Module 6 Lab: Dilutions and Recrystallization
Lab 6 Chemistry
All of the below was generated from the net, to which I added my own personal points to. I have lost the majority of my web pages, along with my references as the paper I had that information on somehow became corrupt, and I was unable to retrieve this. However, while much of this is from other authors, the point is that I have learned a lot about these elements, labs, and chemistry as well as biology and wanted to give thanks to the people that I was unable to put into my reference section, and thanks to the many people that have helped me through out the class in chemistry. Thank you for your information, pictures, and help when I felt I was at the end of striving on. Because of all of you and your dedication to a person you do not know in real life, I hope to go on to being an excellent doctor some day. However it probably wont be in the medical field *laughs* God Bless you, and best to you and yours!
Use volumetric glassware when doing dilutions for chemistry or analytical methods. Serological pipettes and graduated cylinders are fine for microbiology dilutions used to reduce micro levels to a countable number.
1. Begin with a liquid stock solution. This may be a straight liquid sample or a solution made from a powder or liquid, diluted to a known volume.
2. Make a chemistry or analytical dilution by taking a volumetric amount of a solution, using a volumetric pipette, into a volumetric flask of the desired final volume. For example, a 1 to 100 dilution in chemistry requires the use of a 1.0 mL volumetric pipette and a 100 mL volumetric flask. The final volume of the dilution will be 100 mL (1 mL of stock solution plus 99 mL of diluent, the solution used for diluting).
3. Perform a microbiology dilution by taking a serological pipette and measuring a volume of the stock solution into a beaker. Then add the diluent using a graduated cylinder and mix in the beaker. A 1 to 100 dilution in microbiology requires the addition of 1 mL of stock solution to 100 mL of diluent for a final volume of 101 mL.
4. Use the proper diluent identified in the method for the dilution. Liquids such as media, buffer and water are common microbiology diluents. Chemistry methods will specify diluents like solvents, acids, bases and water.
5. Swirl the flask halfway through the dilution to mix. Then continue to add the remaining solution.
6. Use a dropper for adding the final amounts of diluent in small drops for an accurate final volume measurement.
7. Read the final volume by looking at the meniscus. View the meniscus by holding up the flask or beaker to eye level. The shape seen at the top of the liquid level that looks like a smile or upside-down umbrella is the meniscus. The bottom, not the sides that stretch up the sides of the glass, of the meniscus in the center should line up with the line drawn on the flask for an accurate measurement.
8. Add a magnetic stir bar to the final dilution and place on a stir plate to mix. Alternatively, stopper the flask and swirl, then hold the stopper on with the thumb and flip the flask upside down and back several times to mix.
9. Perform a serial dilution, which are a series of dilutions, when the final volume is a large value like 10,000 mL, for example. In this case, make a 1 mL to 100 mL dilution first and from that solution take another 1 mL into another 100 mL. The final solution is a 1 to 10,000 mL (100 mL x 100 mL) dilution.
10. Perform acid dilutions differently by adding a little water in the flask before adding the smaller volume of acid. Dilute to volume as normally required.
Recrystallization is a useful purification method for most organic compounds that are solids at room temperature. The selection of a proper solvent is the most critical part of the recrystallization procedure. Organic solids are usually more soluble in hot solvent than in a comparable volume of cold solvent. The process of recrystallization involves dissolution of the solid in an appropriate solvent at elevated temperature and the subsequent reformation of the crystals upon cooling. This way, many impurities will stay in solution and your target compound is purified. However, it is not possible to recover your entire compound following a recrystallization. To maximize your yield, it is very important to only use a minimum amount of hot solvent.
Choosing the appropriate solvent is the most challenging part of the recrystallization. In choosing a solvent, remember that “like dissolves like”, which means that a nonpolar compound will dissolve well in a nonpolar solvent, and a polar compound will dissolve well in a polar solvent. Also note whether a solvent is flammable or not and find its boiling point. This will enable you to select an appropriate heating device. Generally speaking, solvents with boiling points below 100°C should be heated in a water bath (NO OPEN FLAME), above 100°C can be heated directly on a hot plate or you may use a sand bath.
Selecting a solvent is crucial for recrystallization of an organic solvent. An ideal recrystallization solvent should:
• Dissolve the entire compound at the boiling point of the solvent.
