Crystallization Flashcards by Sarah Jennings

Crystallization

A common PURIFICATION method used when a soild product (precipitate) forms after a reaction

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What is the property that purification in crystallization is based upon?

Separation/purification based upon differences in SOLUBILITY

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Crystal

A piece of HOMOGENOUS SOLID SUBSTANCE with symmetrically arranged geometry

–> Highly specific how the molecules come together

–> PURE

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Crystal formation from a HOT saturated solution:

What are the two steps in this process?

1) Nucleation

2) Crystal Growth

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Nucleation

Process by which a small number of atoms, ions, or molecules arrange themselves into a crystalline pattern, forming a nucleus that can grow into a macroscopic crystal

Solute molecules gather together into clusters (clustering) to reach a “critical size”

–> Once critical size is reached, the crystals become STABLE and do not redissolve into solution!! (the nucleus is formed at this point)

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Precipitate

Also crystalline, but they are broken down (much smaller) and may contain impurities

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How are crystals grown?

Grown from HOT SATURATED SOLUTIONS

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Crystal Growth

Following the production of crystal nuclei, these nuclei grow to achieve critical cluster size

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How does solubility relate to temperature?

Most solids have better solubility in HOT solvents rather than cold

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Overall steps of crystallization (7):

1) Solvent trial
2) Dissolving the solid (formation of hot saturated solution)
3) Removing INSOLUBLE impurities (gravity filtration)
4) Cooling the solution
5) Collecting the crystals (vacuum filtration)
6) Washing the crystals
7) Drying the crystals

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What is a solvent trial?

A method for finding a proper solvent for crystallization process

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Properties of good solvent for crystallization

1) Solid is soluble in the HOT solvent

2) Solid is NOT SOLUBLE in the COLD or ROOM TEMP solvent

–> These two properties allow for the crystals to form when cooled!!

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Solvent trial process

Uses SMALL amount of solid (~30-50mg) in a test tube with 1 mL of the solvent being tested

1) If solid dissolves at room temp = SOLVENT NOT GOOD

2) If solid DOESNT dissolve at room temp –> Heat up the solution:

2A) If solid is still NOT dissolving –> SOLVENT NOT GOOD
2B) If solid IS dissolving (completely) –> SOLVENT MAY BE GOOD

–> 3) RECOVERY CHECK: Plunge the tube from 2B into ice cold water and examine if a solid forms
3A) Solid FORMS = GOOD SOLVENT
3B Solid does NOT form = not good solvent

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When choosing a solvent to crystallize a substance, you should pick a solvent that:

A. Dissolves the substance quickly.
B. Makes a colorless solution of the substance.
C. Dissolves the substance better hot than cold.
D. Does not dissolve the substance.
E. Makes a cloudy mixture when the substance is added.

C. Dissolves the substance better hot than cold.

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Ideal Crystallization Solvent

1) Nonreactive
2) Nontoxic
3) Inexpensive
4) Nonflammable
5) MODERATE BOILING POINT (65-95C)
–> Easier to control the experiment

–> If there are multiple potentially good solvents, choose the one with the more moderate BP

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Crystallization: Dissolving the solution

What type of solution are we trying to make?

A saturated solution

–> Add preheated solvent to the solid a little at a time until all has dissolved (to ensure it is saturated and that there isn’t then too much solvent)

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Insoluble Impurities

Impurities that will NOT dissolve into solution even with the addition of more solvent

How are INSOLUBLE impurities removed in crystallization?

HOT FILTRATION

Crystallization:

Hot filtration set up

(gravity filtering)

1) Hot plate

2) Beaker on top of the hot plate (with a few mL of solvent!!!!!)

3) STEMLESS FUNNEL inserted into beaker

4) Fluted filter paper pressed into the funnel

5) Sample in flask ready to pour

6) Watch glass to put on top of funnel to keep heat inside!

Crystallization: Hot filtration

Why is it important to have a few mL of solvent in the receiving beaker on the hot plate?

To keep the receiving chamber hot to ensure that no solid precipitates out prematurely!

–> Ensure that we don’t lose any of the desired product

Hot filtration (crystallization)

–> Where do the insoluble impurities end up?

The insoluble impurities remain caught in the filter paper

The HOT solution with the desired product filter through to the beaker (soluble impurities go with it!)

How is yield maximized during hot filtration process for crystallization?

By washing the filter paper several times with HOT solvent

–> Will redissolve any of the wanted solid that precipitated out of solution and stuck to the filter paper

Hot filtration (crystallization)

–> What happens if system is not kept hot?

