Exp. #3: Preparation, Characterization and Potentiometric Titration of an Unknown Aromatic Carboxylic Acid Exp. #3-1MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Chemistry 5.311 Introductory Chemical Experimentation Experiment #31 PREPARATION, CHARACTERIZATION, AND POTENTIOMETRIC TITRATION OF AN UNKNOWN AROMATIC CARBOXYLIC ACID I. Purpose This experiment introduces basic manipulative procedures and techniques of preparative chemistry and quantitative volumetric analysis, and provides experience with pH meters. A sample of an alkali or ammonium salt of an unknown aromatic carboxylic acid is converted to the acid, isolated, purified and the yield and melting point determined. Carbonate-free sodium hydroxide solution, which is prepared and standardized against pure potassium acid phthalate, is used to determine the equivalent weight of the purified acid by titration. The apparent pKa of the unknown acid is determined by potentiometric titration with a glass electrode. Finally, a proton NMR spectrum of the acid is recorded and interpreted to verify the proposed structure. II. Safety You will be handling a number of chemicals during this experiment, some of which must be treated with care to avoid damage to yourself or your surroundings. None of these chemicals should be ingested or allowed to contact your skin or eyes. The chemicals are described in this section and denoted by an asterisk when they are used in the procedure. A more comprehensive description of each chemical is included in Appendix A. 1. NaOH 50% w/w: Concentrated NaOH is very caustic and should not be allowed to contact hands or clothing. If it gets on your skin, rinse immediately with plenty of water. Dilute NaOH is less hazardous. 2. Ether: Ether is a highly volatile, flammable liquid. It is used as an anesthetic and should not be inhaled. Prolonged breathing of vapors causes headaches. Ether should be used only in the hood to avoid exposure. 3. HCl: Concentrated HCl is a very acidic liquid. Contact with the skin will cause burns. If any gets on you or your clothing, rinse immediately with water. Dilute HCl (3N or less) is less hazardous. 4. Ethanol: Ethanol is a flammable liquid. The type used in this laboratory is not safe to drink, even if it were legal for you to do so (which it is not for a number of reasons!). 1 The second derivative and the Gran’s plot segment added by Mirce D. GheorghiuExp. #3: Preparation, Characterization and Potentiometric Titration of an Unknown Aromatic Carboxylic Acid Exp. #3-25. Melting Point Standards: the standard compounds used to calibrate your thermometer are organic chemicals varying in melting point. None are particularly hazardous, but they should handled with appropriate care. 6. Soda Lime: Soda lime is a mixture of calcium oxide and sodium hydroxide. The same precautions as apply to sodium hydroxide in solution apply here. These chemicals are caustic and prolonged skin contact will cause burns. If any gets on your skin, rinse well with water. 7. CaCl2: Calcium chloride is a hygroscopic salt, which is not particularly hazardous in small quantities. It is used on roads to prevent icing and is hazardous to cars over the long term. If any gets on your skin, rinse well with water. 8. Potassium Acid Phthalate: KHP is not particularly hazardous and should be handled with the usual precautions. LD50 orally in rats 8.0 g/kg 9. Phenolphthalein: This chemical is an organic dye. It should be handled with the usual precautions. Medicinally it is a potent laxative. III. Procedure for Preparation and Characterization The preparative directions will give satisfactory yields and purity with any of the compounds supplied. They are not necessarily optimal for any given compound. The analytical procedures will give precise and accurate results with any of the compounds supplied. Careful attention to detail is necessary to get good yields and accurate analysis. One of the keys to success in this course is efficient use of your time. For example it is valuable to recognize which operations may be conducted simultaneously. Reading assignments pertaining to both theoretical and technical aspects of the experiment are presented at the beginning of each section (TM refers to Techniques Manual; SWH, Skoog, D.A. West, D.M., and Holler, F.J., Fundamentals of Analytical Chemistry, 8th edition, Saunders: Fort Worth, 2004) A. Isolation and Purification of Acid 1. General References a. Reading Collecting, washing, and drying crystals TM 17:13-15 Procedure of extraction TM 16:11-13; Mohrig 8:56-71 Drying agents TM 10:42-43; Mohrig 8:72-76 Simple distillation TM 15: 6-9; Mohrig 11:109-118 Ground glass equipment TM 10: 7-10: Mohrig 2:16-23 Recrystallization TM 17:1-15; Mohrig 9:78-92 b. DemonstrationsExp. #3: Preparation, Characterization and Potentiometric Titration of an Unknown Aromatic Carboxylic Acid Exp. #3-3 Crystallization and top-loading balance Extraction c. Digital Laboratory Techniques Manual 2. Titration 5. Reaction Work-Up I 6. Reaction Work-Up II 7. Filtration 9. Recrystallization 11. Balances 12. Melting Point 14. DistillationExp. #3: Preparation, Characterization and Potentiometric Titration of an Unknown Aromatic Carboxylic Acid Exp. #3-42. Preparation of the Acid from its Salt. A 6 g sample of the salt of an unknown aromatic carboxylic acid is provided. Record the sample number and weight in the laboratory notebook. Transfer the salt to a 150-mL beaker containing 25 mL of distilled water. Rinse out the vial with 2-3 mL of water and add the rinse water to the beaker. Heat the mixture on a hot plate until a homogeneous solution is obtained. Filter the solution if any undissolved material remains. If additional water is required to effect solution, it may be added in 15-20 mL increments with continued heating. Slowly pour the solution into a 250-mL Erlenmeyer flask containing 5 mL of conc. (s.g. 1.19) hydrochloric acid diluted with 25 mL of ice water. (CAUTION! Avoid breathing the fumes of concentrated HCl. The dilution should be performed in the hood.) Swirl the flask continuously during the addition to ensure good mixing. Check the pH of the solution using pH paper from your desk. It should be pH 2. If not, add more acid. When the addition has been completed, let the Erlenmeyer flask