• Dissolve very little or none of the compound when the solvent is at room temperature.
• Have different solubilities for the compound and the impurities
• Have a boiling point below the melting point of the compound so that the compound actually dissolves, not melts, in the hot solvent.
• Have a relatively low boiling point.
The best way to be certain that a solvent will be a good recrystallization solvent it to try and see it. Test the solvent on a small amount of the compound. If a solvent doesn’t dissolve the crystals when hot, or it readily dissolves the crystals at room temperature, try another solvent. Remember to always use only a minimum amount of hot solvent.
Once you have selected your solvent you are ready to recrystallize your sample. The process of recrystallization can be broken into the following steps:
1. Selection of an appropriate solvent or mixture of solvents (consult table)
2. Dissolution of the solute using a minimum amount of hot solvent
3. Decoloration of the solution if necessary (with an activated form of carbon)
4. Filter: Removal of suspended solids (through filtration of the hot solution)
5. Crystallization of the solid from the solution as it cools
6. Collecting the purified solid by filtration
7. Washing the crystals with an appropriate solvent to remove impurities
8. Drying the crystals
Following is a flow chart for the recrystallization procedure:
Describes the use of recrystallization as a method of purifying a solid. How does it work? What do you do? Be sure to discuss/describes, in detail, the technique of diluting a concentrated solution to a more dilute concentration using a volumetric flask.
(a) the process of making a supersaturated solution of acetyl salicylic acid.
Acetylsalicylic acid (aspirin) is poorly soluble in neutral water; it dissolves in alkaline water by virtue of changing chemically, and in any case, acidic or neutral water will cause this ester to decompose. If you want to make reasonably-sized crystals, you need to use a solvent other than water. Acetone is a suitable solvent - it does not cause decomposition, and the aspirin is highly-soluble in it.
? Warm some acetone (beware of fire);
? Add excess aspirin;
? Filter the solution;
? and let it cool slowly;
If the container is clean and smooth the liquid will not have crystallized and it will be supersaturated. If you need to trigger crystallization, scrape a glass stirring rod over the surface of the vessel or add a small crystal of the compound.
(b) the process of recrystallization.
In recrystallization, you dissolve the material in the minimum of solvent, then cool to form the crystals. You can not let too much solvent evaporate or the impurities will tend to coat the crystals. You want the crystals insoluble and the crud soluble. You pour off the impurities, then rinse the crystals in a minimum of cold solvent. For really good x-ray quality crystals, you would dissolve them again, and repeat the process until the crystals were very pure.
During the synthesis of Asprin, adding ethanol during the recrystallization process removes organic contaminants. It also evaporates easily, which cools the solution.
How to Perform a Recrystallization:
Recrystallization is a laboratory technique used to purify solids based on their different solubilities. A small amount of solvent is added to a flask containing an impure solid. The contents of the flask are heated until the solid dissolves. Next, the solution is cooled. A more pure solid precipitates, leaving impurities dissolved in the solvent. Vacuum filtration is used to isolate the crystals. The waste solution is discarded.
Summary of Recrystallization Steps:
1. Add a small quantity of appropriate solvent to an impure solid.
2. Apply heat to dissolve the solid.
3. Cool the solution to crystallize the product.
3. Use vacuum fitration to isolate and dry the purified solid.
Choose a solvent such that the impure compound has poor solubility at low temperatures, yet is completely soluble at higher temperatures. The point is to fully dissolve the impure substance when it is heated, yet have it crash out of solution upon cooling. Add as small a quantity as possible to fully dissolve the sample. It's better to add too little solvent than too much. More solvent can be added during the heating process, if necessary.
After the solvent has been added to the impure solid, heat the suspension to fully dissolve the sample. Usually a hot water bath or steam bath is used, since these are gentle, controlled heat sources. A hot plate or gas burner is used in some situations.
One the sample is dissolved, the solution is cooled to force crystallization of the desired compound...
Slower cooling may lead to a higher purity product, so it's common practice to allow the solution to cool to room temperature before setting the flask in an ice bath or refrigerator.
