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titration lab - Potentiometric Titration of a Halide...

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Potentiometric Titration of a Halide Mixture ABSTRACT The purpose of this lab determine the identities and the concentrations of halide ions in an unknown solution. Under this purpose, the premise for this experiment was to become familiar with potentiometric titration through four trials of the titration of these unknown halide ions with a standard silver nitrate solution. As a result of the experiment, it was gathered that the first equivalence points of the trials occurred at 0.01245L, 0.01374L, 0.01412L, and 0.01376L of AgNO 3 added with E meas of -0.23401V, -0.17111V, -0.1631V, and -0.14959V respectively . The second equivalence points occurred at 0.04055L, 0.04098L, 0.04135L, and 0.04112L of AgNO 3 added, with E meas of +0.13071V, +0.20002V, +0.2304V, and +0.23429V, respectively. It was thus determined that the halides in the unknown solution were I and Cl, with concentrations of 0.055757M±0.002137 and 0.109652M±0.000722, respectively. INTRODUCTION Potentiometric titration in a titration technique in which rather than an indicator, the voltage across the analyte is measured using a two electrodes. One electrode is placed in the analyte in order to transfer electrons to or from the analyte, making it a half cell. This half cell is connected to another half cell by a salt bridge, and a second electrode is placed in the second half cell. The second half cell has a fixed composition, thus constant potential; hence, the second electrode is known as the reference electrode. The measured voltage is the difference between the potential of the analyte half cell and the constant potential of the second electrode. In this experiment, the first electrode was a silver wire while the reference electrode was a saturated calomel electrode, or SCE. A calomel electrode is based on the reaction Hg 2 Cl 2 (s) + 2e 2Hg(l) + 2Cl (aq) . When a calomel electrode is saturated with KCl, making it a SCE, the [Cl - ] does not change even if some of the liquid evaporates. The E o for SCE is +0.242V. In the analyte beaker, depending on what the halide ions were in the solution, two of the following reactions occurred: AgNO 3 (aq) + Br (aq) → AgBr(s) + NO 3 (aq), AgNO 3 (aq) + I (aq) → AgI(s) + NO 3 (aq), or AgNO 3 (aq) + Cl (aq) → AgCl(s) + NO 3 (aq). The order of precipitation can be determined by using Ksp of the reactions. The three possible precipitates are AgBr, AgI, and AgCl. The reactions for these precipitates are AgBr(s) → Ag + (aq) + Br (aq), AgI(s) → Ag + (aq) + I (aq), and AgCl(s) → Ag + (aq) + Cl (aq) with Ksp of 7.7*10 -13 , 8*10 -17 , and 1 1.6*10 -10 respectively 1 . With the equation Ksp=[Ag + ][halide], if the concentration of the halide ions 1 Atkins, Peter, Jones, Loretta. Chemical Principles The Quest for Insight. Fourth Edition. 2008.
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were known, the concentration of the silver ions could be calculated and the reaction with the lowest concentration of silver ions would occur first. Although in this experiment the concentration of the halide ions are initially unknown, to make a prediction, the concentrations of the halide ions are assumed equal or at least very close. Therefore, the order of precipitation would occur in the order of the Ksp least to greatest. The hypothesis for this experiment then, is that AgI will precipitate first,
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