Learning Objectives: Acid + Base BOOM- Teacher Guide Obtain knowledge on common environmental cleanup problems Obtain a general knowledge of ph and solubility Perform a ph test and analysis Perform a wet chemical test and analysis Work with complex instrumentation to identify an unknown Background: This experiment demonstrates an environmental problem where a food supplement factory has been shut down and some of their products are in unlabeled tubes that are scattered throughout the plant. The company makes both acids and bases and an environmental crew must come in to clean up the mess. However, the chemicals they make look the same, a solid white powder. They need to do a test on the products because acids and bases cannot be disposed of together, since they can react violently with each other. In this experiment, the identity of the acids will be determined through three steps: ph test with litmus paper, wet chemical test with sodium bicarbonate, and advanced technological analysis with infrared spectrometer. An acid is a solution that contains an excess of hydrogen ions and a base is a solution that contains an excess of hydroxide ions. PH measures the acidity and basicity of a solution from the concentration of hydrogen and hydroxide ions. On the ph scale, an acid is less than 7, a base is more than 7, and 7 is neutral. In this experiment, carboxylic acids will be the only type of acids used. These acids contain a COOH functional group which is important for the instrumental analysis. For this experiment, the students will be testing the ph by using litmus paper. There are two types of litmus paper: red and blue. When used together the litmus paper can tell the ph range of the solution. If the blue litmus paper turns red and the red litmus paper stays red, then the solution is acidic. If the blue litmus paper stays blue and the red litmus paper turns blue, then the solution is basic. If the blue litmus paper stays blue and the red litmus paper stays red, then the solution is neutral. This is the step where the students will be able to eliminate the non-acidic compounds from the analysis. The next experiment is a reaction of sodium bicarbonate and carboxylic acids. Since all of our carboxylic acids are soluble in water, adding a saturated sodium bicarbonate solution will cause fizzing and bubbling because the reaction creates a byproduct of carbon dioxide gas and water. RCOOH + HCO 3 - RCOO - + H 2O + CO 2 This is the step in which the students will confirm that their acid is, in fact, a carboxylic acid and they can now figure out which carboxylic acid it is exactly with the IR.
The infrared spectrometer (IR) is the advanced instrumentation that will be used in this experiment. The instrument sends infrared light through a compound causing the molecule s bonds to vibrate, bend, and stretch. The IR identifies functional groups in organic compounds through the specific bond vibrations that are visible in the peaks of the spectrum. In this experiment, the focus is on the -COOH functional group. There are many different bonds that can be focused on in this functional group, but the most prominent will be the carbon double bonded to the oxygen bond. This peak in the spectra should be in the 1700 cm -1 range. Each compound will have at least one of these peaks; however each carboxylic acid has a unique spectrum, which is how the students will be able to identify their unknowns. Unknowns: Station 1: 1-1 Sodium carbonate 1-2 Malonic acid Station 2: 2-1 Tartaric acid 2-2 Mannitol Station 3: 3-1 Sodium bicarbonate 3-2 Citric acid Station 4: 4-1 Sucrose 4-2 Ascorbic acid Station 5: 5-1 Malic acid
5-2 Sodium chloride Station 6: 6-1 Maleic acid 6-2 Sodium carbonate Experimental Procedure: Litmus Paper Test for Acids 1. You should have two vials in front of you. The first number is your station number and the second number is the number of your unknown chemical. Label 2 of your test tubes with these numbers as written on the vials. 2. Use the line on your spatula to transfer a small amount of each of your unknown samples into the labeled test tubes. Clean spatula with tissue between samples. 3. Pipet the deionized water into each test tube until it is about 1/5 of the way full.(about a fingernail full) 4. Put your finger on the top of the test tube and vortex until all the solid has dissolved. 5. Take out one red and one blue litmus paper 6. Dip your glass rod into one of the test tubes and then rub it on both the red and blue litmus papers. (You may have to do this a few times.) 7. See chart to determine if you have an acid or a base. Red paper Blue Paper acid/ base/ neutral Red red Acid Blue Blue Base Red Blue Neutral 8. Mark your results on your worksheet. 9. Clean off glass rod with isopropanol over the waste bin and then wipe off with a tissue. 10. Repeat steps 6-9 with second sample. Sodium Bicarbonate Test for Carboxylic Acids 1. Using your spatula, add a small amount of your acid to the third test tube. Add a drop of the saturated sodium bicarbonate (NaHCO 3) solution into this test tube. 2. If the solution begins to fizz and bubble then this is a positive test for a carboxylic acid. We now want to figure out which carboxylic acid it is. 3. If the solution does not react with the sodium bicarbonate, then we do not know if it is a carboxylic acid or not. We need to do further analysis of it as well. 4. Mark your results on your worksheet. A positive will be if the solution fizzed and a negative will be if the solution did not react. Instrumental Analysis with Infrared Spectrometer 1. Obtain a small amount of your unknown acid powder by using the line on your spatula and transfer it to the agate mortar.
2. Grind the sample to a very fine powder using the agate pestle. 3. Add 3 drops of Nujol (mineral oil) and grind to disperse the sample (forms Nujol mull). 4. Place one salt plate flat on the bench top. 5. Place one drop of Nujol on one face of the salt plate with your small pipette. 6. Place a second salt plate over the first to spread the Nujol. 7. Carefully place the salt plate sandwich into the sample holder. 8. Place the cover over the plates and insert the holder into the IR spectrometer. 9. Record the Nujol spectrum. a. In order to run the IR sample, go to Instrument then Scan Sample b. A Scan Sample window will pop up. Label the file name of the number on the vial and test tube of your unknown. Label the description as the name of one person in your group. Then click OK. c. A spectrum such as this should be the result. 10. Now we want to compare the result with the library. a. Go to Process and then Compare. b. Two new windows will pop up comparing the result spectrum to the spectra that are preset in the library. 11. Click on the window labeled Compare. Under the Correlation column is a decimal number that shows how close the unknown spectrum is to the ones in the library. The closer the decimal is to one, the greater the correlation is to the unknown spectrum. Thus, the top spectrum in the compare window reveals the identity of the unknown. 12. Disassemble the holder and rinse with isopropanol. Wipe both plates with a tissue. 13. On your worksheet, write down your unknown chemical. Instructor Notes: Unknown Acids:
For more information on IR can be found: http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/infrared/in frared.htm