Lab4-acid-base

Lab4-acid-base - Chem 227 Fall 2010 Lab 4: Acid/Base...

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Chem 227 – Fall 2010 Lab 4: Acid/Base Equilibria Objectives and Overview: This experiment will reinforce concepts in complex equilibria and pH measurements and introduce you to the concept of modeling experiments. You will perform several acid/base titrations using a pH/SCE combination electrode and a computer-controlled buret. You will need to prepare a sodium hydroxide solution and standardize it against potassium hydrogen phthalate. You will compare your observed curve to a model (i.e., calculated) curve. Then you will determine the composition of an unknown acidic solid. Finally you will measure the three acid dissociation constants for phosphoric acid. Safety Notes Sodium hydroxide is highly corrosive and will attack biological tissue rapidly. It is especially dangerous to the eyes. The 50% (by mass) NaOH solution that will be used to prepare your titrant is particularly reactive, but even dilute solutions can quickly cause damage. You must wear your safety goggles at all times. If any solution gets in your eyes, immediately flush them with water for 15 minutes using a drench hose at a sink or an eyewash station. Given enough time, NaOH solutions can also cause severe skin damage. Because NaOH is difficult to wash off, you should rinse thoroughly any skin that contacts NaOH solutions. Use of the Glass Electrode and pH/mV Meter Your lab team will be issued a combination glass-membrane/SCE electrode. Handle this combination electrode with care—the replacement cost is about $100. At the beginning of this experiment, you should remove the electrode from the solution in which it has been stored and place the electrode into fresh standard pH 7 solution so that the pH-sensing membrane will be well conditioned. Be sure that the internal filling solution level is above that of the external pH 7 solution in which the electrode is stored. Also be sure to open the cover over the KCl filling port on the upper part of the electrode body. Frequent recalibration of pH electrodes is necessary. You should calibrate your electrode with standard pH solutions prior to each titration. Take care not to contaminate or dilute solutions with liquid adhering to the electrode when you move the electrode between solutions— the standard “buffer” solutions are not necessarily true acid/base buffers. Also, remember that pH/mV meter readings may drift for several minutes after the power is first turned on. Calibration is performed from within the “ acid base titration program, as described in the last section of this procedure. NaOH, KHP and Unknown Solution Preparation Among the first tasks you should perform as a group are preparing CO 2 -free water and drying your solid unknown. De-aerate about 500 mL water by gentle boiling. Ensure that your unknown is dry by following the procedure described in Harris ; the oven temperature should be set to 110 ºC—higher temperatures will cause the phthalate in the unknowns to decompose. The standard KHP has already been dried for you.
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Lab4-acid-base - Chem 227 Fall 2010 Lab 4: Acid/Base...

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