06MANtitrationcurves - Titration Curves Revised TITRATION...

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Titration Curves Revised 5/18/10 1 TITRATION CURVES, INDICATORS, AND ACID DISSOCIATION CONSTANTS Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 OBJECTIVES For you, the student, to write…. SAFETY Wear safety goggles and lab aprons at all times in lab. Acids and bases are caustic solutions and should be handled with care. If spills occur, wash affected areas immediately and inform your TA or course instructor. INTRODUCTION Titration Curves One objective of this lab is to observe differences in shapes of titration curves when various strengths of acids and bases are combined. In this experiment you will react the following combinations of strong and weak acids and bases (all solutions are 0.10 M). Trial #1: Hydrochloric acid, HCl, with sodium hydroxide, NaOH. Trial #2: Acetic acid, HC 2 H 3 O 2 , with sodium hydroxide, NaOH. Trial #3: Hydrochloric acid, HCl, with ammonium hydroxide, NH 4 OH. Trial #4: Acetic acid, HC 2 H 3 O 2 , with ammonium hydroxide, NH 4 OH. (Note: Ammonia (NH 3 ) reacts with water to form NH 4 OH.) A computer-interfaced pH electrode will be placed in one of the acid solutions and a solution of one of the bases will slowly drip from a buret into the acid solution at a constant rate. As base is added to acid, a gradual change in pH will occur until the solution gets close to the equivalence point. Near the
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Titration Curves Revised 5/18/10 2 equivalence point, a rapid change in pH occurs. At the equivalence point, equal numbers of moles of acid and base have been added and the pH will reflect which species are present. Beyond the equivalence point, where more base has been added than acid, you should again observe more gradual changes in pH. A titration curve is normally a plot of pH versus volume of titrant. In this experiment, however, we will monitor and plot pH versus time, and assume that time is proportional to volume of base. The volume being delivered by the buret per unit time should be nearly constant. Indicators Indicators are weak organic acids (HIn) that change color when deprotonated (In - ). A few drops of indicator added to the analyte solution before the beginning an acid-base titration. When enough base titrant is added to the analyte solution the equilibrium expressed in equation 1 will shift toward products. (1) HIn(aq) In (aq) + H + (aq) The result is the formation of more of the deprotonated indicator (In ) and a corresponding color change of the analyte solution (the endpoint). An good indicator for a specific acid-base titration has an endpoint with a pH at or near the pH of the equivalence point. In this experiment, phenolphthalein indicator will be used for each titration. The pH range of the color change will be observed and compared with the pH of the equivalence point to determine if the indicator is an appropriate choice for each titration.
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06MANtitrationcurves - Titration Curves Revised TITRATION...

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