Lab2-FeCe - Chemistry 227 Fall 2010 Lab 2 Potentiometric...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Chemistry 227 – Fall 2010 Lab 2: Potentiometric Titration of Iron(II) With Cerium(IV) Objectives and Overview: You will learn about electrochemical potential measurements through the determination of the concentration of a Ce(IV) solution. For this determination, you will titrate a known Fe(II) solution with the unknown Ce(IV) solution while monitoring electrochemical potential with a platinum wire as a “working” (or “indicator”) electrode and a saturated calomel electrode (SCE) as a reference electrode. In addition to improving your understanding of redox equilibria, this experiment will demonstrate the advantages of titration and require that you apply what you have learned about calibration, statistics, and propagation of uncertainty. Background Chapter 16 section 5 of Harris describes the chemistry of cerium solutions as analytical reagents, and section 1 covers the basics of potentiometric titration. (Some information in this procedure was also taken from E.H. Swift and E.A. Butler, Quantitative Measurement and Chemical Equilibria ). Solutions of ceric compounds were first used as analytical redox agents about 130 years ago, and the advantages of ceric sulfate solutions were already firmly established in the late 1920s. These advantages include, (1) Ce(IV) sulfate solutions appear to be stable indefinitely, (2) Ce(III) is the only reduction product, (3) titrations can be done in hydrochloric acid solutions without significant oxidation of chloride during the time needed for a titration, and (4) although the oxidizing power of Ce(IV) is less than that of permanganate, it can often be used to replace the more troublesome permanganate in analytical procedures. Standard solutions of Ce(IV) can be prepared from primary standard ceric compounds, but common practice is to prepare a solution from less expensive and readily available reagent grade ceric sulfate and then standardize the solution against another standard. Ferrous ethylene diammonium sulfate (FEDS) can be used as a primary standard for this purpose. The titration reaction can be represented as Fe(II) + Ce(IV) = Fe(III) + Ce(III) The notation used in this equation emphasizes the fact that +3 or +4 (as well as even many +2) ions don’t really exist in aqueous solution as the simple ions, but rather as complexes with water or other available ligands. Hydrolysis by such high-oxidation state metal ions is extensive unless the solution is made quite acidic, so solutions of Ce(IV) are made up in sulfuric acid, or some other strong acid, to prevent precipitation of hydroxides. The choice of acid affects the oxidizing strength of the ceric ion. Its oxidizing strength decreases in going from nitric to sulfuric to hydrochloric acid solutions (the formal reduction potential becomes less positive) because ceric ion forms complexes of increasing stability with nitrate, sulfate, and chloride. No significant complexation of ceric ion with perchlorate appears to occur, but perchloric acid is used only in special circumstances because
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/15/2012 for the course CHEM 227 taught by Professor Stevengoates during the Fall '10 term at BYU.

Page1 / 5

Lab2-FeCe - Chemistry 227 Fall 2010 Lab 2 Potentiometric...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online