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1 EXPERIMENT 8 POTENTIOMETRY: DIRECT-MEASUREMENT OPTION I. INTRODUCTION This experiment introduces the direct-measurement approach to potentiometry. Principal purposes of the study are a) to understand quantitative relationships between electrode potential and concentration and b) to use that information to understand how absolute and relative concentration errors depend on errors in measured potentials. A secondary purpose is to use potential measurements to quantify the hydrogen peroxide concentration in an unknown sample and to determine the confidence interval for the results. II. OVERVIEW A. Chemical reactions The half reactions involved in this experiment are as follows: O H 2 e 2 H 2 O H 2 2 2 + + + ( 1 a ) + e 2 I I 3 3 ( 1 b ) Hydrogen peroxide is reduced to water and iodide is oxidized to triiodide. The net reaction is the sum of these two half reactions as follows: O H 2 I H 2 I 3 O H 2 - 3 2 2 + + + + ( 1 c ) This reaction is slow in the absence of a catalyst but quite fast in the presence of catalysts such as molybdate (Mo(VI)) and an excess of iodide that forces the reaction to completion by the formation of triiodide, I 3 - . B. Rationale In the presence of a high iodide concentration, changes in triiodide concentration can be monitored potentiometrically. Accordingly, hydrogen peroxide concentration can be quantified potentiometrically by measuring the potential of an inert electrode immersed in the solution and using a calibration relationship to compute triiodide concentration, which is then converted to hydrogen peroxide concentration. C. Standardization In practical situations, the hydrogen peroxide concentration should be standardized. However, the main thrust of this experiment focuses on characteristics of potentiometric methods. Accordingly, to save time and to permit you to focus on the main thrust of the experiment, you will use the nominal concentration of the hydrogen peroxide in a 3 % solution. D. Potentiometric detection It will be helpful to review some features of potentiometric detection before proceeding with the experimental discussion. 1. General . Potentiometry exploits electrochemical properties of chemical systems. In potentiometry, the aim is to measure the potential of a system without drawing any current from it, e.g. to measure the potential without causing electrochemical reactions that would change the concentration at the electrode surface. Typical examples of potentiometric sensors include the glass electrode used for pH measurements and ion-selective electrodes used for a variety of ionic analytes such as sodium, potassium, calcium, sulfide, etc. Relationships between electrode potential and concentration are described by the Nernst equation (see text). Potentiometry is frequently used in one of two modes, namely a
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This note was uploaded on 02/16/2012 for the course CHEM 125 taught by Professor Na during the Fall '11 term at Purdue.

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