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melting point refractive index

Melting Point

benjamin
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melting point refractive index

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    1. Melting Point & Refractive Index The Theory and use of Melting Point and Refractive Index to Verify or Identify Organic Compounds Study Materials Slayden – pp. 17-22 Pavia – Tech 2; 3.9, 24 – Tech #9 (9.1 – 9.5; 9.7 – 9.9) Dr. Schornick Web Site http:/classweb.gmu.edu/jschorni/meltpoint 9/5/2011 1

    2. 9/5/2011 2

    3. Melting Point Theory & Background Melting Point Temperature at which a transition occurs between solid and liquid phases Temperature at which an equilibrium exists between the well-ordered crystalline state and the more random liquid state Melting Point Range The Onset point (lower temperature) is the temperature at which the liquid phase first appears in coexistence with the crystals The Meniscus point is when a solid phase is at the bottom and a liquid phase on top with a well defined meniscus – Used as “Pelting Point” in Europe The Clear point is when the substance becomes completely liquid – Used as “Melting Point” in USA 9/5/2011 3

    4. Melting Point Uses Identify Compounds Establish Purity of Compounds Melting Point Depression Pure compounds display little, if any, “melting point” range, i.e., they have “sharp” melting points Mixtures of substances, i.e., the contamination of one compound by another, whose components are insoluble in each other in the liquid phase, display both a melting point depression and, instead of a sharp melting point, a melting point range The size of the melting point depression depends on the composition of the mixture Generally, a 1% impurity results in a 0.5oC depression 9/5/2011 4

    5. Melting Point Melting Point Indicates Purity in Two Ways The Purer the Compound, the Higher the Melting Point The Purer the Compound, the Narrower the Melting Point Range Melting point of A decreases as impurity B is added Eutectic Point is the Solubility Limit of B in A; Thus, it is the Lowest Melting Point of an A/B mixture(Note: Sharp melting point, i.e., no range, at eutectic point) 9/5/2011 5

    6. Melting Point The Experiment Determine the melting point range of: Two Known Compounds A Mixture of the Two Known Compounds An Unknown Compound Mixture of Unknown Compound and a Known compound.Note: The Unknown might have to be mixed with additional known compounds until the melting point of the known and the known/unknown mixture match. Identify the unknown compound. Equipment Capillary Tubes Mel-Temp Melting Point Apparatus (Obtain from Prep Room) 9/5/2011 6

    7. Melting Point Procedure Obtain: Mel-Temp Melting Point apparatus from Prep Room Two known samples in sequence from table on page 20of the Slayden manual Unknown sample from Prep room(Note: Record unknown No. in your report) Loading the Capillary Tube Crush sample using spatula or open end of Capillary tube Tap open end of tube into sample (1-2 mm of sample) Drop tube (closed end down) down a length of glass tubing letting it bounce on table – sample is transferred to closed end of capillary tube. Repeat, if necessary 9/5/2011 7

    8. Melting Point Obtaining the “Melting Point Range” Place capillary tube with sample at the bottom of the tube in a Mel-Temp apparatus Adjust temperature knob until temperature rises about (2-3 oC per minute) Determine rough melting point Allow capillary tube to cool until liquid solidifies Reset temperature knob for a slower rate of temperature increase Allow temperature to rise to 10oC below “rough MP” Reset temperature knob so that temperature rises no more than 0.5oC/Min Record “Melting Point Temperature Range, i.e., the temperature when the “initial drop of liquid forms” and the temperature when the entire mass turns to clear liquid 9/5/2011 8

    9. Melting Point Prepare capillary tubes for the following: Two of the known compounds in sequence from theTable 1, p 20, in Slayden manual. Sample of a 1:1 mixture of the two known compounds. Sample of your unknown compound. Determine melting point range of each sample. Select from Table 1 a compound with a melting point close to the melting point of your unknown. Create a 1:1 mixture of your unknown and the known compound Determine melting point range of known/unknown mixture. 9/5/2011 9

    10. Melting Point If the melting point range of the unknown/known mixture and your unknown differ by several degrees or more, create a new known/unknown mixture and determine its MP range. Repeat process with a new known for the mixture until the difference in the two ranges is minimal. Compare your results against literature values. Give IUPAC (formal chemical name) and synonyms for the unknown Provide Molecular Structure of unknown, e.g., CaHbXc 9/5/2011 10

    11. Refractive Index Refractive Index Study Materials Uses Background Measurement & Equipment Temperature Correction Experiment – Refractive Index of Known Compounds & Unknown Compound Study Materials Slayden – pp. 20-22 Pavia – Tech #24 pp. 845 – 850 Dr. Schornick Web Site http:/classweb.gmu.edu/jschorni/meltpoint.ppt 9/5/2011 11

    12. Refractive Index Uses Identification Measure of Purity Background Refractive Index is a physical property of liquids & solids Light travels at different velocities in condensed phases (liquids or solids) than in air. Light travels more slowly through a denser substance. The Wavelength of light is also different in condensed phases. As the velocity decreases, the wavelength decreases. The Frequency of light in condensed phases does not change. 9/5/2011 12

