Potentiometric_acid_base_titration_procedure_&_calculations_Fall_2011

Potentiometric_acid_base_titration_procedure_&_calculations_Fall_2011

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Potentiometric Titration of an Acid Mixture, Page 1 POTENTIOMETRIC TITRATION OF AN ACID MIXTURE Background Reading: Harris, 7 th ed., Chapter 11. Skoog, West, Holler and Crouch, 7 th ed., Chapters 11, 13 Introduction In this experiment a mixture of two acids is titrated with standard base. Because the pH does not change abruptly enough at either equivalence point to permit estimation with visual indicators, a potentiometric titration with a pH meter and glass pH electrode -reference electrode pair is employed. A plot of pH vs. volume of base added yields a curve similar to Figure 13-2 in Skoog or Figure 11-6(a) in Harris. From the pH vs. volume curve the two end points, and thus the amount of acid present, can be found. However, a more accurate estimate of the end points can be made from a derivative plot; that is, d(pH)/d(vol) vs. volume of base added. A mixture of two acids can be analyzed with a precision of a few parts per thousand, provided the dissociation constants of the two acids differ by at least 10 3 to 10 4 and the weaker acid is not so weak that its end point is completely obscured. The normality of the sodium hydroxide titrant is determined by titration of a standard acid. For highest precision the end points of the standardization and the acid being titrated are taken at the same pH; this procedure corrects for any error due to carbonate that may be present in the base. Because two end points at substantially different pH values are obtained in this experiment, calculation of two different effective concentrations for the same sodium hydroxide titrant is required. Procedure Operation of a pH meter. Your TA will demonstrate the proper use of a pH meter and glass pH electrode. Important points to remember include the following: For accurate pH measurements, a pH meter must be calibrated using buffer solutions of known pH prior to measurement. For a titration this is less important since it is the change in pH, not the actual pH value, that is used to detect the end point in the titration. Always rinse the electrodes with distilled water and wipe dry when transferring them from solution to solution. When finished with the last titration, rinse the electrodes and then immerse them in the buffer reservoir provided.
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This note was uploaded on 03/18/2012 for the course CHM 3120L taught by Professor Dr.gormin during the Spring '12 term at Florida State College.

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Potentiometric_acid_base_titration_procedure_&_calculations_Fall_2011

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