RECRYSTALLIZATION

Purification Methods Recrystallization Sublimation Extraction Chromatography Distillation

A pure compound … - is a homogenous sample consisting only of molecules have the same structure. - can be characterized by its physical properties, such as a narrow melting range melting point that matches the accepted value.

Solvent Recrystallization Dissolve the solid in an appropriate solvent at an elevated temperature to form a saturated solution. Cool the solution slowly to recover the solid.

Criteria for Solvent Selection Solvent should have a favorable temperature coefficient – Solid should be soluble in the hot solvent at elevated temperatures (1g/20mL). Solid should be insoluble or nearly insoluble at cold temperatures .

Criteria for Solvent Selection 2. Impurities should either be: insoluble in the hot solvent --or-- at least moderately soluble in the cold solvent.

Criteria for Solvent Selection 3. Boiling point of solvent should be: lower than the melting point of the solute low enough that it can be easily removed from the crystals 4. Solvent should NOT react with the material being separated.

Mixed Solvents Sometimes a mixture of solvents is required to achieve these criteria. For example: ethanol and water Dissolve in the smallest amount of hot ethanol Add hot water until the solution is just turbid (saturated). Cool

Dissolution Weigh the solid to be purified Place in an Erlenmeyer flask (not a beaker) Add boiling stones Add a few mL of solvent and heat to boiling Continue adding solvent in small amounts until no more solid will dissolve

Dissolution If there are insoluble impurities in the hot solution, filtering the solution while hot will remove them. If a hot filtration is to be preformed, add an additional 2-5% volume of solvent and be sure to keep the solution hot while filtering.

Hot Filtration Gravity filtration – fluted paper – preheated stemless glass funnel Keep the solvent hot Observe the material that remains on the paper

Crystallization Protect from airborne contaminants Cool slowly to ambient temperature… Then, cool in an ice bath Crystals larger than 2 mm may occlude solvent and impurities. Very small crystals increase the surface area and retain impurities

Vacuum Filtration Filter paper must fit the funnel. Wet paper with cold solvent first. Add cold slurry of crystals and solvent slowly. Wash with a small volume of cold, pure solvent.

Drying Transfer crystals to watch glass or weigh boat Protect from airborne contamination Air dry in a safe location

Procedure & Suggestions Make sure that you’ve read Chapters 1 and 2 (this was last week’s assignment on the lab schedule). Do the reading for this lab (pg. 93-101, & Sec. 2.17) The procedure for this lab is the Miniscale Procedure on pgs. 103-104 for Benzoic Acid only! The compounds that you need to have in your Table of Chemical Properties are: benzoic acid, water, and decolorizing carbon (aka “activated charcoal”) Suggested illustrations: Figure 2.49 (pg. 67) Figure 2.50 (pg. 67) Figure 2.52, b (pg. 68) – use a piece of folded filter paper, instead of a small stick Figure 2.54, Buchner funnel only (pg. 69)

Procedure & Suggestions, cont’d. Correction to procedure in book: Decolorizing carbon is already mixed in with the impure benzoic acid, so you will NOT be adding it. Suggestion: During hot filtration, add the dissolved benzoic acid in SMALL INCREMENTS to the filter paper in stemless funnel, whilst keeping the remainder of the solution HOT! (This decreases the chances of benzoic acid precipitating out of solution during hot gravity filtration.) YouTube video to give you an idea of how to perform a hot gravity filtration: http://www.youtube.com/watch?v=u-O2VDM9n5E (I would suggest using a piece of folded filter paper, instead of the ring stand.)

What data do I need to collect to complete the discussion? What is the goal? To recover as much of the product as possible in a more purified form. How much? Need to know the mass of dry benzoic acid so you can determine how much was recovered. Recovered mass (g) --------------------- * 100 = % Recovery Original mass (g)

What data do I need to collect to complete the discussion? What is the goal? To recover as much of the product as possible in a more purified form. How pure? Observe the appearance of the substance before and after crystallization. What should pure benzoic acid look like? Determine the melting point. This can not be done until the sample is dry. You will collect this data next lab period for discussion in this experiment.

Discussion of Results You will not be able to write up this section until AFTER NEXT WEEK’S LAB! Your purified benzoic acid needs time to dry, in order to get an accurate mass for the %-recovery calculation. You need to know the MP of your purified benzoic acid, in order to access how effective your recrystallization was. You CAN get a qualitative idea of how pure your benzoic acid is, at the end of lab today, by simply observing its color before and after recrystallization.