Crystals usually begin forming on the bottom of the flask. It's possible to aid crystallization by scratching the flask with a glass rod at the air-solvent junction (assuming you are willing to purposely scratch your glassware). The scratch increases the glass surface area, providing a roughened surface on which the solid can crystallize. Another technique is to 'seed' the solution by adding a small crystal of the desired pure solid to the cooled solution. Be sure the solution is cool, or else the crystal could dissolve. If no crystals fall out of solution, it's possible too much solvent was used. Allow some of the solvent to evaporate. If crystals do not spontaneously form, reheat/cool the solution.
One crystals have formed, it's time to separate them from the solution.
Crystals of purified solid are isolated by filtration. This is usually done with vacuum filtration, sometimes washing the purified solid with chilled solvent. If you wash the product, be sure the solvent is cold, or else you run the risk of dissolving some of the sample.
The product may now be dried. Aspiring the product via vacuum filtration should remove much of the solvent. Open-air drying may be used as well. In some cases, the recrystallization may be repeated to further purify the sample.
Definitions:
(a)Saturated solution:
Definition #1: as in a saturated compound. A substance in which the atoms are linked by single bonds. A fully saturated compound contains no double or triple bonds. Example: ethane (C2H6)
Definition #2: as in a saturated solution. In this context, saturated refers to a point of maximum concentration, in which no more solute may be dissolved in a solvent.
(b) saturation point:
saturation point
1. Chemistry The point at which a substance will receive no more of another substance in solution.
2. The point at which no more can be absorbed or assimilated.
1. the point at which no more (people, things, ideas, etc.) can be absorbed, accommodated, used, etc.
2. (Chemistry) Chem the point at which no more solute can be dissolved in a solution or gaseous material absorbed in a vapour.
The point at which a substance, under given conditions, can receive no more of another substance in solution.
(c) supersaturated solution:
1. To cause (a chemical solution) to be more highly concentrated than is normally possible under given conditions of temperature and pressure.
2. To cause (a vapor) to exceed the normal saturation vapor pressure at a given temperature.
You had asked use to give an example of how we use this every day in our lives. I would say that we use them when we add any kind of substance to our coffee. For instance adding sugar to coffee or tea could be one example. As for filtration, I cant think of any that we use regularly however one may be through a coffee filter to filter the water and get the coffee solution to drink. In the order of decaffeinated coffee there is a process that removes the caffeine. According to my reading of the subject, “The extraction process is simple. Supercritical carbon dioxide is forced through green coffee beans. Its gaslike behavior allows it to penetrate deep into the beans, and it dissolves 97-99% of the caffeine present.
Coffee manufacturers recover the caffeine and resell it for use in soft drinks and medicines. The caffeine-laden CO2 is sprayed with high pressure water and caffeine is then isolated by a variety of methods, including charcoal adsorption, distillation, recrystallization, or reverse osmosis”(Basic lab, 2010).
References:
(Some I was able to find, and others were newly added)To all that were lost in the paper, know that I thank you for all of your inputs.
Mz24k aka Shannon Marie
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
Joesten M and Wood, J (2007) World of Chemistry Essentials, 4th ed.: Thomson: Brooks/ Cole,
2007.
Senese, F., (2010) How Is Coffee Decaffeinated? Retrieved from:
http://antoine.frostburg.edu/chem/senese/101/consumer/faq/decaffeinating-coffee.shtml Thinkwell (2001) Lectures, Labs, Notes
Thinkwell (2001) Unit: Lab Instructional Demonstrations Module: Lab Demos
UCDAVIS Department of Biological and Agricultural Engineering, Ecological Engineering (2011)
http://bae.engineering.ucdavis.edu/pages/Research/eco.html Videos online
www.thinkwell.com .
Visited sites:
http://www.basiclab.com/?r=aw http://www.teoma.com/ans/advance-chemistry http://www.alcoa.com/howmet/en/service_category.asp?cat_id=13 http://www.richmanchemical.com/services/toll-manufacturing.html?gclid=CPrO3ZyvuasCFWYbQgod3F93dw Anne Marie Helmenstine, Ph.D How to Perform a Recrystallization found at
http://chemistry.about.com/od/chemistryhowtoguide/ss/recrystallize_4.htm and Saturated Definition: Chemistry Glossary Definitions
Online Chemistry HelpView Step-By-Step Solutions Online For Your Actual Textbook Problems!www.Cramster.com
http://www.thefreedictionary.com/
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