4-Lab4-Acid-Base and precipitation TitrationAS

4-Lab4-Acid-Base and precipitation TitrationAS - Chemistry...

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1 Chemistry 223: Experiment 4 Acid- Base and Precipitation Titration References : 1. C, Chap. 7 . 2. H7, Chapter 11, 3. Kellner, Chap 4 4. SWH, Chaps. 10,11,12,13 5. Harvey, C5 and C7 6. Kealey, pp. 139-143 This experiment is partly based on the experiment - chem. 213 – Winova State University and Chemistry 3.1- University of Canterbury A. Determination of total alkalinity of soda ash I. Introduction: Crude sodium carbonate, Na 2 CO 3 , is commonly called soda ash. It is frequently used as a commercial neutralizing agent. Besides the carbonate, small amounts of sodium hydroxide, NaOH, and sodium hydrogen carbonate, NaHCO 3 , may also be present. Titrating with standard acid, usually HCl, makes it possible to determine the total alkalinity of the soda ash. It is common practice to report the total alkalinity as percent sodium carbonate or sodium oxide, Na 2 O. Since samples are frequently non-homogeneous, the method of aliquot portions is usually employed. Instead of weighing out three separate samples of soda ash, one accurately weighs out a larger amount. This is then transferred into a volumetric flask, dissolved in water and then diluted to an accurately known volume. From this solution are then taken samples or aliquots on which the titration is carried out. One-tenth molar hydrochloric acid is standardized against primary standard sodium carbonate. Phenolphthalein is used to approximate the halfway point of the titration, and then either modified methyl orange indicator or bromcresol green indicator is used to detect the final end point; the last indicator is used with boiling of the solution near the end point to remove CO 2 . Altematively, the hydrochloric acid is standardized against standardized sodium hydroxide using phenophthalein indicator. The soda ash sample is titrated with the hydrochloric acid solution, with the addition of 2 mol hydrogen per mole Na 2 CO 3 [AS1] . Equations CO 3 2- + 2H + H 2 CO 3 H 2 CO 3 H 2 O + CO 2 II. Solutions and Chemicals Provided. 0.2% phenolphthalein in 90% ethanol modified methyl orange (mixture of methyl orange and xylene cyanole FF available from Eastman Kodak Co.) 0.1 % bromcresol green in 0.001 M NaOH, -- Converted from Word to PDF for free by Fast PDF -- www.fastpdf.com --
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2 primary standard Na 2 CO 3 dried at 160°C for 2 h or more. Cool at least 30 min in a desiccator before weighing. standardized 0.1 M NaOH. To be prepared. 0.1 M HCl. Concentrated hydrochloric acid has a density of 1.18 g/mL and contains 37% by weight HCl. Hence, about 4 mL concentrated acid should be diluted to 500 mL to make 0.1 M acid. Measure about 0.5 mL more than this amount in a 10-mL graduated cylinder and pour into water in a 500-mL glass-stoppered bottle that is filled to the shoulder with distilled water. Shake until the solution is homogeneous. III. Experimental Procedure
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This note was uploaded on 02/12/2011 for the course CHEMISTRY 223 taught by Professor Tathithao during the Spring '10 term at Hanoi University of Technology.

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4-Lab4-Acid-Base and precipitation TitrationAS - Chemistry...

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