Manual For Recrystallization Essay, Research Paper
The purification process:
Recrystallization & Filtration
Table of Contents
Chapter 1 Equipment involved and preparation
Glassware and accessories
Proper sterilization of equipment
Chapter 2 Setup procedure before purification
Choosing of solvent
Chapter 3 Gathering of an unknown compound and the first step: filtration
Repeat Filtration and Cleaning
Collection and measuring of crude sample
Chapter 4 Recrystallization process
Induction of Crystals
Chapter 5 Evaluation methods for purity
Small scale test
More accurate tests
Chapter 6 Frequently asked Questions and trouble shooting
To begin, the intent of this booklet is to give instructional advice. The instructions given will provide a basis to allow an individual to perform the necessary task to undergo the purification process. The design of the manual is to instruct students in a college laboratory setting. In particular, the area of science that the manual will focus on is Organic Chemistry. Students will be able to refer to this instruction guide throughout the semester because this process will be put in use often. The purification process if an essential part of chemistry, and the student must recognize its importance. Without a purified product, many of the drugs on the market would not be on sale. Even though these purification procedures are on a small scale, it will provide a starting point for those who are just beginning Organic Chemistry. The skills needed to purify a product will come with practice and help from the instructor, but this manual will give the initial directions to help the student on purification. Once mastered, more techniques will help the student to gain a product that is pure enough to put on the market.
The Equipment involved and preparation
The first step in understanding any laboratory process is understanding the equipment needed. Also, there is a need for the proper preparation of the tools needed before any of the steps can begin.
Glassware and accessories
The main pieces of glassware needed to conduct the filtration and recrystallization will be at least two 250ml flasks, a 100ml flask, and at least three 50 ml flasks. In addition, a glass stirring rod and a case of micropipettes would be useful. For the filtration procedure, a funnel with a rubber stopper, a trapper flask, and a small piece of filter paper is important to have. There is also a need for vacuum hoses in the filtration process as well. The student will also need a hotplate. The lab will provide all the necessary pipes needed to setup a vacuum and also the air current needed later on in the collection phase. Each lab will have different types of equipment, and it is the students responsibility to understand and learn how to work each tool.
For any experiment to have the proper results, cleaning and sterilization is critical part of preparation. The best way to have the glassware cleaned is to steam clean the many pieces in a autoclave or a dishwasher. This may be done in advance by the instructor or the students. It is also good practice to clean the equipment by hand, even after it has come out of a machine cleaner. The funnel and the filter paper must also be free of any impurities so that it will not reflect in the result. Because the purification process is so important, there has to be a high standard of cleanliness for all the tools that the student will use.
*It is extremely important that the student make sure all the pieces are dry as well or the weight of excess water will show up in the results. Thus exaggerating the final calculations*
Above all else, it is important to take all the proper methods to be safe in the laboratory. All Students must wear safety goggles at all time in the lab. It would also be a good idea to wear pants and shoes in case of spillage. Also, the student might also consider wearing gloves, but the instructor will notify the student if the chemicals require a student to wear gloves. It is also important that all safety related issues be reported to the instructor immediately.
WARNING: DO NOT TRY TO CLEAN UP CHEMICALS WITHOUT NOTIFYING THE INSTRUCTOR
Setup Procedure before purification
The setup for the process is simple, but choosing the right solvent to do the job is critical.
The setup of the glassware for the filtration procedure is as follows. First one hose must be firmly on the vacuum pipe on the water facet in the lab. The other end of the hose will go to a flask will connect to the top end of a trapper flask. ( The purpose of the trap is prevent water from the pipe to flow into the filtrate after the process is complete.) Next the on the side of the trapper flask is a small projection for another hose. The other end of this hose will go to the 250ml flask with the vacuum projection on the side. This is the flask where the filtrate will fall into. Place a funnel with a small piece of filter paper in on the top of the flask. Make sure a rubber stopper is on the top of the flask as well in order to have a proper seal. The only other setup required is for the air drying process that is done in the collection phase after recrystallization. A micropipette attached to a hose, which will provide a stream of air to allow the solvent to evaporate off of the crystals. The air stream will come from the pipes in ventilator hoods in the lab.
Choosing of Solvent
The most crucial part of any recrystallization is the need for the correct solvent. It is important because it must dissolve the crude sample while it is hot but not when it is cold. In addition, it must dissolve the impurities in the sample at both temperatures, so that they remain in solution. This process is usually a matter of trial and error. However, the instructor may have already chosen the solvent before hand. Students should refer to their textbooks and understand the concepts of polarity and solvation before they proceed to choose a solvent. *If problems arise ask the instructor for help.*
Gathering of an unknown compound and filtration
Without proper filtration, the rest of the purification steps will be pointless. It is crucial that the student take care when transporting the sample from each piece of equipment to another.