    13. Refractive Index The Refractive Index for a given medium depends on two (2) variables: Refractive Index (n) is wavelength (?) dependent. Beams of light with different wavelengths are refracted to different extents in the same medium, thus, produce different refractive indices. Refractive Index (n) is temperature dependent. As the temperature changes, the density changes; thus the velocity (?) changes. Density of a medium decreases as temperature rises. Speed of light in medium increases as temperature rises and density decreases. Ratio of speed of light in vacuum vs. speed of light in medium decreases, thus, the Refractive Index decreases as temperature rises. 9/5/2011 13

    14. Refractive Index For a given liquid and temperature, the ratio of the speed of light in a vacuum (c) and speed of light in the medium (?) is a constant (n). The speed of light ratio is also proportional to the ratio of the sin of the angle of incidence and the sin of the angle of refraction. 9/5/2011 14

    15. Refractive Index Consider two (2) media: air (or vacuum) & organic liquid Frequency of light in both media remains constant Divide ?1 by ?2 9/5/2011 15

    16. Refractive Index Since: Then: Substitute in original refractive index equation Note: n1 for air (or vacuum) = 1.0 9/5/2011 16

    17. Refractive Index The Instrument – Abbe Refractometer (Bausch & Lomb) Clean prisms with tissues & Methyl Alcohol – BE GENTLE!! Do not touch prism with fingers or other hard objects, use tissues Use 3 – 4 drops of sample Close hinged prisms together - Gently Turn on the light - Preferred light source is a sodium discharge lamp producing yellow light at 589 nm – also called Sodium “D” light. Move hinged lamp up into position 9/5/2011 17

    18. Refractive Index Abbe Refractometer (Con’t) Rotate coarse and fine adjustment knobs on the right side of instrument until the horizontal dividing line (may not be sharp at first) between the light upper half and dark lower halve of the visual field coincide with the center of the cross-hairs. Use eyepiece to focus cross-hairs If horizontal line dividing light & dark areas appears as a colored band (chromatic aberration), adjust with the knurled drum knob on the front of the instrument Press small button on left side of instrument to make the scale visible. Read refractive index value to 4 decimal places 9/5/2011 18

    19. Refractive Index The Measurement Place 3-4 drops of sample on Prism. Close Prism and raise lamp in front of Prism Portal. Flip switch on left side to turn on light. Use large dial on right to bring light/dark image into view. If image cannot be found, flip switch on left down and use large dial on right to bring the Scale into view around 1.4000 Release switch on left and use large dial on right to bring light/dark image into view Sharpen line of demarcation using Drum dial on front of instrument. Use Eyepiece to sharpen Cross-Hairs Align the line of demarcation with the Cross-Hairs Flip switch on left down and read value to 4 decimal places, e.g., 1.3875 9/5/2011 19

    20. Refractive Index Reading the Instrument Index of Refraction (ND) decreases with increasing temperature, i.e., velocity of light in medium increases as density decreases. Measured values of (ND) are adjusted to 20oC Temp Correction Factor = ?t * 0.00045 = (Room Temp – 20) * 0.00045 For temp > 20oC (?t is positive), i.e., add correction factor For temp < 20oC (?t is negative), i.e., subtract correction factor The following equation automatically accounts for temperature correction: ND20 = NDRm Temp + (Rm Temp – 20) * 0.00045 Ex: For an observed value of 1.5523 at 16oC, the correction is: ND20 = 1.5523 + (16 – 20) * 0.00045 = 1.5523 + (-4) * 0.00045 = 1.5505 Note: Instrument can be read to “4” decimal places 9/5/2011 20

    21. Refractive Index Procedure Use the ABBE refractometer to measure the Refractive Index of a compound with a known refractive index. Note the temperature using the thermometer on the right side of the refractometer. Record the refractive index value to 4 decimal places Repeat the measurement Obtain an unknown sample from Instructor’s desk. Determine Refractive Index, noting temperature. Repeat the measurement In your lab report correct the Refractive Index value for Temperature. Identify unknown from list of unknowns given in the Slayden lab manual. Confirm values with literature values. 9/5/2011 21

    22. Melting Point & Refractive Index The Laboratory Report (Review Points) The report must reflect the appropriate number of procedures. A new procedure is defined when the experimental process changes to a logically different series of steps. Remember that each unique computation is considered a new procedure. When the procedure involves a computation, the equation must be set up in the procedure description and must include the definition of each variable. 9/5/2011 22

    23. Melting Point & Refractive Index The laboratory Report (Review Points) (Con’t) When the results for a computation are reported in the “Results” section, the calculation of each result must by shown along with the applicable units and appropriate precision, i.e., decimal places & significant figures. When multiple samples or sub-samples are processed with the same procedure, it is not necessary to set up a separate procedure for each sample. Setup a suitable template in “Results” to report all of the results obtained. 9/5/2011 23

    24. Melting Point & Refractive Index The laboratory Report (Review Points) (Con’t) Literature references for specific compounds are usually cited in the “References” section of the lab report and must include the page number and the item no., if available. Note: The Slayden manual and the Pavia text are not citable references for compounds. Use the following sources for compound citations: CRC handbook of Chemistry & Physics The Merck Index The CRC Handbook of Data on Organic Compounds 9/5/2011 24

    25. Melting Point & Refractive I The laboratory Report (Review Points) (Con’t) Summarize in paragraph form, all of the results obtained in the experiment. Use a logical organization and order of the results. The “Conclusion” for the Melting Point & Refractive Index experiment must present arguments, using applicable results, that support the identification of the melting point and refractive index unknowns. 9/5/2011 25

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