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8- lab8-potentiometric titration of acid mixture

8- lab8-potentiometric titration of acid mixture - Chem 223...

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1 Chem 223: Experiment 8 THE POTENTIOMETRIC TITRATION OF AN ACID MIXTURE References: SWH. pp. 212-218;224-229; 402-439; Harris. Chapter 15 This experiment is revised from Expt. 5- Chem 5.310. Massachusetts Institute of Technology. I. PURPOSE OF THE EXPERIMENT In this experiment the quantitative composition of a solution, which is a mixture of a monoprotic strong acid and a weaker, triprotic acid, will be determined by potentiometric methods. This experiment will introduce you to quantitative volumetric analysis and potentiometric titrations. (A) A carbonate-free sodium hydroxide solution is prepared and standardized against pure potassium hydrogen phthalate (KHP), and is then used in a potentiometric titration of the acid mixture. (B) Three or four 1.0 mL aliquots of your acid will be used for potentiometric titrations. Each titration must be continued through two equivalence points. II. INTRODUCTION Strong acids, like hydrochloric acid, are completely dissociated in water, but weak acids, like acetic acid, are only partially dissociated. The extent of dissociation can be calculated from the value of the equilibrium constant and the amounts of weak acid and strong base added to the solution. The relative acidities of acids and bases are commonly expressed in terms of pK a = - logK a , where K a is the dissociation constant for the reaction HA H+ + AIn the following derivation, a A - , [A-] , and A - represent, respectively, the activity, the molar concentration, and the activity coefficient of the conjugate base, A-.
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2 Since pK a - log K a then pK a = - [log (a H + ) + log (a A - ) - log (a HA )] since pH - log (a H + ) then pK a = pH + log (a HA ) - log (a A -) Making the approximation that Making the approximation that This equation is referred to as the Henderson-Hasselbalch equation. It is very useful in the buffering region of the titration of a weak acid. The pK a is -log K a at the ionic strength of the solution. Accordingly, the value of K a obtained will deviate slightly from values listed in standard references since they report the thermodynamic value at zero ionic strength.
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3 Figure 1. Typical Plot of a Potentiometric Titration to Determine the Equivalence point and pK a value (monoprotic acid).
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