Experiment 1
"Who's Got What I've Got"

Purification of an Unknown Solid by Recrystallization and Identification by Melting Point

Learning Objectives

attempting to solve an identification of an unknown compound (limited to solid unknowns and to just one physical property: a compound's melting point)
learning basic techniques for the purification and identification of organic compounds

Goals

find other students who have the same compound
determine the identity of your compound from the list of possibilities

* Before initiating the comparisons, it is necessary to purify your unknown by recrystallization from water

Lab Equipment

Erlenmeyer flask
Hot plate
Funnel
Fluted funnel paper
Bent stick
Watch glass
Buchner funnel or Hirsch funnel
Thermometer (Mel-Temp)

Make sure:

Identify particular compound
confirm predictions about who has same substance
confirm no others have same substance
finally, determine the identity of the compound

Outline of Procedure

A. Recrystallization

Obtain small sample of an impure solid (unknown will later be identified from Table 1)
Recrystallize sample from hot water (*If unknown or hot water is colored, decolorize with charcoal)

[Technique 2]
Steps:
A. Find a suitable solvent for recrystallization;
- Compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature
- Unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent
- Solvent should not react with the compound being purified
- Solvent should be volatile enough to be easily removed from the solvent after the compound has been crystallized
B. Dissolve the impure solid in a minimum volume of hot solvent;
- Place impure solid into an Erlenmeyer flask
Flask's conical shape decrease amt. of solvent lost to evaporation, prevents formation of a crust around the sides of the glass, and makes easier to swirl without splashing
- Add small volume of hot solvent
- Keep warm on hot plate or in water bath
C. Remove any insoluble impurities by filtration;
- Hot filtration; prevent solute from crystallizing out
- Fluted filter paper inside funnel, small amt. of solvent (few ml.) in flask
- Heat until solvent boils and vapors warm filter *Add extra solvent to keep solid from crystallizing out during filtration
- Rapidly filter the hot solution
- After... wash the residue on the filter paper with a small volume of fresh, hot solvent to dissolve any compound that may have precipitated
D. Slowly cool the hot solution to crystallize the desired compound from the solution;
- Cover flask containing the hot filtrate with a watch glass
E. Filter the solution to isolate the purified solid compound
- Line bottom of Buchner funnel or Hirsch funnel w/ a piece of filter paper large enough to cover the holes in the bottom plate without curling up
- Place neoprene adapter on stem & insert it in the top of the filter flask
- Connect side-arm of filter flask to water
- Run water full force -> create vacuum
- Slowly pour recrystallization solution into the funnel allow suction
- Rinse w/ small vol. of recrystallization solvent
- Wash w/ few millimeters of fresh, cold recrystallization solvent to remove any remaining mother liquor and dissolved impurities
***Remove the vacuum from the system by disconnecting the vacuum tubing from the aspirator BEFORE turning the water off***

B. Melting Point

1. Determine the melting point
Melting point: temperature at which the solid phase is in equilibrium with the liquid phase
- be sure that it is dry and finely powdered
2. Compare melting point with unknowns listed
3. Carry out mixed melting point determinations with as many other students' unknowns to determine which have the same unknown
4. Carry out mixed melting point determinations using authentic samples of the unknowns to confirm identity

	Created by McKenna Killets about 8 years ago