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Melting Point & Refractive Index
The Theory and use of Melting Point andRefractive 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 aknown and unknown compounds
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Melting Point Theory & Background
Melting Point
Temperature at which a transition occurs between solidand liquid phases
Temperature at which an equilibrium exists between thewell-ordered crystalline state and the more random
liquid state Melting Point Range
The Onset point (lower temperature) is the temperatureat which the liquid phase first appears in coexistencewith the crystals
The Meniscus point is when a solid phase is at thebottom and a liquid phase on top with a well definedmeniscusUsed as Pelting Point in Europe
The Clear point is when the substance becomes
completely liquidUsed as Melting Point in USA4/27/2012 3
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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
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)4/27/2012 5
Liquid A + B
Solid A + B
RangeClear Point
Onset Point
mp A
mp B
0% B 0% A
Temperature
Eutectic Point
mpB > mpA
MP Range
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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 Knowncompound.Note: The Unknown might have to be mixed withadditional known compounds until the melting point ofthe known and the known/unknown mixture match.
Identify the unknown compound. Equipment
Capillary Tubes
Mel-Temp Melting Point Apparatus (Obtain from Prep
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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 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 glasstubing letting it bounce on table sample is transferred
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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 (2-3oC per minute)
Determine rough melting point
Allow capillary tube to cool until liquid solidifies
Reset temperature knob for a slower rate of temperatureincrease
Allow temperature to rise to 10oC below rough MP
Reset temperature knob so that temperature rises no morethan 0.5oC/Min
Record Melting Point Temperature Range, i.e., thetemperature when the initial drop of liquid forms and thetemperature 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 theTable 1, p 20, in Slayden manual.
Sample of a 1:1 mixture of the two knowncompounds.
Sample of your unknown compound. Determine melting point range of each sample.
Select from Table 1 a compound with a melting pointclose to the melting point of your unknown.
Create a 1:1 mixture of your unknown and the knowncompound
Determine melting point range of known/unknownmixture.
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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 anddetermine its MP range.
Repeat process with a new known for the mixture untilthe difference in the two ranges is minimal.
Compare your results against literature values.
Give IUPAC (formal chemical name) and synonyms forthe 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 condensedphases (liquids or solids) than in air.
Light travels more slowly through a densersubstance.
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
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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 refractedto different extents in the same medium, thus, producedifferent 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 risesand density decreases.
Ratio of speed of light in vacuum vs. speed of light inmedium decreases, thus, the Refractive Index
decreases as temperature rises.4/27/2012 13
f d
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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).
The speed of light ratio is also proportional to the ratioof the sin of the angle of incidence and the sin of theangle of refraction.
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nv
c
nV
Vliquid
air Constant
sin
sin
2
1
1 - Angle of Incidence (air)
2 - Angle of Refraction (sample)
(Index of Refraction)
(Refractive index)
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Refractive IndexConsider two (2) media: air (or vacuum) & organic liquid
Frequency of light in both media remains constant
Divide 1 by 2
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1 1v f
2 2=v f
v f
v f
1 1 12 2 2
2 2
= =f f f
(velocity) (Frequency * Wavelength)v f
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Refractive Index Since:
Then:
Substitute in original refractive index equation
Note: n1 for air (or vacuum) = 1.04/27/2012 16
1 1 1
2 2 2
sin v Refractive Index
sin v
2
1
nn
n
1 2
1 2
&c c
= =n n
v v
c
v ncv
n
1 112 2
2
n n1 2 2
2
1
n
n
1
2
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Refractive Index The Instrument Abbe Refractometer (Bausch & Lomb)
Clean prisms with tissues & Methyl Alcohol BEGENTLE!!
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 sodiumdischarge lamp producing yellow light at 589 nmalso called Sodium D light.
Move hinged lamp up into position
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Refractive Index Abbe Refractometer (Cont)
Rotate coarse and fine adjustment knobs on the rightside of instrument until the horizontal dividing line(may not be sharp at first) between the light upperhalf and dark lower halve of the visual field coincidewith the center of the cross-hairs.
Use eyepiece to focus cross-hairs
If horizontal line dividing light & dark areas appears asa colored band (chromatic aberration), adjust with theknurled drum knob on the front of the instrument
Press small button on left side of instrument to makethe 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 PrismPortal.
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 leftdown 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 onfront 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 4decimal places, e.g., 1.38754/27/2012 19
Dark Half
Light Half
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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 20
oC
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 = ND
Rm 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
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1.5500 1.5523 1.56001.5550 1.5580
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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 Instructors desk.
Determine Refractive Index, noting temperature.
Repeat the measurement
In your lab report correct the Refractive Index value forTemperature.
Identify unknown from list of unknowns given in theSlayden 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 anew 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) (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.
When multiple samples or sub-samples are processed with
the same procedure, it is not necessary to set up aseparate 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) (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
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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 applicableresults, that support the identification of the melting point
and refractive index unknowns.