Module 5: RECRYSTALLIZATION AND SUBLIMATION Flashcards
- also known as fractional crystallization
RECRYSTALLIZATION
- involves the dissolution of the solid in an appropriate solvent at an elevated temperature and the subsequent “re-formation of the crystals upon cooling”, so that any impurities remain in solution
RECRYSTALLIZATION
- desired compound should be ____________________________
+ soluble in the hot solvent
+ insoluble or nearly insoluble in the cold solvent
RECRYSTALLIZATION PROCESS
= Selection of an appropriate solvent
= Dissolution of the solid to be purified in the solvent
= Decoloration with activated charcoal
= Filtration of the hot solution to remove insoluble impurities
= Formation of the crystals as it cools
= Isolation of the purified crystals by filtration
= Drying the crystals
_____________ should either be insoluble in the solvent at all temperatures or must remain at least moderately soluble in the cold solvent
impurities
- “most critical step” in the process: selected to form a product of high purity and in good recovery or yield
selection of an appropriate solvent
- the best solvent will dissolve the solute in hot solution but not in the cold solution); vice versa, it is called a poor solvent
good solvent
the boiling point of the solvent should be low enough so that it can readily be removed from the crystals
(True or False)
True
- the solvent should not react chemically with the substance being purified
(True or False)
True
- most ______________ dissolve in hot solvents than in cold ones
organic compounds
high degree of microporosity; has large surface area
activated charcoal
▪ using excessive amounts may decrease recovery; use “dropwise method”
too much solvent is used
too less solvent will not yield crystals
supersaturated solution
“removes high molecular weight impurities”, which are often
colored and relatively less soluble
decolorization with activated charcoal
- a ______________ should be used to minimize crystallization in the funnel
short-stemmed or stemless glass funnel
- sometimes the sample is ______________ with small quantity of insoluble impurity that will never dissolve
contaminated
- using ______________ will minimize crystallization on the filter
fluted filter paper
- it is important to filter the solution while hot to prevent _____________ during filtration
premature crystallization
reduce the temperature to induce crystal formation; generally the solution should not be disturbed as it cools, since this also leads to production of small crystals
Cooling
crystal of the original solid is added to the solution to induce crystallization, which may then be quite rapid
Seeding
▪ _____________ the solution will induce bumping of crystals which causes to reformation of more crystals
Stirring and Agitation
a 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
Scratching
- during cooling and crystallization, the solution should be protected from _________________ by covering the opening with a piece of filter paper, an inverted beaker, or by loosely plugging it with a clean cork
airborne contaminants
solution should not be ___________ as it cools
disturbed
crystals normally are washed with a small amount of
pure, cold solvent
crystals easily form at a ___________
low temperature
be careful not to use too low temperature as it may form ____________
small crystals
- the solid product is isolated by ____________ using a Büchner or Hirsch funnel and a clean, dry filter flask or by _____________
- vacuum filtration
- gravitational filtration
- removing the last traces of solvent from the crystalline product may be accomplished by _______________
air or oven drying
percentage recovery formula
% 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 =
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑟𝑦𝑠𝑡𝑎𝑙𝑠 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
——————————————- × 100
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒
- the purity of the crystals is usually assessed using ______________, as the presence of the impurities decreases the melting point of the product
melting point determination
___________ is employed to detect any sodium chloride impurity left on the purified crystals (white precipitate or turbid solution indicates presence of NaCl)
AgNO3
(l) + NaCl (aq) → AgCl (s) + NaNO3 (l)
silver nitrate test
- defined as the “direct conversion” of a substance from its “solid phase to its gas phase” without passing through the liquid phase
SUBLIMATION
- this process entirely “relies on the equilibrium” between the solid and gas phases rather than solid and liquid phases
SUBLIMATION
SUBLIMATION SET-UP
- an “outer or lower vessel” is where the ____________ is placed while an “inner or upper vessel” is where the ____________ is formed again
- impure sample
- pure solid
enough “energy builds up” in the
molecules during heating and then escape into gas phase
SUBLIMATION
- the crystals may appear in different forms or habit:
- light and feathery crystals
- round and scaly crystals
- needle-like crystals
- blocks of crystal
SUBLIMATION SET-UP
- the upper vessel must be cold for the pure solids to form on it
note!
- the crystals, deposit, or material obtained when a substance is heated
SUBLIMATE
- has high vapor pressure, low melting point, and more volatile
SUBLIMATE
__________, it also depends on volatility
PURIFICATION
- has low vapor pressure, high melting point, and less or non-volatile
RESIDUE
- the solid particle that remained on the dish
RESIDUE
- “less volatile or non-volatile impurities” remain as a __________, while the sublimate is being formed
residue
- the process of sublimation is very convenient to apply; however, its application is limited since it :cannot be used: for separating compounds that have “similar vapor pressures”
LIMITATION
__________ was employed to differentiate the organic substance from the inorganic residue
barium chloride test
- BaCl2 will react with ___________ producing __________, thereby identifying the residue from the sublimate
BaCl2 (l) + Na2SO4 (aq) → BaSO4 (s) + 2 NaCl (aq)
- Na2SO4
- white precipitate
RECRYSTALLIZATION
= BENZOIC ACID
* Appearance:?
* Chemical formula:?
* Molecular Weight:?
* Melting Point:?
* Boiling Point:?
* Use:?
* Synonym:?
* Density:?
* Toxicity:?
- Appearance: colorless crystalline solid
- Chemical formula: C7H6O2 or
C6H5COOH - Molecular Weight: 122.12 g/mol
- Melting Point: 122-123°C
- Boiling Point: 249°C
- Use: antifungal and preservative
- Synonym: Carboxybenzene, Benzene carboxylic acid
- Density: 1.32 g/cm3
- Toxicity: toxic to the lungs & nervous system (prolonged exposure may lead to target organ damage)
sodium sulfate
amorphous powder
polar
inorganic
3.4 × 10-5 mmHg at
25°C
RESIDUE
NaCl was identified if separated through ____________
! the crystals, upon the test must not yield white precipitate to be able to conclude a complete separation
silver nitrate test
methylene blue was separated upon filtration after the addition of ____________
! the crystals will remain blue if methylene blue is incompletely separated
activated charcoal
SUBLIMATION
= SALICYLIC ACID
* Appearance:?
* Chemical formula:?
* Molecular Weight:?
* Melting Point:?
* Boiling Point:?
* Use:?
* Synonym:?
* Density:?
* Toxicity:?
- Appearance: colorless needle-like crystals
- Chemical formula: C7H6O3
- Molecular Weight: 138.12 g/mol
- Melting Point: 158°C
- Boiling Point: 211°C
- Use: keratolytic, used to treat acne, seborrhea, corn, calluses and warts
- Synonym: 2-hydroxybenzoic acid
- Density: 1.44 g/cm3
- Vapor pressure: 8.2 × 10-5
mmHg at 25°C - Toxicity: salicylism (through
ingestion)
salicylic acid
needle-like crystal
non-polar
organic
8.2 × 10-5 mmHg at
25°C
SUBLIMATE
