Exp. 3 Prelab - standards to a high, constant ionic...

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Jennie Senter CH. 319 Sec. 1 1/23/08 Experiment 3: Determination of Fluoride in Water With Ion Selective Electrodes Prelab Exercises 5.22 In the standard addition method, it is best to use a small volume of concentrated standard because using a large volume of a dilute standard could appreciably alter the sample matrix. 15.29 Analyte ions equilibrate with ion-exchange sites at the outer surface of the ion- selective membrane. Diffusion of analyte ions out of the membrane creates an electric potential difference across the interface between the membrane and analyte solution. Changes in analyte concentration change the electric potential difference. 15.33 The electrodes respond to the activity of uncomplexed analyte ion, so ligands must be absent or masked. Because we’re usually looking to derive concentrations, not activities, we add an inert salt to bring all samples and
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Unformatted text preview: standards to a high, constant ionic strength. If the activity coefficients are all constant, the electrode potential gives concentrations directly. 1. Concentrations measure the number of moles per liter of solution, while activities account for the effect of ionic strength. 2. Benefits of using ISE: they do not consume unknowns; the introduce negligible contamination; fast response times; color and turbidity do not hinder their effectiveness; they can be used inside living cells. Disadvantages of using ISE: precision is rarely better than 1%, and is usually worse; they can be fouled by proteins or other organic solutes, which can lead to a sluggish, drifting response; certain ions interfere with or poison particular electrodes; some are fragile and have a limited shelf-life....
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