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1 EXPERIMENT 2 ACIDITY OF VINEGAR 07F ANALYSIS OF ACETIC ACID (VINEGAR) Although other organic acids and other organic compounds may be present in vinegar, the major acidic component is acetic acid, CH 3 COOH. Acetic acid and the other organic acids in vinegar are weak acids, and as such they are titrated with standard aqueous NaOH to an end point of pH > 7, generally with phenolphthalein as the indicator. This analysis is a standard one for quality control of vinegar samples. A similar procedure is used for the acidity of wine, either as the total acidity, including the non-volatile di- carboxylic acids, or the volatile acidity, which is primarily acetic acid with smaller amounts of higher molecular weight mono-carboxylic acids. Titration of rainwater samples with dilute standard solutions of NaOH is used to determine the acidity of rainwater. 1. STANDARDIZATION OF NaOH Standardize the ~0.1 F NaOH against potassium hydrogen phthalate, KHP or KC 8 H 4 O 4 H, with phenolphthalein as the indicator. KHP is a weak monoprotic acid and reacts quantitatively with OH - C 8 H 4 O 4 H - + OH - C 8 H 4 O 4 2- + H 2 O The burets have been used for other experiments and by other students. Although they may be clean and dry, burets should always be rinsed with a small amount of the titrating solution (titrant). Do NOT fill the buret with the ~ 0.1 F NaOH and discard. The rinse solutions may be discarded in the sink with a little added water. Weigh ~0.8 g (to ± 0.1 mg, analytical balance) of dried (100 - 110 o C) KHP (MW = 204.23 g/mol) into an Erlenmeyer flask and dissolve in 50 - 75 mL of distilled water. (Graduated cylinder OK) Use enough water that the stirring bar doesn’t splash the solution badly. Remember that you titrate an amount, not a concentration. Add 2-3 drops of phenolphthalein (thymol blue OK) indicator and titrate to the first permanent pink (blue) color. Near the end point, wash the sides of the flask with a little distilled water from a squirt bottle. Use the magnetic stirrer and stirring bar during the titration, but don’t let the stirring bar revolve so rapidly that a lot of the solution is splashed up the sides of the Erlenmeyer flask. Do not spin the stopcock during the titration. This procedure does not allow accurate control of the flow. Be sure that there is a continuous column of sodium hydroxide from the stopcock to the buret tip. That is, always check for, and eliminate, air bubbles in the tip of the buret. Do not waste time adjusting the initial reading to 0.00 mL. You can do the subtractions, quickly and easily. However, be sure to record the initial buret reading. Remember that you cannot estimate readings less than 0.00 mL or greater than 50.00 mL. Near the end point "half drops" may be added to allow the most accurate determination.
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This note was uploaded on 04/13/2008 for the course CHEM 119H taught by Professor Munson during the Fall '07 term at University of Delaware.

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