|Melting Point and Thermometer Calibration|
PLEASE VIEW THE RELEVANT VIDEO BEFORE ATTENDING THE LAB CLASS
Example of a Typical Experiment
FAQ: When you observe a melting point, you are watching a solid change into a liquid. Do you record the temperature when you see the first drop of liquid and record a second temperature when the sample is a liquid?
FAQ: Suppose I have two pure compounds, one which melts at 100°C and the other melting at 50°C. Shouldn't the mixed melting point of a 1:1 mixture be depressed for both compounds, i.e., even less than 50°C?
Melting point is the temperature at which the solid phase is in equilibrium with the liquid phase.
In practice, however, a melting point is actually recorded as a temperature range: the first temperature, when the first drop of liquid forms; the second temperature, when the last crystal dissolves.
The melting point provides reliable information about the purity and identity of a compound.
Different factors can influence the accuracy of measured melting ranges (crystal size, crystal quantity, rate of heating, dryness of crystals).
Due to variations in the size of their internal bore, laboratory thermometers differ in accuracy and need to be calibrated.
Melting point standards (known compounds with known, sharp melting points) are used to calibrate a thermometer.
Calibration of a thermometer involves determining the deviation of the measured temperature from a published value for a series of known compounds. This allows you to find the thermometer errors and thus the correction factors.
Two kinds of graph showing the relationship between the temperature a thermometer measures and the actual temperature may be used to correct a temperature measurement:
Earl N. Meyer realized that his thermometer was inaccurate. He and his partner, Phil Terpaper, decided to "calibrate" it, that is, find the error in temperature reading at specific temperatures.
Phil gathered together the necessary equipment and "melting point standards" (known compounds with known sharp melting point ranges).
He decided to follow the advice of Todd, the Super TA, and measure the temperature ranges of the low melting componds first. After nearly embarrassing himself by trying to fill a melting point tube with ice, Phil inserted his thermometer in a beaker containing ice-water to obtain the 0° C reading. To prevent the thermometer from falling down while in the beaker, he clamped a universal clamp to the monkey bars and secured his thermometer with it.
Phil then packed a melting point tube with ca. 3 mm of finely powdered chemical obtained from the available melting point standard.
Using the Gallenkamp apparatus and controlling the heating rate by adjusting the rheostat, he measured the melting point range for each compound. He noted the temperature when the first drop of liquid appeared and when all of the solid had melted.
From his data, Phil prepared two thermometer calibration graphs:
* First he plotted the midpoint of his observed melting point range versus the published melting point for the known compounds (melting point standards). Using this graph Phil was able to directly interpolate the corrected temperature for a specific observed melting point.
Later Phil can use either graph to correct for inaccuracies in his thermometer.
Practical Lab Tips
Make sure your sample is dry and crushed to a fine powder.
Pack the solid sample in the bottom of the melting point tube by tapping the tube on the desk.
Insert the closed end of the melting point tube into the Gallenkamp block.
Do not fill too little or too much sample into the melting point tube. Approximately 1-2 mm is right.
If you are unsure about your melting point measurement, prepare another sample and measure the melting temperatures again. Do not reuse your old mp tube, since melting often causes decomposition.
The crystals will shrivel up (simmer) before you will see a liquid drop forming. Do not take the first temperature yet, but wait for the first drop to form before noting the temperature. Likewise, wait for the last crystal to dissolve before noting the second temperature of the melting point range.
Be careful with your thermometer. Try to avoid exposing it to sudden temperature drops. DO NOT place it into ice-water after it has been heated. The thermometer will break! Wait until the thermometer bulb cools to room temperature.
A calibration graph should have:
Use one of the two calibration graphs to correct temperature measurements made with this thermometer.
Observed melting point temperature vs. actual (literature) temperature:
Observed melting point temperature vs. difference to the actual (literature) temperature:
The dial (rheostat) is used to vary the heating rate when measuring a melting temperature.
- A low rheostat setting (e.g., '3') allows for gradual heating, suitable for low melting compounds ( e.g., < 80°C).
However, each Gallenkamp apparatus is somewhat unique and a rheostat setting of for example '6' on one Gallenkamp may result in a faster heating in comparison to another.
