Lab_2 - Fall 2007 Chemistry 300 Cornell University Lab 2...

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Unformatted text preview: Fall 2007 Chemistry 300 Cornell University Lab 2 Analysis of the Calcium Content of Materials I. INTRODUCTION The goal of this three-week lab is to measure the cal- cium content of different materials: a standard sample to calibrate your solutions, an antacid tablet with a pub- lished calcium content to test the analysis method for gross systematic errors, natural materials with variable calcium content (an oyster shell or limestone), and an unknown powder to test your technique. II. MATERIALS AND METHODS You will need First day Equipment * analytical balance Glassware * volumetric flask: 100 mL * volumetric pipet: 10 mL and 25 mL * burette: 50 mL * graduated cylinder: 10 mL * Erlenmeyer flasks: 50 mL and 125 mL * glass storage bottle Misc * mortar and pestle * plastic squirt bottle * hotplate Chemicals * calcium carbonate * 0.01 M disodium ethylene diamine tetraacetic acid (ETDA) solution * 0.01 M magnesium sulfate solution (Mg SO 4 ) * Eriochrome Black T (EBT) indicator * ammonium chloride / ammonium hydroxide pH = 10 buffer solution * 6 M hydrochloric acid (HCl) * 2 M sodium hydroxide (NaOH) Materials * anatacid tablets Second day . You will need all the supplies from the first day, and in addition: Materials * limestone * oyster shell Third day . You will need all the supplies from the first day, and in addition: Chemicals / Materials * powdered sample with unknown calcium content Before coming to lab you should read the sections from Harveys text covering propagation of uncertainty (Chapter 4C), the overview of titrimetry (Chapter 9A), and his description of titrations based on complexation reactions (Chapter 9C). In Chapter 9C you will find a detailed description of the hexadentate ligand disodium ethylene diamine tetraacetic acid (EDTA) and a brief description of the metallochromic indicator Eriochrome Black T (EBT). In this experiment you will determine the amount of calcium in various samples by complexometric titration of calcium with EDTA. The indicator for the titration is a secondary magnesium- EBT complex. Since EDTA binds Ca 2+ more strongly than it binds Mg 2+ , as EDTA is titrated into a solution containing Ca 2+ and the indicator complex, the EDTA binds up all the calcium in solution first. Once all the calcium in solution is complexed with EDTA, excess EDTA is at liberty to bind to magnesium, dislodging Mg 2+ from the magnesium- EBT complex. Since the magnesium- EBT complex is red and the free EBT is blue, the endpoint of the titration is indicated by a change in the color of the solution from red to blue. The source of EDTA for this experiment is the soluble salt disodium EDTA dihydrate Na 2 H 2 Y 2 H 2 O, where Y 4- represents fully ionized EDTA anion; the free acid is virtually insoluble. This EDTA salt dissolves only slowly in water; it has been provided for the lab at the approx- imate concentration indicated on the bottle. You will need to determine its concentration to high precision by...
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This note was uploaded on 10/04/2008 for the course CHEM 3000 taught by Professor Ta during the Fall '07 term at Cornell University (Engineering School).

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Lab_2 - Fall 2007 Chemistry 300 Cornell University Lab 2...

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