Melting Point

1
Melting Point Refractive Index

• The Theory and use of Melting Point and
• Verify or Identify Organic Compounds
• Study Materials
• Slayden pp. 25 - 30
• Pavia Tech 2 3.9, 24
• Tech 9 (9.1 9.5 9.7 9.9)
• Dr. Schornick Web Site
• http/mason.gmu.edu/jschorni/me
  ltpoint

2
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
• Uses
• Establish Purity of Compounds
• The Purer the Compound, the Higher the Melting
  Point
• The Purer the Compound, the Narrower the Melting
  Point Range
• Identify Compounds

3
Melting Point

• Melting Point Range
• The Onset point (lower temperature) is the
  temperature at which the liquid phase first
  appears in coexistence with the crystals
  (crystals will begin to glisten)
• The Meniscus point is when a solid phase is at
  the bottom and a liquid phase is on top separated
  by a well defined downward pointing meniscusThe
  Meniscus Point is used as the Melting Point in
  Europe
• The Clear Point is when the substance becomes
  completely liquidThe Clear Point is used as the
  Melting Point in USA

4
Melting Point

• 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

5
Melting Point

• Melting Point Depression and Degree of Purity
• The Melting Point of a mixture decreases as the
  concentration of one component increases relative
  to the other compound, e.g., antifreeze in
  coolant system
• As the melting point of a mixture decreases with
  increasing contamination of one of the
  components, the melting point range initially
  increases then decrease until the range is minimal

6
Melting Point

• The Lowest Melting Point of an A/B mixture The
  Eutectic Point - is reached when the
  contaminating compound (B) reaches its solubility
  limit in component A

7
Melting Point Refractive Index

• Elements of the Melting Point Experiment
• Pre-lab report
• Melting Point
• Melting point of
• 2 known compounds
• A mixture of the two known compounds
• An unknown compound
• A mixture of the unknown compound and a known
  compound from the list of knowns (repeated with
  other known compounds until one is found that
  matches the unknown
• Final Report

8
Melting Point

• Procedure
• Equipment
• Capillary Tubes
• Mel-Temp Melting Point Apparatus orSRS Digimelt
  Apparatrus
• Obtain
• Two known samples in sequence as listed in the
  table on page 27 of the Slayden manual i.e., two
  samples with similar melting points
• Unknown sample from Prep room or Instructors
  desk (Note Record unknown No. in your report)

9
Melting Point

• Procedure
• Loading the Capillary Tube
• If necessary, crush the sample using a spatula,
  pestle, or open end of Capillary tube
• Tap the open end of the capillary tube into the
  sample until 1-2 mm of sample is obtained
• 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
• Prepare capillary tubes for the following
• Two of the known compounds in sequence from
  theTable 1, p 27, in Slayden manual
• Sample of a 11 mixture of the two known
  compounds
• Sample of your unknown compound

10
Melting Point Mel_Temp

• 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

11
Melting Point SRS DigiMelt

• Press Start/Temp button and then use the ?/2 and
  ?/3 buttons to set the starting temperature about
  5 degrees below the lowest expected melting point
  of the two known compounds
• Note For an unknown compound set the Start/Temp
  at 100oC (applies to this experiment
  only)
• Press the Ramp/Rate button and use the ?/2 and
  ?/3 buttons to set the ramp rate to 0.5oC/min for
  compounds of known melting point
• Note For an unknown compound set the initial
  ramp rate to 5oC/min
• When the approximate melting point of
  the unknown compound is
  determined, reset the ramp
  rate to 0.5oC and retest the sample for
  the actual melting point

12
Melting Point SRS DigiMelt

• Press the Stop/Temp button and use the ?/2 and
  ?/3 buttons to set the Stop Temperature to at
  least 5oC above the expected melting point
• Note For an unknown set the Stop/Temp to
  175oC
• Press the Stop/Temp button again to return to the
  current temperature display
• Insert the capillary tube, closed end down, into
  one of the chassis holes near the Tube Tapper
  button (right side)
• Press the Tube Tapper button to transfer the
  sample to the bottom of the tube (repeat as
  necessary)

13
Melting Point SRS DigiMelt

• Press the Start/Stop button to preheat the block
  to the starting temperature. The Preheat LED will
  light
• When the Ready LED becomes lit, the oven is
  holding at the start temperature
• Insert capillary tube containing sample into
  heating block
• Note The heating block can accommodate up to 3
  capillary tubes
• Each slot is associated with a keypad button
• Left slot (keypad 1)
• Middle slot (keypad ?2)
• Right slot (keypad ?3)

