Melting PointsMelting Points

Background InformationBackground Information

Melting point:Melting point: temperature at which solid is in equilibrium with its liquidtemperature at which solid is in equilibrium with its liquid

Melting point range:Melting point range: span of temperature span of temperaturess from the point at which the from the point at which the crystals first begin to liquefy to the point at which the entire sample is crystals first begin to liquefy to the point at which the entire sample is liquid liquid

Pure compounds usually melt over a narrow temperature range of 1-2Pure compounds usually melt over a narrow temperature range of 1-2°C°C

Presence of soluble impurity almost always decreases and broadens the Presence of soluble impurity almost always decreases and broadens the melting point rangemelting point range

Melting Point of Mixture (X Melting Point of Mixture (X and Y)and Y)

Eutectic temperatureEutectic temperature:: lowest possible melting point for a mixture of lowest possible melting point for a mixture of 2 compounds 2 compounds

Eutectic composition:Eutectic composition: composition at which both solid X and solid composition at which both solid X and solid Y are in equilibrium with the liquid mixture (at eutectic temperature)Y are in equilibrium with the liquid mixture (at eutectic temperature)

Eutectic pointEutectic point

Taking a Melting Point: Taking a Melting Point: PackingPacking

Pack the capillary tube by Pack the capillary tube by pressing the open end gently into pressing the open end gently into a sample of the compound to be a sample of the compound to be analyzed.analyzed.

(Capillary tubes break easily!)(Capillary tubes break easily!)

The solid should fill the tube to a The solid should fill the tube to a depth of depth of ~~2 mm. Tap the bottom 2 mm. Tap the bottom of the capillary on a hard surface of the capillary on a hard surface so that the crystals pack down so that the crystals pack down into the bottom of the tube.into the bottom of the tube.

Melting Point ApparatusMelting Point Apparatus

Pack the crystals at the bottom of Pack the crystals at the bottom of the tube and place the tube in the the tube and place the tube in the slot behind the eye-piece on the slot behind the eye-piece on the Mel-Temp. Turn on the Mel-Temp Mel-Temp. Turn on the Mel-Temp and turn the knob to adjust heating and turn the knob to adjust heating rate (not too fast or too slow). rate (not too fast or too slow).

Watch through eyepiece and record temperature at which the crystals first begin to liquefy and record temperature at which the entire sample is liquid (mp range)

Experimental Procedure Preview

Part I Practice melting range:

Measure the mp of only one known compound, benzoic acid

Part III Identifying an unknown compound by mixture melting point range: Measure the mp of an unknown compound and identify it by testing

with known compounds

Experimental Procedure: Experimental Procedure: Part IPart I

Use benzoic acid (mp = 122-123Use benzoic acid (mp = 122-123°C).°C).

Use proper size of sample (not more than 2 mm in capillary) Use proper size of sample (not more than 2 mm in capillary) just just enough to see!!!enough to see!!!

Pack firmly by dropping through a long glass tubePack firmly by dropping through a long glass tube

Do not move the Mel-TempDo not move the Mel-Temp to your bench or remove to your bench or remove the probethe probe

Increase Temperature by 10°C/min until 10-15°C below the known Increase Temperature by 10°C/min until 10-15°C below the known mp range, then reduce rate to 1-2°C/minmp range, then reduce rate to 1-2°C/min

Allow mp apparatus to cool down naturally after experimentAllow mp apparatus to cool down naturally after experiment ( (no wet no wet paper towels or ice – may use airpaper towels or ice – may use air))

Part II – Eutectic Temperature Part II – Eutectic Temperature (skip this)(skip this)

Unknown mp = 133-135Unknown mp = 133-135°C°C

Benzoin (mp = Benzoin (mp = 135-137°C) or 135-137°C) or transtrans-Cinnamic Acid (mp = 133-134°C)-Cinnamic Acid (mp = 133-134°C)

mp =mp = 135-137°C135-137°C

Unknown is BenzoinUnknown is Benzoin

mp mp <<<<133°C133°C

Unknown is not BenzoinUnknown is not Benzoin

mp mp <<<<133°C133°C

Unknown is not Unknown is not transtrans-Cinnamic Acid-Cinnamic Acid

mp mp = 133-= 133-134°C134°C

Unknown is Unknown is transtrans-Cinnamic Acid-Cinnamic Acid

Mix Unknown and BenzoinMix Unknown and Benzoin

Mix Unknown and Mix Unknown and transtrans-Cinnamic Acid-Cinnamic Acid

Part III: Identifying Your Part III: Identifying Your UnknownUnknown

Unknown is given to you by TA (see list - page 20) Unknown is given to you by TA (see list - page 20) Record the code.Record the code.

Take 2 melting points of your unknown (2 different samples):Take 2 melting points of your unknown (2 different samples):- - 11stst one one taken quickly to approximate range (~10°C/min) taken quickly to approximate range (~10°C/min)- - 22ndnd one one taken slower (1-2°C/min) when 10°C below mp range taken slower (1-2°C/min) when 10°C below mp range approximated from 1approximated from 1stst run run

Get a spatula tip of the compound that could match the mp of your Get a spatula tip of the compound that could match the mp of your unknown (from labeled samples on TA bench) and add it to a spatula tip unknown (from labeled samples on TA bench) and add it to a spatula tip of your unknown on a watch glass (1:1 mixture). of your unknown on a watch glass (1:1 mixture). Mix thoroughly by Mix thoroughly by grinding with a scoopula or spatula.grinding with a scoopula or spatula.

Take mp of the mixture:Take mp of the mixture:- If mp is the same as mp of unknown, unknown is identified.- If mp is the same as mp of unknown, unknown is identified.- if mp of mixture is different from mp of unknown, repeat - if mp of mixture is different from mp of unknown, repeat

process process with another compound.with another compound.

Waste HandlingWaste Handling

Don’t discard capillaries in the trash. Put them in the Don’t discard capillaries in the trash. Put them in the collection beaker labeled “3221 Melting Capillaries”.collection beaker labeled “3221 Melting Capillaries”.

Any unused unknown is to be returned to the TA in the Any unused unknown is to be returned to the TA in the original sample vial (properly capped).original sample vial (properly capped).