Recrystallization

Recrystallization

•  technique used to purify a solid compound

What makes an ideal solvent for recrystallization?

Ideal characteristics (name a few):

• Compound should be insoluble in cold solvent, but soluble in hot solvent

• Impurities should either never dissolve (and are thus removed by filtration) or remain dissolved even in cold solvent forever

• Solvent should be volatile

• Boiling point of solvent must be below melting point of solid

• Solvent should be non-toxic/non-hazardous

What is a solvent pair and how does it work for recrystallization?

Solvent pair - combination of two solvents used together in recrystallization to achieve optimal purification of a solid compound.

• The pair consists of a "good" solvent = compound dissolves at high temperature, and a "bad" solvent = compound doesn’t dissolve at any temperature.

• When the compound is dissolved in the "good" solvent and the "bad" solvent is added, the solution becomes saturated with the compound, leading to the formation of crystals upon cooling.

• The addition of the "bad" solvent causes the compound to become less soluble, promoting the precipitation of pure crystals while leaving impurities behind.

• This technique helps in achieving higher purity and better yields compared to using a single solvent alone.

Summarized steps in a recrystallization:

1. Choose solvent by testing for solubility at room temp, high temp, and ice bath. Add solvents to solid to see which is the most suitable

2. Once solvent is chosen, add warm solvent to flask with 1g of your impure solid to dissolve it

3. Hot gravity filtration this mixture (beaker, stemless funnel, filter paper, pour in)

4. Vacuum filtrate this mixture, collect crystals

5. Use the mother liquor to collect second crop
   
   

What is the minimum amount of solvent you need to dissolve your solids in the Erlenmeyer flask?

There is no minumum, you need to determine how much solvent should be used:

• Slowly add hot solvent to your warm Erlenmeyer flask containing your solids, 1-2 mL at a time

• Be cautious of insoluble impurities since they will never dissolve.

• do not add too much solvent!!!!

Why use boiling chips?

• prevents bumping or splashing during boiling

• promotes even boiling throughout the liquid

What is oiling out?

• when the solvent is not well-chosen or when the concentration of the solution is too high, an oil is formed

Why use fluted filter paper?

• used in hot gravity filtration process

• provides a larger surface area for filtration compared to flat filter paper

What are “supersaturated solutions” and how do you fix this for recrystallization?

Supersaturated solution - solution that contains more than the average solvent that can be dissolved at a given temperature

• The recrystallization of the excess dissolved solvent in a super-saturated solution can be started by inserting a tiny solute crystal, called a seed crystal.

What is the mother liquor and how do you use it to collect a second crop?

Mother liquor - the liquid left behind in the filtered flask after vacuum filtrating the hot solvent

• can be used to collect a second crop by evaporating to half the amount, cooling, placing in ice bath, and vacuum filtrating. 

Formula for percent recovery

• percent recovery = (amt recovered)/(starting amt) x 100% 

how does decolorizing charcoal work?

Used to remove highly colored impurities

• has large surface area to adsorb colored impurities

• should be used carefully since it can adsorb the compound we want to purify

What are the steps for using decolorizing charcoal?

• remove solution from heat source after solid is dissolved in desired amount of hot solvent

• add small amount of decolorizing charcoal, swirl to mix, reheat until color disappears

• decolorizing agent is removed by hot gravity filtration