Module 5: RECRYSTALLIZATION AND SUBLIMATION Flashcards by Aho Tei

also known as fractional crystallization

RECRYSTALLIZATION

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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

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desired compound should be ____________________________

+ soluble in the hot solvent
+ insoluble or nearly insoluble in the cold solvent

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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

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_____________ should either be insoluble in the solvent at all temperatures or must remain at least moderately soluble in the cold solvent

impurities

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“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

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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

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the boiling point of the solvent should be low enough so that it can readily be removed from the crystals

(True or False)

True

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the solvent should not react chemically with the substance being purified

(True or False)

True

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most ______________ dissolve in hot solvents than in cold ones

organic compounds

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high degree of microporosity; has large surface area

activated charcoal

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▪ using excessive amounts may decrease recovery; use “dropwise method”

too much solvent is used

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too less solvent will not yield crystals

supersaturated solution

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“removes high molecular weight impurities”, which are often
colored and relatively less soluble

decolorization with activated charcoal

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a ______________ should be used to minimize crystallization in the funnel

short-stemmed or stemless glass funnel

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sometimes the sample is ______________ with small quantity of insoluble impurity that will never dissolve

contaminated

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using ______________ will minimize crystallization on the filter

fluted filter paper

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it is important to filter the solution while hot to prevent _____________ during filtration

premature crystallization

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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

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

Seeding

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▪ _____________ the solution will induce bumping of crystals which causes to reformation of more crystals

Stirring and Agitation

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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 _____________

1. vacuum filtration 2. 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

1. impure sample 2. 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)

1. Na2SO4 2. 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