What personal protective equipment is required for a recrystallization lab?

Students must wear a lab coat, chemical splash goggles, and gloves during recrystallization experiments. These items protect against chemical exposure and potential splashes from organic solvents and hot solutions. Ensure all equipment is in good condition before beginning work to maintain a safe laboratory environment.

How do you prepare a 95% ethanol solution for recrystallization?

Measure 38 mL of absolute ethanol and pour it into a 50-mL glass bottle using a funnel. Add 2 mL of deionized water to create the 95% ethanol solution. Mix thoroughly with a glass rod or by shaking the closed bottle until the solution appears homogeneous, then label and store in a fume hood.

What glassware and equipment should be set up at each student lab station?

Each station requires a lab stand, stirring hotplate, two 600-mL beakers, one 250-mL beaker, Erlenmeyer flasks, graduated cylinders, glass stirring rods, Büchner funnels, a filter flask, and a vacuum pump. Additionally, include flask tongs, tweezers, pipette bulbs, and a medium stir bar. This setup enables students to perform heating, cooling, and vacuum filtration steps efficiently.

Why are high-purity crystals set aside separately in recrystallization preparation?

High-purity acetanilide and trans-cinnamic acid are reserved as seed crystals to initiate crystallization during the experiment. Seed crystals provide nucleation sites that promote controlled crystal growth. Reagent-grade compounds are used for the bulk of the experiment, while pure seeds ensure quality crystal formation.

What waste containers and materials should be prepared before the lab begins?

Prepare a glass waste container and separate containers for solid acetanilide and trans-cinnamic acid waste. Confirm these containers are available and properly labeled before students arrive. Proper waste segregation prevents contamination and ensures safe disposal of chemical materials used during recrystallization.

How should the laboratory be organized for efficient student workflow?

Place boxes of laboratory wipes, medium filter papers, and Pasteur pipettes in a central area for shared access. Set up the melting point analyzer with capillaries nearby for analysis. Fill an insulated cooler with crushed ice and position it on the central bench. This organization minimizes congestion and allows students to access common supplies quickly.

What backup glassware and equipment should be available during the recrystallization lab?

Keep extra 125-mL Erlenmeyer flasks, stemless or wide-stem glass funnels, large filter papers, small beakers, and boiling chips available. These backups accommodate students who need to perform hot filtration or encounter breakage. Having contingency supplies ensures the lab can continue smoothly without interruption.

Recrystallization

Concepts Instructor Prep Student Protocol

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Most products contain impurities. One way to purify these products is by recrystallization. Recrystallization begins with dissolving the impure product in just enough hot solvent to form a saturated solution, where as much solute is dissolved in the solvent as possible. Any additional solute will not seem to dissolve.

A hot solvent is used because solubility typically increases with temperature. As temperature increases, the amount of solute that can be dissolved in the solvent increases. As the solution cools, the solubility of the product decreases, and solute molecules come together to form small stable crystals called nuclei.

This is the first step of crystallization, called nucleation. Additional crystal growth happens on the nuclei because solute molecules have a greater affinity for joining existing solute crystals than forming new crystals. Soluble impurities are left in solution.

Crystallization can happen spontaneously or can be encouraged by scratching the inside of the flask, agitating the solution, or adding a seed crystal of the compound, all of which provide a surface for further growth.

Recrystallization into large, pure, regularly shaped crystals only works when an appropriate solvent is used. The compound should be insoluble in the solvent at room temperature and soluble at high temperatures. Ideally, the impurities should be either insoluble in the solvent at high temperature or soluble in the solvent at room temperature.

If the impurities are insoluble in hot solvent, they are filtered out before crystallization. After recrystallization, the crystals are filtered out and washed with cold solvent to remove impurities from the surfaces. Now, the purity of the crystals can be analyzed.

When choosing a solvent, keep in mind that the greater the difference in solubility between high temperatures and low temperatures, the more likely the solute will come out of solution as it cools to form crystals. The rate of cooling is also important in recrystallization.

Rapid cooling favors the formation of many nucleation sites and the growth of smaller crystals, whereas cooling slowly induces the formation of fewer nucleation sites and the growth of fewer, larger but purer crystals. So, slower cooling is preferred.

In this lab, you will recrystallize two impure organic compounds, acetanilide and trans-cinnamic acid, and then assess the purity of the recovered compounds by comparing their melting point range to values in the literature.

At the end of this lab, students should know...

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What is recrystallization used for?

Recrystallization is a method to purify solid compounds based on the differences in solubility in a solvent.

What are the steps of recrystallization?

1-Choosing the solvent or solvent pairs, 2-dissolving the sample in the minimum volume of the hot solvent, 3-cooling the saturated solution, 4-collecting and washing the crystals, and 5-drying.

What is an ideal solvent with respect to the solute?

An ideal solvent has the following characteristics: it completely dissolves the solute only at high temperatures (near the boiling point of the solvent); it does not react with the solute; it has approximately the same polarity as the solute; it precipitates the solute at low temperatures (near room temperature or lower); and it only dissolves impurities at low temperatures.

When is a solvent-pair used in recrystallization?

A solvent-pair is used when a single solvent with ideal properties cannot be found.

What are the two phases of crystallization?

The two phases of crystallization are nucleation and crystal growth. During nucleation, solute molecules come together to form small stable crystals called nuclei. Then, additional crystal growth occurs on the nuclei, increasing the size of the crystals. This is because solute molecules have a higher affinity for joining existing solute crystals than forming new crystals.