Lab+4+Sublimation


 * Lab 4 Sublimation: **


 * Intro: **

Sublimation is a process used commercially to dehydrate food. In an organic chemistry lab, sublimation is used for the purification of solid substances. It occurs when a solid goes through a direct transition to a gaseous state. By skipping the transition of the solid melting to liquid, a large amount of substance can be recovered in condensing crystals. The heated substance evaporates and condenses on the cooled surface above. Impurities are left behind with pure crystals forming on the cold surface. In a microscale experiment, this proves to be a very successful technique. The forming crystals,however, become too heavy and fall back to the bottom of the flask. This will minimize recovery in experiments exceeding 100mg. After the vacuum sublimation of an unknown substance, Acetanilide was recovered and identified through determination of melting point.

please cite the source of this lovely figure.
 * Procedure: **

Experiments used: Sublimation of an unknown Ch. 7, p.123

Williamson K.L., 2003. Macroscale and Microscale Organic Experiments 4th Edition. Boston (MA): Houghton Mifflin Company, p.122-125 Anything that happened (past tense) during your time in lab should NOT be in the procedure, but should instead be DATA. For instance, the mass of unknown you record below is an experimental measurement and does not belong in this section. Initially, 0.051g of an unknown substance was weighed out and placed into a 25mL flask. The walls of the flask were covered with a tin foil wrap in order to contain and distribute heat. Additionally, the openings at the top and sides of the vessel were contained to create a closed system. Also, a centrifuged tube with ice water referred to as a cold finger, was suspended in the center of the flask 5-8mm above the base. The entire system was placed on a hot sand bath for sublimation.

During the heating procedure, cautious inspection of the system as sublimation began contributed to the accumulation of crystals at the tip of the finger. Melted ice cubes were pippetted out and replaced one time during the procedure.

Once sublimation completed, the 25mL flask was removed from the hot sand bath and ice water was again pippetted out from the cold finger. Cautious removal of the finger was necessary to prevent condensation forming on the recovery. The crystals were placed on a filter paper and weighed to determine the percentage of recovery

A capillary tube was used in concordance with a Mel Temp apparatus to determine the melting point of this particular unknown. Crystals from sublimation were placed inside the apparatus as the temperature of the substance was recorded.

Thanks for including this structure.
 * Data: **

Weight of the unknown prior to being placed in the25mL flask was 0.051g. Upon initiating experimentation, the entire 25 mL flask was covered with tin foil in order to disperse the heat evenly in the vessel. Crushed ice was added to the cold finger and placed in the center of the flask. The drop in pressure through the sublimation apparatus caused crystallization around the base and peak of the cold finger.

When placed in the sand bath, the tin foil covering enhanced the time period of sublimation. The tin foil was also used in preventing crystallization on the beaker. Heated flask walls ensured the formation of crystals on the cold finger. Upon the presence of vapors in the process, a moderate amount of crystallization began to appear on the side walls of the flask. In order increase observed trends, fresh ice cubes were added to replace the melted ice. After making the switch, crystallization began to accumulate distinctly at the base of the finger.

The cold finger was removed from the 25mL flask and excess ice water extracted by a pipette. Crystals appeared as needle projections both sharp and pointy. A piece of filter paper and flask was weighed before and after adding the crystals. Percentage of recovery was calculated from the following: Percent purity?? Doto the low percentage of product recovered an analysis of possible sources of error will be included in the Analysis/Discussion section below.


 * Analysis/Discussion: **

After the sublimation process of the unknown compound, the product is that of a purified substance. The process by which sublimation works is a volatile substance condenses the technical term is "deposits" onto a cold finger apparatus and leaves behind a purified substance that can be identified by testing the melting point. The melting point of a substance is one of the physical properties which can identify an element or substance. It is important to purify a substance prior to identification through testing its melting point. Impurities will reduce the temperature at which a substance melts. When the melting temperature of the unknown was tested during the lab procedure then compared to a list of sublimation unknowns, it was discovered that the identity of the sublimated product was Acetanilide. Acetanilide has a melting point ranging from 113-115 degrees Celsius which matched the melting point of the sublimated unknown.

It should be noted that sublimation, though effective in separating a pure substance from a contaminated substance, is not as reliable as distillation or recrystallization. Heat is applied to transition the solid substance into a gaseous state via evaporation. After reviewing the phase diagram it was noted that in conjunction with raising the temperature of the system, if possible the pressure must be kept close to constant. Unfortunately the properties of a closed system, when heat is applied, result in an increase in pressure. The pipet bulb you put on the side of your filter flask should have expanded to keep the pressure more or less constant....though you are correct that it probably rose a bit. This will shift the physical state of certain chemical compounds into a liquid instead of shifting directly from a solid into a gas. Therefore, when choosing to process a chemical compound for sublimation the specifics of that chemical compound will determine whether true sublimation is possible. This is primariy because every chemical compound has a unique phase diagram associated with it. Please comment on how the narrowness of the range of melting temperatures indicated purity (or lack of purity). Why the low yield?


 * Post Lab Question: **

Q: What is Lyophilization?

A: Lyophilization is accomplished by a purification technique called sublimation. Sublimation is an endothermic transition from the solid to a liquid solid to gas phase at temperatures and pressures below that substances triple point. The triple point on a phase diagram represents the point at which a substance’s three phases; solid, liquid and gas; exists together in equilibrium. This triple point can exist in pure substances as well as mixed compounds. The purpose of Lyophilization is to recover the solid that remains after the solvent is removed during the process of sublimation. The process of Lyophilization requires that the solution of which is to be sublimated is frozen prior to the process. During sublimation the water collects onto the very cold surface of a condenser which is used to remove the heat from fluids. The condenser would be similar to that of the “cold finger” used in the micro scale process of sublimation. The sample that forms on the condenser, in which is now free of water, is the final product of the Lyophilization process.

Q: Why is it done?

A: Lyophilization is more universally known as Freeze drying and is used on macro scale to dehydrate (the removal of an aqueous solution) substances such as proteins, vitamins, microorganisms, vaccines, pharmaceuticals or food to make the product more stable for storage or distribution. Once the final product is extracted it is sealed airtight to prevent the re absorption of moisture back into the solute. Without moisture, products can be stored at room temperature for long periods of time. This is possible due to the fact that microorganisms require moisture to cultivate and therefore are unable to spoil the substance that has been dried. This report has earned the following scores for: format (2/2) style (1/2) data (3/3) quality of the result (1/1) quality of reported data (1/1) conclusion (1/2) error (0.5/1) post-lab question (1/2) for a total score of 10.5.