The compound will precipitate out on the filter paper and get filtered out along with the insoluble impurities

–> You will lose some sample!

What is the purpose of the hot filtration step?

A. To remove insoluble impurities.

B. To dilute the solution with additional solvent.

C. To test if you can properly fold filter paper.

D. To separate the solute from solvent.

E. To separate the solute from soluble impurities.

A. To remove insoluble impurities.

(the stuff that won’t dissolve)

Cooling the solution (crystallization) --> Why is it important to cool the solution slowly?

Because if cooling occurs too quickly a precipitate will form instead of crystals

Polycrystalline

Multiple nucleated parts of growing crystals begin to come together to form clusters that will eventually create a singular large crystal

How do soluble impurities contaminate crystal formation?

When nucleated parts of a forming crystal begin coming together to form a polycrystalline, in the initial steps there are gaps between the segments --> In these gaps, impurities that can fit within will get trapped as the polycrystalline begin to close in on one another, sealing the impurity within the crystal

How do soluble impurities get removed from crystals with time?

With time, the impurities DIFFUSE out of the polycrystalline structure due to different intermolecular forces --> THIS REQUIRES TIME and for the polycrystalline structure to NOT be fully formed yet (ie. the solution must be still kind of warm; which is why slow cooling is important)

Why is slow cooling important for proper crystal production?

To allow time for the removal of any soluble impurities that may get stuck within the crystalline structure --> Produces a PURE crystal!

What is formed if cooling of the solution occurs too quickly?

A precipitate

Crystallization: What method is used to collect the crystals once formed?

VACCUUM FILTRATION --> Because we want to keep the SOLID but get rid of the LIQUID!

What is vacuum filtration usually used for?

Used for when you want to SAVE THE SOLID rather than the liquid!

Crystallization: How are the SOLUBLE impurities removed?

Removed through vacuum filtration --> They get sucked through with the liquid into the receiving flask

Vacuum Filtration Setup

1) Two filter flasks clamped horizontally to each other 2) Buchner funnel with FILTER VAC inserted into one of the flasks 3) Filter paper in the buchner funnel 4) TRAP adapter inserted into the other filter flask 4) THICK walled tubing: a) One tube attaching the side arms of the two flasks together b) One tube attaching the TRAP adaptor to the aspirator 5) Pinch clamp: to clamp the air INLET to initiate or break vacuum

Vacuum Filtration Setup: Which knob is for the correct aspirator?

The MIDDLE KNOB (lower green knob that is level with the aspirator height)

Vacuum Filtration Setup: What happens if the air-inlet is not clamped?

Vacuum will NOT start (the system is open) --> Air inlet must be clamped in order to start the vacuum (to close the system)

Crystallization: How do we wash the crystals?

Once vacuum filtration is complete, the crystals are washed with COLD solvent --> Vacuum filtration apparatus then sucks this extra solvent with the soluble impurities out into the receiving flask

Crystallization: Why must the crystals be washed?

If the crystals are left to dry without washing, there may still be residue of soluble impurities left on the crystals --> To increase purity, we wash the crystals several times to get off any of these residual impurities deposited on top of the crystals

Crystallization: Drying the crystals process

Leave them to dry; depending on the solvent used, the drying time may vary

At 100 ºC, the solubility of A in water is 2 g/10 ml; at 20 ºC, the solubility of A in water is 0.5 g/10 ml. When 10 ml of the saturated water solution is cooled from 100 ºC to 20 ºC , how much A will crystallize out of the solution? A) 0.5 g B) 1.5 g C) 2.0 g D) 10 g

B) 1.5 g At 100C the solid can dissolve up to 2g in 10 mL of the solvent BUT at 20C the solid can dissolve up to 0.5g in 10 mL of the solvent. If the solution is saturated at 100C that means there is 2g of solid dissolved in it (as this is the max amount that can dissolve at that temperature) SOOOO when the solution is cooled and the saturation point for the solvent DECREASES, an amount of solid will precipitate out until the new solution is saturated (which occurs at 0.5g of solid dissolved) Therefore since there is 2g of solid present in the mix overall, and only 0.5g of it can dissolve at the lower temp, **1.5g of it will be left not able to dissolve**

Percent Recovery =

(FINAL MASS / INITIAL MASS) x 100

Crystallization vs Precipitation

Crystallization: Solubility is decreased gradually (cooled slowly) = production of large, pure crystals Precipitation: Solubility is decreased quickly (cooled fast) = production of microcrystals (as crystals weren't given enough time to form) --> typically contains impurities