Experiment 1
Purification of an organic solid by recrystallization
Objective: To purify a sample of benzoic acid contaminated with sand by recrystallization from water
Crystalline state of matter is a highly organized state of matter. To form crystals, the molecules of the substance must arrange themselves in an orderly manner. Any impurity present in the sample of the substance will prevent the orderly organization of molecules of the substance. Thus, a sample obtained in a crystalline form is likely to be a pure substance. A sample of an impure solid substance can be purified by recrystallization.
Recrystallization Method:
Identify the impurity in the sample and the approximate degree of contamination
Identify a solvent suitable for purifying the impure sample of benzoic acid by crystallization. [An ideal solvent for recrystallization for a substance is a solvent that will (1) partially dissolve the substance at room temperature, and (2) completely dissolve the substance while hot . In addition, the hot solution of the substance in the solvent, when cooled will allow the substance to form crystalline solid.]
Separate the impurity by mixing the impure sample with minimum amount of the hot solvent and filtering off the insoluble impurity by gravity filtration.
Allow the filtrate to cool slowly to room temperature or below, for the pure substance to crystallize.
Isolate the recrystallized benzoic acid by vacuum filtration, dry the residue and determine the amount of sample recovered. Calculate the % recovery of benzoic acid
Determine the purity of the recrystallized benzoic acid by determining its melting point and comparing this melting point to the value reported in literature.
Procedure:
Weigh out approximately 200.0 mg of the impure benzoic acid sample to be purified into a 50mL Erlenmeyer flask (A). In another Erlenmeyer flask(B) containing a boiling chip, heat the solvent of crystallization (distilled water) on a hot plate. Heat the solvent to boiling. With a pasture pipet (plastic), add a small portion of the hot solvent into the solid in the flask (A). Heat flask A on the hot plate. Add hot solvent in small amounts to flask A and swirl to dissolve as much of the solid as possible. Watch carefully to make sure that only the impurity is left undissolved. Filter the hot solution by gravity filtration into another small Erlenmeyer flask(C) using a fluted filter paper. Wash the residue of the impurity with minimum amount of hot solvent. Allow the filtrate to cool, undisturbed, to form crystals of the pure compound. After the flask, has cooled to room temperature, place it in an ice bath to complete the crystallization (10 minutes). Filter the crystals by vacuum filtration. Wash the residue with ice-cold water and dry the residue between wads of filter paper. Weigh the dried crystals and determine its melting point.
PreLab Substance Information:
Impure sample information: Benzoic acid sample contaminated (20%) with sand
Information about benzoic acid – Molecular formula, structure, melting point
Information about sand – Formula
Solvent of recrystallization, its formula and boiling point
Solubility of benzoic acid in the chosen solvent of crystallization (g/100 mL) at Room Temperature(RT) and at 700C (use Merck Index for the higher temperature data)
Solubility of the impurity in the solvent of crystallization (g/100 mL) at RT and at a higher temperature (Guess based on your observations!)
Compare solubility of benzoic acid and the impurity in the solvent of choice (cold and hot) and determine if the chosen solvent would be a good solvent for recrystallization of benzoic acid
Safety Precautions:
Use eye protection
Use caution while using ALL chemicals
Carry out the procedure in the fume hood
Do not heat the flask with the solid before adding solvent
Use caution while filtering the hot solution of the substance – flask will be hot – use a paper wrap around the neck of the flask to hold the flak.
Use caution while using hot plate – Ceramic hot plates heat quickly and do not “appear’ hot. Do not turn on the hot plate without the container on it. Start heating at lower setting and increase incrementally – DO NOT start on the highest setting first!
Data Analysis Guidelines:
Calculate the % recovery
Compare the melting point determined for the purified benzoic acid to that of the literature value.
Determine the efficiency of this method to purify the impure sample based on the % recovery of benzoic acid after recrystallization (30% recovery is considered good recovery) and its purity (a pure sample will have a melting point range within one degree of the reported melting point for the authentic sample).
