analy lab 3 intern - Titration Curves Revised TITRATION...

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Behavior Modification: Principles and Procedures
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Chapter 12 / Exercise 16
Behavior Modification: Principles and Procedures
Miltenberger
Expert Verified
Titration Curves Revised 10/21/14 1 TITRATION CURVES, INDICATORS, AND ACID DISSOCIATION CONSTANTS Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 INTRODUCTION Titration is the volumetric measurement of a solution of known concentration when it reacts completely with a measured volume or mass of another substance. In this experiment, the titrant , which is placed in the buret, will always be a base (NaOH or NH 4 OH). (Ammonia (NH 3 ) reacts with water to form NH 4 OH.) The analyte , which is placed in a beaker below the buret, will always be an acid (KHP, HCl, or HC 2 H 3 O 2 ). Titration Curves The differences in shapes of titration curves when various strengths of acids and bases are combined will be observed. In this experiment you will react the following combinations of strong and weak acids and bases (all solutions are approximately 0.10 M). Titration #1: Hydrochloric acid, HCl, with sodium hydroxide, NaOH. Titration #2: Acetic acid, HC 2 H 3 O 2 , with sodium hydroxide, NaOH. Titration #3: Standardization of NaOH: Potassium Hydrogen Phthalate, KHP, with sodium hydroxide, NaOH. Titration #4: Hydrochloric acid, HCl, with ammonium hydroxide, NH 4 OH. Titration #5: Acetic acid, HC 2 H 3 O 2 , with ammonium hydroxide, NH 4 OH. A 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. 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.
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Behavior Modification: Principles and Procedures
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Chapter 12 / Exercise 16
Behavior Modification: Principles and Procedures
Miltenberger
Expert Verified
Titration Curves Revised 10/21/14 2 As base is added to acid, a gradual increase in pH will occur until the solution gets close to the equivalence point . Near the 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. The titration curve will be sigmoidal with the inflection point (the point where the curvature changes direction) is the equivalence point. Beyond the equivalence point, where more base has been added than acid, more gradual increases in pH are observed. 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) D 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). A 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

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