If you know the melting point of the compound:
If you do not know the melting point of the compound:
Theoretically, yes. However, this strategy will not be accurate because you may observe a broad melting range and falsely conclude that your compound is impure when, in fact, it is pure. Rapid heating does not allow sufficient time for your compound to melt completely at a given temperature.
If you are running out of time at the end of a lab class, it is a better practice to prepare a melting point tube, and measure the melting point later in the Melting Point Room. Do not forget to take your thermometer with you!
Q: When you observe a melting point, you are watching a solid change into a liquid. Do you record the temperature when you see the first drop of liquid and record a second temperature when the sample is a liquid?
Yes. That is the correct technique for measuring since the so-called melting point is actually a melting range.
Well, let's look at some examples. What do you think will happen in each case?
Rate of heating:
A wet sample:
When you are measuring the melting point and turn the dial on the Gallenkamp, it is the Gallenkamp apparatus which is heating up. The thermometer only indicates the temperature of the heating block.
If you need to do other things between melting point measurements and want to stop the temperature of the Gallenkamp from increasing, then simply turn it off or set the rheostat to '0'.
Removing the thermometer from the Gallenkamp between measuring melting points will not change the temperature of the apparatus. It is even a good idea to leave the thermometer in the Gallenkamp instrument during your lab class, because the thermometer is secured and less likely to be accidentally broken.
Did you plug it in and turn it on? You can easily verify this by checking that the Gallenkamp lamp is on. If this is not the case, then report the problem to your TA.
During the lab class you can store your used melting point tubes in the black storage tube on the right side of the Gallenkamp.
At the end of the lab class, please dispose the used melting point tubes into the yellow waste container labelled "broken glass".
When measuring the melting points of multiple solids in succession you don't need to wait for the Gallenkamp apparatus [melting point apparatus] to cool to room temperature if you plan your work!
First measure the melting temperatures of lower-melting solids, then measure the melting temperatures of the higher-melting solids.
The Gallenkamp apparatus should be allowed to cool approximately 20°C below the anticipated melting point of the next sample.
Yes. Connect the rubber tubing (hose) to the air outlet fixture located on the student bench. Blow a gentle stream of cold air onto the heating block, but do not let the rubber tubing touch the plate. Observe the changing temperature on the thermometer placed in the Gallenkamp.
Fill the melting point tube with enough sample to cover about 2 mm of the tube. Tap the melting point tube a few times onto the desk to ensure that the sample is packed evenly at the closed end of the tube.
Make sure your sample is dry. A wet sample will NOT move easily down the tube, if at all!
Sometimes a melting point tube breaks inside the Gallenkamp apparatus. Usually you can remove it by inverting the Gallenkamp. However, be careful that you first remove other melting point tubes and the thermometer before turning over the apparatus.
Q: Suppose I have two pure compounds, one which melts at 100°C and the other melting at 50°C. Shouldn't the mixed melting point of a 1:1 mixture be depressed for both compounds, i.e., even less than 50°C?
The typical melting behavior for most 1:1 two-compound mixtures will result in a depression of the melting temperature below that of the individual compounds and a broadening of the melting range. Different molar ratios of the two compounds give different melting temperatures, the lowest melting point of any mixture being the so-called eutectic temperature.
The temperatures you will measure throughout the lab course may differ from the "actual" temperatures. This is because the thermometer you use is relatively cheap and its internal bore is not very uniform.
By measuring the melting ranges of several so-called "melting point standards" (known compounds with sharp, known melting points) you can find the relationship between the temperature your thermometer measures and the actual temperature.
This relationship can be plotted in form of a graph. Two sorts of graphs are possible:
1. You can plot the observed melting point temperature on the x-axis and the actual (literature) temperature on the y-axis. In practice, find your measured temperature on the x-axis, then look for the corresponding actual temperature on the y-axis.
In subsequent experiments you can correct your temperature measurements (e.g., m.p. or b.p.) for errors inherent in your thermometer by using the data obtained in your graph. REMEMBER though: Calibration is individual!
Look in the lab manual for further information on how to calibrate, plot, and use the calibration graph.