A melting point apparatus is a specialized scientific instrument designed to determine the temperature range over which a solid substance transitions into a liquid state. This temperature is a fundamental physical property that provides two pieces of information in a chemistry setting. It helps in the positive identification of an unknown compound, as this melting temperature is unique to a pure substance. Measuring the melting range is also a standard method for assessing the purity of a sample, since even minor impurities can alter this characteristic temperature range. A pure compound will exhibit a sharp, narrow melting range, while contaminants cause the substance to melt at a lower temperature and over a broader range.
Preparing the Sample and Capillary Tube
The accuracy of the melting point determination depends heavily on careful sample preparation before using the apparatus. The solid substance must first be completely dry, as residual solvent or absorbed water acts as an impurity that will artificially broaden and lower the melting range. Once the sample is dry, it should be ground into a very fine powder using a watch glass and a glass rod to ensure uniform heat transfer. Coarse crystals or granular solids will heat unevenly, which can lead to a misleadingly wide melting range.
Next, a thin-walled glass capillary tube, which is sealed at one end, must be loaded with the powdered sample. The open end of the tube is pressed gently into the pile of powder to collect a small amount of the solid. The tube is then inverted, and the sample must be compacted tightly into the sealed bottom. This packing is typically achieved by repeatedly tapping the tube on a hard surface or by dropping the tube closed-end down through a long, narrow glass tube onto the benchtop.
The resulting packed sample column should be uniform and not exceed a height of about two to three millimeters. If the column is packed too high, a temperature gradient will form across the sample, meaning the bottom will melt before the top, which artificially broadens the observed melting range. A loosely packed sample will contain insulating air pockets that hinder efficient heat transfer. After packing, the outside of the tube must be wiped clean to ensure no stray particles interfere with the viewing window or the heating block.
Step-by-Step Apparatus Operation
After preparing the capillary tube, the next step involves inserting it into the heating mechanism of the melting point apparatus. Most modern instruments use a heated metal block with slots designed to hold one or more capillary tubes simultaneously. The sealed end of the tube containing the sample must be placed into the heating cavity with the open end facing up.
Prior to starting the heating process, the apparatus must be programmed with the correct temperature settings. If the compound’s approximate melting point is known, the initial starting temperature should be set approximately \(10-20\) degrees Celsius below this expected value. This pre-heating step rapidly brings the apparatus up to a temperature near the melting point.
Once the apparatus reaches the initial set temperature, the heating rate must be significantly reduced for the final approach. For a precise measurement, the temperature should increase very slowly, typically at a rate of only \(1\) to \(2\) degrees Celsius per minute. This slow rate allows the sample and the thermometer to reach thermal equilibrium with the heating block, which is necessary for the recorded temperature to accurately reflect the sample’s true melting point.
The operator should use the viewing light and magnifying eyepiece to clearly observe the sample within the capillary tube as the temperature rises. If the melting point of the compound is completely unknown, an initial rapid run (e.g., \(5-10\) degrees Celsius per minute) is often performed first to quickly estimate the general temperature range. A fresh, unused sample must always be prepared for the final, slow-rate determination.
Observing and Recording the Melting Point Range
Observing the sample through the apparatus’s eyepiece requires careful attention to define the two temperatures that make up the melting range. The first temperature, known as the onset of melting, is recorded the instant the first visible droplet of liquid appears in the solid sample. This moment may be signaled by the solid glistening or a slight movement within the packed column.
The second temperature, which completes the range, is recorded when the last trace of opaque solid disappears, and the entire sample column becomes a clear, transparent liquid. This two-point range is what is reported, for example, as \(120.5-122.0\) degrees Celsius.
A pure compound is expected to melt over a narrow range, typically no wider than two degrees Celsius. A range that is wider than this, or a value that is lower than the literature value, is a strong indication of impurities within the sample. If a compound darkens or changes color as the temperature increases, this is a sign of thermal decomposition. Record the temperature at which this change occurs and note it with the letter ‘d’.
Once the melting range has been recorded, the apparatus must be allowed to cool significantly before another trial is run. The temperature of the heating block should drop at least \(20\) to \(30\) degrees Celsius below the next expected melting point. This cooling prevents the next sample from being heated too quickly at the start.
Ensuring Accuracy: Common Errors and Corrections
The most frequent source of error in melting point determination is heating the sample too quickly, especially as the temperature approaches the melting point. A rapid temperature ramp, exceeding \(2\) degrees Celsius per minute in the final stage, prevents the thermometer and the sample from achieving thermal equilibrium with the heating block. This thermal lag results in a recorded melting point that is artificially higher than the true value.
Errors in sample preparation also significantly compromise the accuracy of the result. Using an excessive amount of sample, such as a packed column taller than three millimeters, creates a temperature gradient that causes the sample to melt over an artificially broad range. Similarly, a loosely packed sample with air pockets leads to inefficient and uneven heat transfer, making the determination unreliable.
The experimenter’s own observation can introduce error, particularly if the initial melting point is recorded too early, such as when the solid merely sinters or shrinks. The initial point must be reserved for the appearance of the first liquid droplet, not just the softening of the solid. To correct for inaccuracies inherent in the apparatus, especially those related to the thermometer, a routine calibration should be performed. This process involves measuring the melting points of certified standard compounds with known, precise melting points to verify the apparatus’s temperature readings.