Melting Point by mikesanye

VIEWS: 68 PAGES: 25

									            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



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            Melting Point & Refractive Index
           Elements of the Experiment
             Pre-lab report
             Melting Point
               Theory and Background
               Uses
               Measurement Techniques & Equipment
               Melting Point Range
               Melting Point Ranges of Known Compounds, Mixtures,
                 Unknown
               Refractive Index
                 Theory and Background
                   Temperature Correction
                 Measurement Techniques & Equipment
                 Refractive Index, with temperature correction for a
                  known and unknown compounds
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                             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
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                                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

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                                 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
                                                            mpB > mpA            mp B

                         mp A                       Liquid A + B




                                                                                  Temperature
                                           
                  Clear Point
      Range                     MP Range
                  Onset Point

                                   Solid A + B
                                                           Eutectic Point
                                0% B                                      0% A
           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)
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                                 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)
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                                Melting Point
           Procedure
               Obtain:
                   Mel-Temp Melting Point apparatus from Prep Room
                   Two known samples in sequence from table on page 20
                    of 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
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                                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
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                             Melting Point
           Prepare capillary tubes for the following:
               Two of the known compounds in sequence from the
                Table 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.
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                            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




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                             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
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                         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.
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                             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.
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                                     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).
                                         c
                                           n   (Index of Refraction)
                                         v

           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.

            V            sin 
                                     Constant  n
                                 1
               air
                                                        (Refractive index)
            V liquid
                         sin    2




            1 - Angle of Incidence (air)
            2 - Angle of Refraction (sample)
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                        Refractive Index
      Consider two (2) media: air (or vacuum) & organic liquid


            Frequency of light in both media remains constant

                              f2 = f2 = f
             v (velocity)    f  (Frequency * Wavelength)

                  v1  f 1               v2 = f λ2
            Divide 1 by 2
                        v1         f 1        1
                                          
                        v2         f 2        2
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                            Refractive Index
           Since:             c              c
                          v1 =         & v2 =
                               n1             n2
                                      c
                             v1       n1       1
           Then:                         
                             v2       c        2
                                      n2
                                1n1   2n2
                                    1   n2
                                       
                                    2   n1
           Substitute in original refractive index equation
            sinφ1       v1      λ1      n2
                                          n  Refractive Index
            sinφ2       v2      λ2      n1
           Note:    n1 for air (or vacuum) = 1.0
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                           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


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                           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

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                                Refractive Index
           The Measurement
               Place 3-4 drops of sample on Prism.
               Close Prism and raise lamp in front of Prism
                Portal.                                             Light Half
               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
                                                                    Dark Half
               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
4/28/2011       decimal places, e.g., 1.3875                                     19
                                  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
                         1.5500      1.5523      1.5550        1.5580   1.5600




               Typical Range of Values for Organic Liquids: 1.3400 - 1.5600
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                          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.

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            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.




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            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.




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            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




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                  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.




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