upper marking the complete liquefaction. A well-purified crystalline substance generally melts within a narrow temperature range of about 1 °C.

Factors Affecting Melting Points

Impurities notably impact melting point ranges. An analysis of a case where a solid mixture consists of 80 mol % substance A and 20 mol % substance B illustrates this effect. As heat is applied:

  • At temperature e, both A and B begin to melt, following a predefined ratio referred to as the eutectic point.

  • Once B has completely melted, solid A remains in equilibrium with the liquid, which upon continued heating leads to melting of A as well.

  • This demonstrates the relationship between equilibrium temperature and the composition of the molten solution, with deviations due to impurities resulting in a depressed melting point, which broadens the observed temperature range.

Identification Using Melting Points

The phenomenon of melting point depression can be capitalized upon for identification purposes. For example, if an unknown compound X displays a melting point of 134–135 °C and comparisons are made with urea or trans-cinnamic acid, different observations arise:

  • If mixed with urea, a lowered melting point indicates that urea contaminated the sample, confirming X is not urea. Conversely, an identical melting point to urea suggests that X is indeed urea.

  • This approach yields a rapid and effective method to determine the purity of a solid, with narrower melting point ranges suggesting greater purity.

  • A methodical approach to purification via recrystallization can significantly reduce the melting point range, indicating successful removal of impurities while remaining cognizant that persistent broad ranges may suggest the need for additional purification steps or changes in solvent utilized.

Micro Melting-Point Methods

Accurate determination of organic compound melting points can be labor-intensive; thus, micro methods provide a more efficient alternative that requires minimal sample volume. These techniques serve various practical considerations as follows:

  • The observed melting-point range is influenced by the sample size, state of subdivision (e.g., crystal size), rate of heating, and inherent chemical characteristics.

  • For instance, larger samples may yield uneven heat distribution, causing inaccurate ranges, just as improperly packed large crystals may also introduce airspaces, resulting in poor thermal conduction.

  • A fast heating rate may exacerbate errors by causing thermometer readings to lag behind the actual temperature.

  • In cases of decomposition upon melting, samples may display discoloration or gas evolution, further complicating external measurements, demanding careful reporting of the melting point, such as “mp 195 °C (dec)” indicating decomposition at melting.

Melting Point Measurement Protocol

To measure the melting point accurately, one must maintain a heating rate of about 2–3 °C/min. To streamline the process, at least two capillary tubes can be prepared:

  1. Conduct a rapid assessment heating one capillary tube to obtain a rough melting point, which is expected to be lower than the true melting point.

  2. Allow the heater to cool 10–15 °C below this preliminary figure, then conduct a careful measurement on the second sample for a refined melting point.

  • As a cautionary note, calibration of thermometers is crucial for accuracy in temperature measurement. Proper calibration involves testing against standard substances to rectify discrepancies across various temperature ranges, ensuring that reported values reflect the corrected measurements.

Melting Points Purpose

The underlying objective in this experiment is to determine melting points utilizing the capillary-tube method.

Safety Precautions

  1. Always wear safety glasses or goggles and appropriate protective gloves during laboratory activities.

  2. Ensure that flammable solvents are not near any burners; keep rubber tubing away from the flame and turn off when not in use.

  3. Verify that any oils used in melting-point devices are clear of water contamination, which can lead to dangerous splattering when heated. Organic solvents should not exceed specific temperature thresholds to prevent spontaneous ignition.

  4. Exercise caution to avoid chemical contact with skin and promptly clean any spills with appropriate materials.

  5. Handle the Thiele tube with care after experiments due to slow cooling, avoiding burns upon removal from support.