Lab3-ISE - Chemistry 227 Fall 2009 Lab 3: Ion-Selective

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Chemistry 227 – Fall 2009 Lab 3: Ion-Selective Potentiometry—Determination of Chloride by Standard Addition Objectives and Overview: In this experiment, you will learn about using an ion-selective electrode (ISE) by making a determination of the percent chloride in an unknown solid, and you will be introduced to the method of standard addition. An essential aspect of your experiment will be an application of the concepts of activity coefficients and ionic strength. Finally, you will apply the t-test to replicate data sets and analyze the effects of noise and drift on your results. Background Membranes that transport or interact preferentially (selectively) with certain ions have made important contributions to chemistry and other branches of science. The electrical potential that develops due to differences in ion concentrations on either side of an ion-selective membrane can be used as a sensitive probe of concentrations of specific ions in complex solutions. Membranes are also used as a method to separate ions (extraction or chromatography) or to help drive certain reactions. Natural ion-selective membranes play key roles in many biological processes, such as the firing of nerve cells (see exercise 14-K for another example). The first manufactured ion-selective electrode was the glass electrode for pH measurements, but many others now exist, and more are designed, used, and marketed each year. Procedure Overview : You will be given solutions of 1 F KNO 3 and standard 0.1 F KCl in 1 F KNO 3 . The KNO 3 solution will be used to keep ionic strength constant and, consequently, to keep the activity coefficient of Cl constant. Your unknown will be a solid unknown from which you will prepare a solution for the determination. (Plan to begin drying your unknown early on the first day.) Before making an unknown determination, you will need to characterize the response of your electrode. Ion-selective electrodes can have a Nernstian factor (slope) that is very different from the ideal value of RT/zF (see the discussion in Harris ). Although manufacturers try to design their electrodes to give a slope close to the ideal value, for careful work you should always check the response of your electrode—not only because the slope may differ significantly from the ideal, but also because the slope changes as the electrode ages. You will determine the Nernstian slope for your electrode by making measurements on a diluted known chloride solution and then adding known amounts of chloride, measuring the change in potential after each addition. To determine your unknown by standard addition, first measure the potential of a solution of
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This note was uploaded on 03/15/2012 for the course CHEM 227 taught by Professor Stevengoates during the Fall '10 term at BYU.

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Lab3-ISE - Chemistry 227 Fall 2009 Lab 3: Ion-Selective

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