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
Pavia – Tech 2; 3.9, 24
– Tech #9 (9.1 – 9.5; 9.7 – 9.9)
Dr. Schornick Web Site

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 curve (the meniscus)
The Meniscus Point is used as the “Melting Point” in Europe
The Clear Point is when the substance becomes completely liquid The 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
The mixture displays both a melting point depression and a melting point range
The size of the Melting Point Depression depends on the relative concentrations of the compounds in the mixture

5. Melting Point Melting Point Depression and Degree of Purity
The Melting Point depression of a mixture decreases as the concentration of one component increases relative to the other compound, e.g., antifreeze in coolant system
Generally, a 1% impurity results in a 0.5oC depression
As the melting point of a mixture decreases with increasing contamination of one of the components, the melting point range initially increases then decreases until the range is minimal

6. Melting Point
The temperature at which the melting point of the mixture is the lowest and the range is at a minimum is called The Eutectic Point
The Eutectic Point represents the solubility limit of the contaminating compound relative to the other compound
Solid + Liquid
Liquid A + B
Solid A + B
Range
Clear Point
Onset Point
mp A
mp B
0% B
0% A
Temperature
Eutectic Point
mpB > mpA 
MP Range

7. Melting Point & Refractive Index
Elements of the Melting Point Experiment
Pre-lab report
Melting Point
Melting point of:
2 known compounds with similar melting points
A mixture of the two known compounds
An unknown compound
A mixture of the unknown compound and a known compound from the list of known compounds
This process will be repeated until the known/unknown mixture produces a melting point similar to the pure unknown
Final Report

8. Melting Point Procedure Equipment Capillary Tubes
Mel-Temp Melting Point Apparatus or SRS 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 Instructor’s 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 the Table 1, p 27, in Slayden manual
Sample of a 1:1 mixture of the two known compounds
Sample of your unknown compound

10. Melting Point – MelTemp Apparatus
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 a 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 20oC 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
Repeat for next compound or mixture

11. Melting Point – SRS DigiMelt
Press Start/Temp button and then use the /2 and /3 buttons to set the starting temperature about 10 degrees below the lowest expected melting point of the two known compounds
Note: For an unknown compound set the Start/Temp at oC (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)
Temporarily remove the capillary tube from the instrument

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: Let the sample to equilibrate for at least minutes before proceeding with the measurement
Note: 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 melting point of the sample.
When the sample reaches the “Onset Point” (the particles will begin to glisten) press the 1st keypad (1) 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 2nd keypad button (2) again to record the 2nd data point
When the sample becomes completely liquid at the Clear (or Liquefaction) Point, press the 3rd keypad button (3) to record the 3rd data point

15. Melting Point – SRS DigiMelt
Sample Sequence
Determine the melting point values for the first known sample:
Onset Point
Meniscus Point
Clear Point
Determine the melting point values for the second known compound
Set the “Start Temp” button to about 20oC below the lowest melting point of the two known compounds
Prepare a 50/50 mixture of the two known compounds
Determine the melting point values of this mixture

16. Melting Point – SRS DigiMelt
Determine the melting point values for an unknown compound obtained from the instructor’s desk
From the table on page 27 of the Slayden lab manual select a compound the closely matches the melting point of your unknown
Prepare a 50/50 mixture of the unknown and selected known
Determine the melting point values for this mixture
If the melting point range 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

17. Melting Point
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

18. The Determination Of The Refractive Index Of Organic Compounds
Study Materials
Slayden – pp
Pavia – Tech #24 pp. 845 – 850
Dr. Schornick Web Site

19. 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

20. 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

21. 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

22. Refractive Index (Refractive Index) (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.
(Refractive Index)
(Refractive Index)
1 - Angle of Incidence (air) 2 - Angle of Refraction (sample)

23. Refractive Index
Consider two (2) media: air (or vacuum) & organic liquid
Frequency of light in both media remains constant
Divide 1 by 2

24. Refractive Index Since: Then:
Substitute in original refractive index equation
Note: n1 for air (or vacuum) = 1.0

25. 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 just enough sample to cover the rectangular prism
Close the hinged prisms together - Gently
Turn on the light by pushing up on the switch on the left side of instrument.
Note: The 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

26. Refractive Index Abbe Refractometer (Con’t)
Rotate coarse adjustment knob 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 demarcation line dividing light & dark areas appears as a colored band (chromatic aberration), adjust with the knurled drum knob on the front of the instrument until demarcation line is as sharp as possible
Press switch on left side of instrument down completely to make the scale visible
Read refractive index value to 4 decimal placese, e.g.,

27. Refractive Index View through eyepiece of ABBE’ Refractometer
Dark Half
Light Half

28. Refractive Index Reading the Refractive ValueI
The Index of Refraction (ND) decreases with increasing temperature, i.e., velocity of light in medium increases as density decreases
Measured values of (ND) must be adjusted to 20oC
Temp Correction Factor = t * = (Room Temp – 20) *
For temp > 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 < 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

29. ND20 = NDRm Temp + (Rm Temp – 20) * 0.00045
Refractive Index
The equation for adjusting the raw Refractive Index value is:
ND20 = NDRm Temp + (Rm Temp – 20) *
Note the equation automatically adjusts the correction factor up or down from 20oC
ND20 = (16 – 20) *
(-4) * =
Note: The scale must be read to “4” decimal places
Typical Range of Values for Organic Liquids:
1.5500
1.5523
1.5600
1.5550
1.5580

30. 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

31. Refractive Index Procedure (con’t)
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 just 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

32. 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

33. 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

34. 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

35. 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

36. 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 it could be)
Create a 1:1 mixture of your unknown and the selected known compound
Determine melting point range of known/unknown mixture