Melting Point Determination Methods PDF

▪ The melting point is the temperature where the solid-liquid phase change occurs ▪ In some reference books it is listed as a single value (e.g. 98˚C), but in chemical catalogs it is more often listed as a range of values (e.g. 96-98˚C) ▪ The melting "point" is therefore more of a melting "rang...

▪ The melting point is the temperature where the solid-liquid phase change occurs ▪ In some reference books it is listed as a single value (e.g. 98˚C), but in chemical catalogs it is more often listed as a range of values (e.g. 96-98˚C) ▪ The melting "point" is therefore more of a melting "range" Ibrahim El-Serafi 29 October 2023 2 ▪ A melting point is determined by loading a small amount of sample into a capillary tube and then slowly heating the sample ▪ The sample is slowly heated through contact with hot vertical metal blocks on either side of the capillary tube ▪ The sample is kept small in this technique to ensure adequate heat transfer between the metal and sample Ibrahim El-Serafi 29 October 2023 3 a. Well below the melting point b. "Glistening" of the solid c. First liquid droplet is seen (the temperature is recorded as the lower value of the melting range) d-g. Melting h. Sample is completely melted (the temperature is recorded as the upper value of the melting range) Ibrahim El-Serafi 29 October 2023 4 Identification ▪ Melting point is a physical constant ▪ It can be used to support the identity of an unknown solid ▪ Literature melting point ▪ It is not uncommon for two different compounds to have coincidentally similar or identical melting points ▪ A melting point should be used as simply one piece of data to support the identification of an unknown Ibrahim El-Serafi 29 October 2023 5 Assessing Purity ▪ impurities lower and broaden the melting range Ibrahim El-Serafi 29 October 2023 6 ▪ There are a variety of methods by which a sample's melting point can be measured, with the newest being electrical probes ▪ Traditional methods use ▪ An electrical melting point apparatus ▪ Thiele tube ▪ Both methods use capillary samples that are prepared in the same manner Ibrahim El-Serafi 29 October 2023 7 Sample Preparation 1. Obtain a glass capillary melting point tube ▪ Has one end sealed and the other end open ▪ Jab the open end of the tube into a pile of the solid to be analyzed (a) ▪ The solid must be dry or the results will be affected as solvent can act as an impurity and affect the melting range ▪ If the solid is granular, pulverize the solid somewhat before packing Ibrahim El-Serafi 29 October 2023 8 Sample Preparation 2. Invert the capillary tube and gently tap the tube on the benchtop to cause the solid to fall to the closed end (b) ▪ Drop the capillary tube closed side down several times through a long narrow tube (glass tube) (c) Ibrahim El-Serafi 29 October 2023 9 Sample Preparation ▪ The capillary tube will bounce as it hits the benchtop, and pack the solid into the bottom of the tube ▪ Failure to pack the solid well may cause it to shrink when heating, which can cause confusion as to the correct melting temperature Ibrahim El-Serafi 29 October 2023 10 Sample Preparation 3. If needed, repeat the previous steps to load sample until it is a height of 2-3 mm in the tube (d) ▪ It is important that the sample be no higher than 3 mm or the melting range will be artificially broad Ibrahim El-Serafi 29 October 2023 11 Melting Point Apparatus 1. Insert the capillary tube containing the sample into a slot behind the viewfinder of a melting point apparatus (a) ▪ There are usually three slots in each apparatus, and multiple melting points can be taken simultaneously after gaining experience with the technique Ibrahim El-Serafi 29 October 2023 12 Melting Point Apparatus 2. Turn on the apparatus and adjust the setting to an appropriate heating rate (b) ▪ The rate of heating is often experimental and should be adjusted by careful monitoring of the thermometer on the apparatus 3. Look through the viewfinder to see a magnified view of the sample in the apparatus (c) Ibrahim El-Serafi 29 October 2023 13 Melting Point Apparatus 4. If the expected melting point of the compound is known ▪ Heat at a medium rate to 20oC below the expected melting point ▪ Then slow the rate of heating such that the temperature increases no more than 1oC every 30 seconds 5. If the expected melting point of the compound is NOT known ▪ Heat the sample at a medium rate the entire time and determine an approximate melting point ▪ Repeat the process with a fresh sample after allowing the apparatus to cool and use the recommendations as described previously Ibrahim El-Serafi 29 October 2023 14 Melting Point Apparatus 6. The solid may be approaching its melting point if the solid is seen pulling away from the walls of the tube to form a cone of solid "sintering“ ▪ The solid may also shrink or compact before melting ▪ Melting will normally occur within a few degrees of this point Ibrahim El-Serafi 29 October 2023 15 Melting Point Apparatus 7. Record the first temperature of the melting range with the appearance of the first visible drop of liquid ▪ The temperature should be recorded when a droplet is seen on the side or bottom of the tube 8. Record the second temperature of the melting range when the entire sample has just melted (transparent liquid) Ibrahim El-Serafi 29 October 2023 16 Melting Point Apparatus ▪ https://www.youtube.com/watch?v=8T0OSYI5pYk Ibrahim El-Serafi 29 October 2023 17 Thiele Tube Method 1. Obtain a Thiele tube and clamp it to a ring stand ▪ The tube is normally filled with clear mineral oil ▪ The oil should be filled to at least higher than the top triangular arm 2. Insert a thermometer into a one-holed rubber stopper with a slit down one side. Attach the capillary sample to the thermometer 3. Place the rubber stopper and thermometer assembly into the Thiele tube, adjusting the height so that the sample is midway inside the tube Ibrahim El-Serafi 29 October 2023 18 Thiele Tube Method 4. Heat the apparatus gently using a back and forth motion ▪ As the oil warms and becomes less dense, it will rise and travel up the triangular portion of the tube ▪ The cooler, denser oil will sink, thereby creating an oil current ▪ This method is an excellent way to indirectly and slowly heat the sample Ibrahim El-Serafi 29 October 2023 19 Thiele Tube Method (Similar to Melting Point Apparatus) 5. If the expected melting point of the compound is known (Melting Point Apparatus) ▪ Heat at a medium rate to 20oC below the expected melting point ▪ Then slow the rate of heating such that the temperature increases no more than 1oC every 30 seconds 6. If the expected melting point of the compound is NOT known (Melting Point Apparatus) ▪ Heat the sample at a medium rate the entire time and determine an approximate melting point ▪ Repeat the process with a fresh sample after allowing the apparatus to cool and use the recommendations as described previously 29 October 2023 20 Thiele Tube Method (Similar to Melting Point Apparatus) 7. Record the first temperature of the melting range with the appearance of the first visible drop of liquid ▪ The temperature should be recorded when a droplet is seen on the side or bottom of the tube 8. Record the second temperature of the melting range when the entire sample has just melted (transparent liquid) Ibrahim El-Serafi 29 October 2023 21 Ibrahim El-Serafi 29 October 2023 22 Thiele Tube Method ▪ https://www.youtube.com/watch?v=6G2x6SjQAh4 Ibrahim El-Serafi 29 October 2023 23 Ibrahim El-Serafi 29 October 2023 24