Crystallization; Decolorization; Identification of an Unknown using Mel-Temp

The purpose of this experiment was to develop new laboratory techniques and apply them by purifying and identifying an unknown.

Table 1: Reagents in the Experiment

Compound Ethanol Acetone Toluene
Other Designation Grain Alcohol Dimethylformaldehyde Methylbenzene
Compound Class Non-Polar Polar Covalent Polar Covalent
Lewis Structure
Boiling Point 78 degrees Celcius 56 degrees Celcius 110.6 degrees Celcius
Melting Point -114.1 degrees Celcius -95 degrees Celcius -95 degrees Celcius
Specific Gravity 0.79 g/cm3 0.79  g/cm3 0.86 g/cm3
Water Solubility Miscible Complete Insoluble
Flash Point 16.6 degrees Celcius -20 degrees Celcius 4 degrees Celcius
Hazards Ethanol is toxic and flammable; a severe eye irritant. Acetone is highly flammable and slightly toxic. Toluene is a highly flammable and toxic chemical.

 

ANALYSIS

Decolorizing Brown Sugar

Amount of Norit (mg) Time (s) Results
50 15 Light amber
250 15 Charcoal; cloudy brown
50 120 Light charcoal; amber overall
250 120 Charcoal; cloudy brown

The purpose of this activity was to remove color impurities from brown sugar. Norit, or activated charcoal has microscopic pores and cavities to which impurities are attracted. A couple molarities and boiling times were selected to find the best conditions for removing color impurities. The results suggest that using Norit in bulk is ineffective and that gravity filtration is not totally effective (see Table A below). The less molar solution (50 mg Norit) produced more favorable results.

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Table A: Decolorizing Brown Sugar with Norit Pellets (Activated Charcoal)

 

Crystallization of Unknown #12

The purpose of this experiment was to purify a sample of solid unknown by crystallization and to identify it by examining its melting point. Crystallization is a fundamental technique for removing impurities from organic compounds. In order to find a viable solvent for the crystallization, several solubility tests were conducted on unknown #12 (see Table B below). An ideal solvent for crystallization will dissolve the solute when heated or in room temperature. Also, it is important to find a solvent in which the solute will precipitate at room or freezing temperatures. The solute is expected to precipitate as a crystal solid, driving impurities out of the compound.

Table B: Solubility of Unknown #12 in Various Organic Solvents

Solvent Room Temperature 0 degrees Celcius Hot Bath
Acetone Soluble Soluble Soluble
95% Ethanol Soluble Soluble Soluble
Distilled Water Insoluble Insoluble Insoluble
Toluene Soluble Soluble Soluble
Ligroin Insoluble Insoluble Insoluble

 

Unfortunately, no solvents were ideal for the crystallization of unknown #12. In this experiment, it was necessary to use a solvent-pair for the crystallization. The solvents chosen were ethanol and water. Unknown #12 was highly soluble in ethanol (95%) and insoluble in distilled water during the solubility tests (see Table B). Also, water and ethanol appeared miscible when mixed, a necessity for using the compounds. This solvent-pair was an excellent candidate for crystallizing unknown #12.

Chemical Melting Point (Celcius) Boiling Point (Celcius)
Urea Trial 1 131.5-133.2
Urea Trial 2 131.7-133.6
Urea & Cinnamic Acid (4:1) 97.6-100.1
Pentane 33.5
Unknown #12 131.9-135.5

 

 

 

 

 

Table C: Melting Point Examinations

Unknown #12 (2.00 grams) was saturated in ethanol (95%; ~5mL) on a hot plate, allowing the heat to nearly reach the boiling point of the solvent (see Table of Reagents). The solute completely dissolved in the solvent. Next the solution was removed from the beaker and several drops of the solvent pair (water) were added, resulting in near instantaneous clumping and precipitation of the solute. The solution was heated a short time longer and the cloudiness receeded. Then, the solution was removed and more of the solvent pair was added (~2mL), giving the solution a permanent, thick white cloudiness. The beaker containing the solution was slowly cooled to sub-room temperature (~5 degrees Celcius). Moist crystals fully developed in the beaker.

The crystals were then vacuum filtrated and set out to dry. During the vacuum filtration, approximately four percent of the crystallized yield was lost in the mother liquor (discarded). Crystallized unknown #12 was transparent with a yellow tint, and smelled spicy. The melting point of the compound was determined using a Mel-Temp apparatus. Unknown #12’s melting range was observed from 131.9-135.5 degrees Celcius (see Table C). Unknown #12’s identity is cinnamic acid, which has an accepted melting point of 132-134 degrees Celcius. The experiment proved highly successful in purifying and indentifying the unknown.

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