Melting Range Determination: Experiment and Analysis - Prof. Bruce Hathaway, Lab Reports of Organic Chemistry

Download Melting Range Determination: Experiment and Analysis - Prof. Bruce Hathaway and more Lab Reports Organic Chemistry in PDF only on Docsity! Melting Range Determination Melting range (sometimes just called melting point) is a fundamental physical property of a substance. A pure substance should have a melting range of 1-2 degrees. The melting range should also be close to the reported value in the chemical literature. Impure substances usually have wide melting ranges (over several degrees), and generally melt lower than the reported value. There are several techniques and pieces of apparatus that can be used to take a melting range. We will be using the Meltemp apparatus. Although rather simple-looking, this apparatus costs over $800 to replace now, so handle it carefully! The melting range of a substance is determined in the following manner. 1. Introduce a small amount (about 1-2 mm) of dry, finely powdered crystals into a capillary tube. Be sure that the crystals pack tightly. This can be accomplished by tapping the closed end of the capillary tube on the benchtop. 2. Place the capillary tube (closed end down) in the slot of the Meltemp®. Be careful not to break the tube in the apparatus. Notice that the Meltemp® can hold three tubes at once. 3. Heat the sample and record the melting range. This range starts when the sample just begins to liquify, and ends when the sample has totally liquefied. You will have to look in the eyepiece and watch the sample. When the sample just begins to liquefy, you will have to look up at the thermometer and record the temperature. You then look back down at the sample and watch it until it has totally liquefied, then look back up at the thermometer, and record this temperature. The rate of heating is critical for obtaining a good melting range. If you heat the apparatus too fast, you may miss the end of the melting range, because the sample will have completely melted before you can look back down at it. Therefore, you should set a rate of heating of no more than about 5 per minute while your sample is melting. This is done by adjusting the voltage appropriately. For example, let’s assume you have a sample that melts at 150. Looking at the table of heating curves, you can see that some voltage settings would not be appropriate. Voltages of less than 40 would never reach 150. Voltages of 80 or higher would have the temperature increasing so rapidly, you would miss the melting range. A voltage in the range of 50-60 would have a reasonable rate of temperature increase, and would probably work well. Notice that at 50 volts, it would take approximately 12 minutes to reach 150. (Note: on the blue apparatuses, the voltage knob only goes from 0-10. Divide the settings mentioned by 10 for the blue apparatuses.) So what do you do if you don’t know what the melting range is supposed to be? One approach is to start with a reasonably low voltage (such as 50), then gradually increase the voltage to maintain a level of heating of about 5 per minute. The disadvantage of this approach is that it will probably take a long time if the melting point is >150. A second approach would be to take a melting range at a high voltage, such as 90, first. This melting range would be inaccurate, but would give you a rough idea of what range you are looking for. You would then go back and take a second melting range, using a more appropriate voltage setting. The disadvantage of this approach is having to use two samples, and the need to let the Meltemp® cool back down to about 20 below the melting point before you could use it again. Of course, if another Meltemp® is available, you could just use it. 1 2 Precautions: 1. The heating block of the Meltemp® gets very hot (surprise!). Therefore, do not touch it with your fingers or nose (yes, your nose can get close to it) when taking a melting range. 2. Check the temperature on the thermometer before you insert your capillary. If the temperature is higher than the melting point of your sample, it will melt immediately, and you will have to prepare another sample. 3. Don’t break off the capillary tubes in the Meltemp®. If you do, or if you find broken tube pieces in a Meltemp®, please notify your instructor so it can be cleaned out. Broken tubes reduce the efficiency of the apparatus.