The collection of the initial sample will come after the student has gone through the desired experiment. Experiments that end in a final compound that is a mixture of solid and liquid will require the filtration process. The collection of the compound will be usually in a flask or another beaker after the chemical reactions have run their course.
Remember: it is important that the experiment is complete before the purification can begin or there would be nothing to purify.
Once the chemical reaction are complete and the experimental procedures are over, the filtration of the crude product can begin. Turn on the water facet and allow the it to produce a vacuum in the hoses and throughout the system. To test if there is a vacuum, the student should place their hand over the funnel and feel for suction. Now the student can slowly begin to pour the initial compound into the funnel very slowly. Pour the compound into the funnel in intervals or it will overflow. Allow for the filtrate to drain completely before pouring more of the sample. When that is complete, a liquid filtrate should accumulate in the flask and a solid sample should build up on the filter paper.
Repeat Filtration and cleaning
If the student recognizes that there is more of the solid left in the filtrate in the flask, he or she should repeat the filtration using the filtrate. The next step is to clean the compound on the filter paper. This can usually be done with cold water so that it does not dissolve the solid or it can be done with a chemical solvent that does not dissolve that particular chemical compound. Such as using a nonpolar solvent for a polar sample.
WARNING: OVER FILTRATION WILL CAUSE THE SAMPLE TO DISSOLVE. BE CAREFUL!
Collection and measuring of crude sample
After the filtration is over, the crude sample will collect onto the filter paper. The student must be careful in removing the sample from the paper and placing it into another flask. At this time the student should record the weight of the sample and make calculations on the percent yield of the final product as compared to the results of a perfect experiment. The student may also choose to run certain purity tests. At this point come of the impurities will be left in the sample and the next purification step will try to eliminate those as well.
The is a most crucial part of any Organic Chemistry experiment. It is also an amazing sight to watch these crystals form from a solution of just liquid.
The first step is to prepare the particular solvent by heating on the hot plate. Once the solvent has almost reached its boiling point, it is time to begin adding it to the sample compound. Place the sample in a small 50ml flask and add the hot solvent dropwise onto the solid. Dropping of the solvent must be done slowly because one drop will could cause the solid to dissolve, and an extra drop could cause the solid to not reform in crystalline form. Once again the student must take extreme care!
Induction of crystals
After the solvent completely dissolves the solid, recrystallization can proceed. Take a clean stirring rod and slowly scratch the inside of the flask containing the solution. Do not completely immerse the rod in the solution. The scratching of the flask will induce crystal formation to begin. Remove the rod and watch as more of the crystals form in the flask. Take the flask and place it in a larger beaker or flask that is filled with ice. This will help promote more crystal growth
Allow enough time for the crystals to grow in the ice bath before collection of the crystals. Now the student should use the air stream setup to evaporate the rest of the solvent. The micropipette should have a slow stream of air coming out of it. Place the pipette above the liquid and allow it to blow onto it until all the solution is completely gone. After this procedure, the student should once again begin to carefully remove all the solid crystals from the flask. Try to get as much as possible. Allow time for the sample to sit and dry as well. The student should record the weight of the purified sample and place it in safe place for further tests. This would be a perfect time for the student to perform more calculations on the purified sample.
Evaluation Methods for purity
Now it is time to find out if all the hard work has paid off. Hopefully, the recrystallization has removed most of the impurities found in the sample. If it has not, more purification method might be an option for the student to consider.
Small scale test
These test are of more generic method to find out what the sample is made up of. A melting point test can help determine the chemical formula of the product. Each chemical compound has a specific melting point, and a student can refer to these melting points in a reference manual provided in the lab. Also, the tool used for melting point tests are also available in the lab. Another small test is Thin Layer Chromatography. Also know as TLC, this test can help determine other chemical compounds that might exist in the sample. The process of TLC is available in the other series of instruction guides titled, Identification Tests.
More accurate tests
The student or instructor to conduct more complicated test to determine the chemical nature of the product. This can be done using Infrared Spectroscopy or Nuclear Magnetic Resonance. Both of these procedures require machines that give computer readouts on the nature of a sample compound. These tools provide exact measurements and formulas.
Frequently asked questions and troubleshooting
Q: How do I know when the reactions are done?
A: Follow the instructions, usually a color change or something will indicate that the reaction is over. Or ask the instructor
Q: Can I skip the Recrystallization process?
A: This process does not have to be done if the student has achieved a sample that is pure after filtration. However, this is highly unlikely.
Q: How do I know when the filtrate is completely clear of product?
A: This is a judgment call. The student should look to see if there could be more product left to filter out. Try to do the filtration at least twice.
Q: What if know crystals form?
A: Keep scratching the glass using the rod. Or try and place the flask in the ice bath earlier, that may induce crystal formation. If nothing happens the solution have been solvated too much to allow the crystals to reform.
IF ALL ELSE FAILS, ASK THE INSTRUCTOR FOR HELP!
Nuclear Magnetic Resonance
Thin layer chromatography