Disease Case

Fifty milligrams of an impure unknown solid (slightly yellowed white solid) was

placed into a 25-mL Erlenmeyer filter flask. (Note: the solid smells like moth

balls).The side arm of the flask was closed off with a pipette bulb. Ice water

was placed in the centrifuge tube. The system was carefully heated on a sand

bath until sublimation began. Sublimation was noted by observing small needles

condensing onto the cooled centrifuge tube. At times, small needles were seen

on the sides of the upper part of the flask. In order to ensure condensation on

the centrifuge (and not on the sides of the flask), a loose fitting of aluminum

foil was placed on the sides of the flask to cause the crystals to sublime from

the sides of the flask and condense on the centrifuge tube.

The temperature was maintained, and sublimation was complete when there was

a small amount (~10-12 mg) left on the bottom of the filter flask. The centrifuge

tube was carefully lifted out of the filter flask, and the white crystals were

carefully scraped off the centrifuge tube and weighed. Note: there was a

small amount of product loss during the collection/scraping of the crystals.

Thirty-two milligrams of white solid compound were collected, and its

melting point was determined to be 79-81oC.Final Reports: Results/Discussion Example

Based on the experimental melting point data, the unknown white solid has

been identified as naphthalene, which has a melting point of 82oC.

The percent recovery is calculated below:

% recovery = 32 mg/50 mg X 100 = 64%

Sublimation has proved to be an effective method for purifying a small amount

of impure solid. The experimental melting point, 79-81oC, is extremely close to

the theoretical melting point of naphthalene. Also, naphthalene is reported to

appear as pure white crystals, which is what was observed after sublimation,

and is the main component of moth balls. The percent recovery, 64%, although

not excellent, is fair; it would have been higher if loss of product did not