Lab 1: Melting Point and Recrystallization

Characteristics of a good recrystallization solvent

• Dissolves analyte well at boiling but poorly when cold (maximizes recovery to allow material to be crystallized out)

• BP greater than 50ºC (can be heated significantly)

• BP less than 100ºC (facilitates drying of crystals making them easy to remove)

• BP below melting point of analyte (prevents ‘oiling out’, crystal melts before it dissolves)

Phase Diagram of Pure Substances

Fixed Composition, variable pressure and temperatures

Phase Diagram of Mixtures

Fixed pressure (1 atm), variable composition and temperature

Melting point depression

effect of impurities on crystal stability, impure mixture easier to melt since not as stable so MP lower than pure components MP

Melting Point Broadening

Caused by incongruent melting

Incongruent melting

starts and finishes melting at different temperatures since not all energy put into melting, starts and finishes at different compositions

Eutectic Point

lowest temperature liquid phase can exist in a binary system

Safe heating of organics

flammable so must use bumper deterrent (boiling stick/spin bar), never place open container of flammable solvent directly on hotplate, use lab jack and hot water bath and place organic solvent in bath

How to induce crystallization

cool in ice bath, scratch flask, use ‘seed crystal’, re-boil to reduce solvent volume

How to isolate recrystallized solvent

Gravity filtration, use cold recrystallization solvent ot wash crystals

How to confirm identity of solid

Heat sample slowly and record temperature at which liquid first appears and temperature at which last solid appears