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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
4/28/2011 1
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)
4/28/2011 5
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
4/28/2011 10
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
4/28/2011 11
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
4/28/2011 16
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
4/28/2011 20
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.
4/28/2011 21
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.
4/28/2011 25
