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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

Room)4/27/2012 6

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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

to closed end of capillary tube. Repeat, if necessary4/27/2012 7

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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

change.4/27/2012 12

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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)

f d

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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.