Experiment 5: Recrystallization & Sublimation

fractional crystallization

recrystallization

recrystallization

dissolution of the solid in an appropriate solvent at an elevated temperature and reformation of the crystals upon cooling, so that any impurities remain in solution

solids

more soluble in a hot than in a cold solvent

no 1: selection of appropriate solvent

most critical step in the process. soluble in hot solvent and insoluble or nearly insoluble in cold solvent

impurities

should be insoluble in the solvent at all temperatures, or must remain moderately soluble in cold solvent

no 2: dissolution of the solid to be purified in solvent

add enough solvent to dissolve the solute (saturated solution)

no 3: decolorization with activated charcoal

removes HMW impurities, which are often colored and relatively less soluble

activated charcoal

high degree microporosity; has large surface area

no. 4: filtration of hot solution to remove insoluble impurities

filter the solution while hot to prevent premature crystallization during filtration. uses short-stemmed or stemless glass funnels

fluted filter paper

minimize crystallization on the filter

cooling

reduce the temperature to induce crystal formation

seeding

crystal of the original solid is added to the solution to induce crystallization, which may be quite rapid

scratching

glass rod is to rub the inside surface of the crystallization vessel at/or just above the air/solution interface; microscopic scratches provide sharp edges where crystals may grow

stirring & agitation

the solution will induce bumping of crystals which causes reformation of more crystals

oiling out

occasionally the solute will separate from solution as an oil rather than a solid; undesirable for purification of solutes because the oils usually contain significant amounts of impurities

no. 6: isolation of purified crystals by filtration

crystals are washed with a small amount of pure, cold solvent

no. 7: drying the crystals

less desirable option however, because fibers of paper may contaminate the product

purity of the crystals

usually assessed using melting point determination

silver nitrate test

employed to detect any sodium chloride impurity left on the purified crystal

sublimation

direct conversion of a substance from its solid phase to its gas phase without passing through the liquid phase

outer or lower vessel

where the impure sample is placed

inner or upper vessel

where the pure solid is formed again

sublimate

the crystals, deposit, or material obtained when a substance is heated. has a high vapor pressure, low melting point, and is more volatile

residue

the solid particle that remained on the dish. it has a low vapor pressure, high melting point, and is less or non-volatile

less volatile substances

do not evaporate stay behind as residuue

more volatile substances

turns into vapor and then solidifies as the purified sublimate

process of sublimation

cannot be used for separating compounds that have similar vapor pressures

barium chloride test

to differentiate the organic substance from the inorganic residue

BaCl2 reacts with Na2SO4

produces white precipitate, thereby identifying the residue from the sublimate

presence of NaCl

can be determined using the silver nitrate test

white

if NaCl is still present, it will react with silver nitrate to form a __ precipitate

white precipitate

to confirm that NaCl has been completely removed or separated, the test should not produce any __

methylene blue

can be removed by adding activated charcoal and then filtering the mixture

incomplete

if the separation is __, the crystals will still appear blue