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Unformatted text preview: Chemistry 135 Clark College Potentiometric Titrations Revised Spring 2007 NF Page 1 of 14 POTENTIOMETRIC TITRATION OF A WEAK ACID A Weak Acid/Strong Base Titration For this experiment: 1. Complete the Prelab and obtain a stamp before you begin the experiment. 2. Write your lab notebook prelab and get it initialed/signed before you begin the experiment. 3. Obtain an unknown organic acid and record the unknown code in your notebook and on the data report sheet. Each student will receive a different unknown! You may pick your own number, dont forget to record the number in your lab notebook and on the Data Report Sheet. Store your unknown in your lab drawer. 4. Titrate a preliminary sample of your unknown roughly with phenolphthalein to determine approximate equivalence point. 5. Perform two trials of your titration using the LoggerPro interfaces and the same batch of NaOH that you standardized. 6. Determine the identity of your unknown from the calculated equivalent weight and estimated pK a . Turn in your Data Report Sheet, Stamped Prelab, notebook pages and the following (labeled and organized graphs for each equivalence point in each titration): pH vs V NaOH , 1 st derivative curve, 2 nd derivative curve, and a blowup of the pK a /buffer region. Introduction In a potentiometric titration one makes a graph of the volume of titrant delivered from a buret against the voltage (or some function directly related to the voltage, such as pH) produced by two electrodes in the solution being titrated. Provided the equilibrium of the analytical reaction is favorable and one chooses the right combination of electrodes there will be a sharp change in voltage at the equivalence point. This change in voltage can be used in place of a color indicator to locate the equivalence point. This is obviously a very useful technique if one is dealing with a colored solution in which an indicator would be useless or in a system for which a suitable indicator does not exist. Although the present experiment deals with an acid-base reaction, potentiometric titrations can also be used with redox reactions, precipitation reactions and complex formation reactions, as well as titrations in non-aqueous systems. Furthermore, although the present experiment may give the impression that the technique is slow, it can, in routine work, be made just as rapid as titrations using visual indicators, and often gives more accurate results. The choice of electrodes is critical. One electrode, called the reference electrode , must hold a constant potential throughout the titration, regardless of the concentration of various reagents in the solution. The other electrode, called the indicator electrode , must have a potential that depends directly on one of the reactants or products in the analytical reaction. The electrodes used in acid-base titrations are almost always a calomel reference electrode and a glass indicator electrode. The potential of the glass electrode depends directly on the hydrogen ion activity in solution, whereas the potential of the...
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