Hawzhin A Salih MSc in medicinal chemistry Hawzhinsalihunivsuleduiq Lab policies Punctuation Safety issues Mobile phone Mark divisions Outline Physical and chemical properties What is MP ID: 1025185
Download Presentation The PPT/PDF document "Determination of melting point" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
1. Determination of melting pointHawzhin A. SalihMSc. in medicinal chemistryHawzhin.salih@univsul.edu.iq
2. Lab policies PunctuationSafety issuesMobile phoneMark divisions
3. Outline Physical and chemical propertiesWhat is MP?How MP is measuredFactors effecting MPEffect of impurity on MP range
4. ObjectiveThe purpose of the Melting Point experiment is to learn to determine melting point (MP) of different solid organic compounds accurately.
5. Physical vs. chemical propertyA physical property is an aspect of matter that can be observed or measured without changing its chemical composition.Examples are color, hardness, solubility, electrical conductivity, density, melting point, and boiling point.
6. Physical vs. chemical propertyA chemical property may only be observed by changing the chemical identity of a substance. In other words, the only way to observe a chemical property is by performing a chemical reaction. This property measures the potential for undergoing a chemical change. A chemical change always produces one or more types of matter that differ from the matter present before the change. Examples of chemical changes are burning, cooking, or reaction between chemicals.
7. What is Melting point (MP)? In a solid, the particles are closely packed together and are vibrating on the spot. When a solid is heated, the particles start to move faster and faster. If enough heat is applied, the particles will have enough energy to move about. They are still close together, but can move over and around each other. At this point, the solid has melted to form a liquid.When a solid turns into a liquid it is called melting. The temperature at which a solid material melts is called its melting point. Different materials have different melting points.
8. What is melting point (MP)?If a solid material is heated to its melting point, it will change state from a solid to a liquid. The melting point of a solid is the temperature at which transition from solid to liquid occurs at atmospheric pressure; or the temperature at which solid and liquid phases are in equilibrium at a pressure of one atmosphere. In practice, a solid usually melts over a range of temperatures rather than at one specific temperature. For this reason it is more useful to speak of a melting point range. Although the term "melting point" is usually used, what is meant is "melting point range".
9. What is the melting point?If the compound melts over a very narrow range, it can usually be assumed that the compound is relatively pure. Conversely, compounds that melt over a wide range are assumed to be relatively impure. An impure solid melts over a wide range and at a temperature lower than that of the pure solid . It should be noted that “insoluble” impurities such as bits of filter paper or dust have no effect on the MP of a substance. To affect the MP the impurity must be soluble in the solid.
10. QuestionYou have two samples of mannitol. One melted between 168º-169º and the other melted between 161º-168º. Which sample has the greater purity? Why?
11. Why MP is important?1. The melting point indicates the level of purity of a sample. An impure compound melts over a wider range of temperatures, usually greater than 2 degrees. 2. The melting point helps to identify unknown samples, narrowing the number of possibilities, because a pure solid melts reproducibly over a narrow range of temperatures. 3. The melting point helps to characterize new compounds.
12. Factors effecting MPThe melting point range is affected by a number of factors in addition to that of purity. Particle sizeAmount of material usedDensity of packing in the capillary tubeThickness of the capillary tube Rate of heating of the liquid bath.
13. Effect of impurity on the melting point An impure sample melts over a wide range and at a temperature lower than that of the pure material. For the purposes of this course, a compound with a MP range of 2°C or less will be considered to be sufficiently pure.Soluble impurities affect the melting point of a solid in the following manner: A. Lower the melting point of the substance with the upper limit considerably below the true melting point. The presence of an impurity in the molten compound, reduces its vapor pressure thus lowering the melting point of the compound.
14. Effect of impurity on the melting pointB. Broaden the melting point range. Depending on the amount of impurity, the melting process may extend over a range of 2-20 C° or more. The lattice energy and consequently Melting point depends on: 1- Molecular weight. 2- Symmetry of the molecules. 3- Polarity of the molecules. 4- The type of bonding.
15. How is melting point measured?Sample is added to a small glass capillary tube, which is closed at one end. It is important to use as small amount of sample as possible so that sufficient heat is present to melt the sample rapidly. The temperature of the sample is measured with a digital thermometer. The sample is heated slowly as the temperature approaches the MP, while the sample is carefully observed. The temperature at which the first drop of liquid is observed is recorded as the beginning of the melting point range.
16. How is melting point measured?The temperature at which all solid has melted is recorded as the end of the melting point range. Upon heating, the solid may expand and move slightly in the tube. This movement should not be interpreted as the beginning of the MP range. The melting point (MP) is recorded in the lab notebook as, for example, MP (compound A) 102.5-104.0° C.
17. Capillary Method:The procedural rules for melting point determinations are defined in the pharmacopeias. The pharmacopeias regard the capillary method as the standard technique for melting point determination. In this methodology, a thin glass capillary tube containing a compact column of the substance to be determined is introduced into a heated stand (liquid bath) in close proximity to a high accuracy thermometer. While determining a melting point, several observations and the temperatures are recorded.
18. ProcedureFilling the capillary tube : Adding sample is accomplished by pushing the open end of the capillary down into the powdered sample, then tapping the sample down into the closed end of the capillary tube by dropping the tube, closed end first, down into a 2’ length of glass tubing so the sample bounces and allows the solid to pack into the closed end. This can also be done by tapping the closed end gently on the desktop but care must be taken to not break the fragile tube. The height of sample in the capillary should be about 2-3 mm (or 0.5 cm). Too much sample will result in poor results.
19. Sample PreparationCareless preparation of a sample is the leading cause of inaccurate and irreproducible results in melting point determinations. Any substance being loaded into a melting point capillary must be: 1. Fully dry 2. Homogeneous 3. In powdered form
20. Sample PreparationMoist samples must be dried first 48 hours in a dessicator, usually gets the job done. The primary requirement for good melting point determination is that the sample be in a fine powder form. This makes the heat transfer into the sample more efficient, and also enhances the overall reflectivity of the sample for easier automated detection of the melt. Coarse crystalline and nonhomogeneous samples must be crushed into a fine powder in a mortar. An agate, glass or alumina mortar and pestle are recommended.
21. Sample PreparationIf your sample is hygroscopic, or sublimates at high temperatures, the open end of the capillary tube must be sealed by heating. Hygroscopic samples must be stored in a dessicator between tests. This is particularly critical in humid environments or even rainy days.
22.