14
Melting Point SRS DigiMelt

• Press the Start/Stop button to begin ramping the
  temperature at the ramp rate The Melt LED will
  light
• Observe the sample(s)
• When the sample reaches the Onset Point (the
  particles will begin to glisten) press the
  appropriate keypad button to record the first
  data point (repeat for each capillary tube if
  multiple samples are being tested)
• When the sample begins to exhibit a meniscus
  (liquid phase on top, solid phase on bottom with
  a well defined downward curved interface) press
  the applicable keypad button again to record the
  2nd data point
• When the sample becomes completely liquid at the
  Clear (or Liquefaction) Point, press the keypad
  button again to record the 3rd data point

15
Melting Point

• If the melting point ranges of the unknown/known
  mixture and your unknown differ by several
  degrees or more, select a new known compound from
  the table and create a new known/unknown mixture
  and determine its MP range
• Repeat this 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

16
Refractive Index

• The Determination Of The
• Refractive Index Of Organic Compounds

• Study Materials
• Slayden pp. 25 - 30
• Pavia Tech 24 pp. 845 850
• Dr. Schornick Web Site
• http/mason.gmu.edu/jschorni/meltpoint.ppt

17
Refractive Index

• Elements of the Refractive Index Experiment
• Pre-lab Report
• Uses Purity and identification of unknowns
• Background
• Measurement Equipment
• Temperature Correction
• Experiment Refractive Index of a Known Compound
  an Unknown Compound
• Final Report

18
Refractive Index

• Uses
• Identification
• Measure of Purity
• Background
• Refractive Index is a physical property of
  liquids solids related to the velocity and
  wavelength of light in a medium
• Refractive Index is the ratio of the velocity of
  light in a vacuum (air) to the velocity of light
  in a medium
• The Velocity and Wavelength of light in a medium
  are functions of temperature, thus refractive
  index is a function of temperature
• The velocity of light in a medium increases as
  the density decreases and decreases as the
  density increases

19
Refractive Index

• The Refractive Index for a given medium depends
  on two (2) variables
• Refractive Index (nD) 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 (nD) is temperature dependent
• As the temperature changes, the density of the
  medium changes, thus, the velocity (?) changes
• As temperature increases, the medium density
  decreases
• As the medium density decreases, the velocity of
  light increases
• As the velocity of light increases, the ratio of
  the speed of light in vacuum vs. speed of light
  in medium decreases
• Thus, the Refractive Index decreases as
  temperature rises

20
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.
  
  

(Index of Refraction)
(Refractive index)
?1 - Angle of Incidence (air)


?2 - Angle of Refraction
(sample)
21
Refractive Index

• Consider two (2) media air (or vacuum) organic
  liquid
• Frequency of light in both media remains constant
• Divide ?1 by ?2

22
Refractive Index

• Since
• Then
• Substitute in original refractive index equation
• Note n1 for air (or vacuum) 1.0

23
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

24
Refractive Index

• Abbe Refractometer (Cont)
• 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

25
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

26
Refractive Index

• 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

27
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 gt 20oC (?t is positive) Correction
  Factor is added to Raw Value, i.e., the
  Refractive index value at 20oC is greater than
  the value determined at a higher temperature
• For temp lt 20oC (?t is negative) Correction
  Factor is subtracted from Raw Value, i.e., the
  value at 20oC is less than the value at a lower
  temperature

28
Refractive Index

• The equation from the previous slide correctly
  accounts for temperature correction factor
• 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
• Typical Range of Values for Organic Liquids
• 1.3400 - 1.5600

29
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 Instructors desk
• Determine Refractive Index, noting temperature
• Repeat the measurement

30
Refractive Index

• Procedure (cont)
• In your lab report, correct the Refractive Index
  value for Temperature
• Identify your unknown from the list of unknowns
  given in Table 2 on page 30 of the Slayden lab
  manual
• Note The values for the unknown possibilities
  in the table are shown to 2 decimal places
• Use Google, CAS nos, and literature resources
  to find the refractive index values (to 4
  decimal places) for the compounds in the table
  that closely match your measured value
• Match values and determine your unknown

31
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\
• 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
• 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

32
Melting Point Refractive Index

• The laboratory Report (Review Points) (Cont)
• 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

33
Melting Point Refractive Index

• The laboratory Report (Review Points) (Cont)
• 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

34
Melting Point Refractive I

• The laboratory Report (Review Points) (Cont)
• 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

35
Melting Point

• 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
• Note Selection of this compound is probably
  not related to either of the original
  known compounds (but could be)
• Create a 11 mixture of your unknown and the
  selected known compound
• Determine melting point range of known/unknown
  mixture