Recrystallization of Aspirin Flashcards
process in which an impure compound is dissolved in a solvent at or near its respective boiling point and slowly cooled to form crystals
recrystallization
Why do we heat the solvent before adding it to the solute?
Solutes are typically more soluble at higher temperatures, especially those with impurities; a much smaller quantity of hot solvent is needed in comparison to room temperature solvent
Why does the solvent matter?
the solute needs to be fairly insoluble in the solvent at room temp, but greatly more soluble at higher temps. At the same time, impurities need to be fairly insoluble at high temps so that they can be filtered out by vacuum filtration
What are the five major steps in recrystallization?
- dissolve solute in solvent
- perform vacuum filtration
- cool resulting solution to form crystals
- perform second vacuum filtration to retrieve crystals
- dry remaining crystals ; Find percent yield
Why do we crush the three aspirin tablets before adding the hot solvent?
increased surface area with result in a faster dissolve
Why do we weigh the crushed tablet
To keep track of how much product we began with
Why do we use a sandbath for heating instead of something else?
more even distribution of heat
Why do we add the hot ethanol dropwise?
To ensure the smallest amount of solvent possible is used to dissolve the aspirin
Why do we perform the first vacuum filtration?
during this step, pure aspirin will be in a liquid form and will collect at the bottom of the flask, while the more insoluble impurities are filtered out onto the filter paper.
Why do we put the resulting solution of ethanol and liquified aspirin into the freezer to cool?
At cold temperature, ethanol is a liquid and aspirin is a solid, so the aspirin will form crystals that may be collected.
What is the purpose of the second vacuum filtration?
To separate the pure aspiring crystals from the ethanol
Why do we let the crystals sit overnight?
to allow excess water and ethanol to evaporate
why do we wash the crystals with cold water?
The separation of ethanol and aspirin is the most pronounced at freezing temperatures, so to prevent from shocking the crystals and them turning back to the liquid state, we wash with ice cold water
Why does recrystallization result in a loss of product?
Filtering out impurities in the tablets causes a loss in mass in comparison to the mass we began with
How do you evaluate a compounds melting point?
- place a small amount of product into capillary tube and place in the slot of the melting point apparatus
- set the apparatus plateau temperature to around 10 degrees below the expected melting point of the solid
- start the machine and allow it to climb to the plateau temperature
- once the machine hits the plateau region, start the slow rise
- frequently check the temperature while watching the product through the viewfinder and record the temperature at which the product begins to gain transparency
- record the temperature at which the product is fully transparent and begins forming bubbles
- these two temperatures are your range
