Melting Point & Refractive Index

The Theory and use of Melting Point and

Verify or Identify Organic Compounds

Study Materials

Slayden – pp. 25 - 27

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/meltpoint

04/18/23 1

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

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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

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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

04/18/23 4

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

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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

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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

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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)

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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 1:1 mixture of the two known compounds

Sample of your unknown compound04/18/23 9

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

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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

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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)

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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)

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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

04/18/23 14

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

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Refractive Index

The Determination Of The

Refractive Index Of Organic Compounds

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Study Materials

Slayden – pp. 28 - 30

Pavia – Tech #24 pp. 845 – 850

Dr. Schornick Web Site

http:/mason.gmu.edu/~jschorni/meltpoint.ppt

Refractive IndexElements 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

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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

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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

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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 ratio of the sin of the angle of incidence and the sin of the angle of refraction.

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air

medium

V

V

c = n

v

1air

medium 2

V nVsin

Constantsin

1 - Angle of Incidence (air)

2 - Angle of Refraction

(sample)

(Index of Refraction)

(Refractive index)

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 λ

v fv f

1 1 1

2 2 2

2 2 = = f f f

fv (velocity) (Frequency * Wavelength)

Refractive Index Since:

Then:

Substitute in original refractive index equation

Note: n1 for air (or vacuum) = 1.004/18/23 22

1 1 1

2 2 2

sinφ v λ Refractive Index

sinφ v λ2

1

n n

n

1 21 2

&c c

= = n n

v v

cv n

cvn

1 11

2 2

2

n n 1 1 2 2

2

1

nn

1

2

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 places04/18/23 24

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

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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

04/18/23 26Dark Half

Light Half

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) 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

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Refractive Index The equation from the previous slide correctly

accounts for temperature correction factor:

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 - 1.560004/18/23 28

1.5500 1.5523 1.56001.5550 1.5580

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 measurement04/18/23 29

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 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

04/18/23 30

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

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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

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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 Compounds04/18/23 33

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

04/18/23 34

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 1:1 mixture of your unknown and the selected known compound

Determine melting point range of known/unknown mixture

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