Experiment5_plus flow chart

Experiment5_plus flow chart - EXPERIMENT 5 ACID/BASE...

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1 EXPERIMENT 5 ACID/BASE TITRATIONS: TITRATION OF A DIPROTIC WEAK ACID WITH STRONG BASE This is a group experiment. To save time, there will be only one unknown per group. I. INTRODUCTION Purposes of the experiment are: to illustrate partial mechanization of the titration process, to understand the titration curve for a diprotic weak acid, to quantify the diprotic acid, to quantify and compare dissociation constants calculated using exact and approximate equations, to compare hydrogen ion concentrations calculated using approximate and exact equations. Of particular interest is to understand errors associated with approximate calculations of dissociation constants and hydrogen ion concentration. II. BACKGROUND INFORMATION There is a table of symbols at the end of this experiment. This experimental description should be accompanied by a description of the constant-flow titrant delivery system to be used in this experiment. A. Reactions and volume/concentration relationships This experiment involves the titration of a diprotic weak acid alone or a diprotic weak acid plus some of the conjugate base of the acid with a strong base (e.g. NaOH). 1. Diprotic acid alone . The reaction of a diprotic weak acid with strong base (e.g. NaOH) takes place in two steps as follows: O H BH Na NaOH BH 2 2 + + + + (Complete at 1 st end point) (1a) O H B Na NaOH BH 2 2 + + + + (Conj. base from 1 st reaction) (1b) The relationship between the initial concentration of the weak acid, C BH2 0 , the volume of the sample, V s , the concentration of the base, C b , and volume of base required to reach the first end point, V ep1 , is given by 0 BH2 s b ep1 C V C V = ( 2 a ) where the symbols are defined before the equation. In the absence of any conjugate base in the sample initially, the same volume of titrant will be required to titrate the sample from the first to the second end point as was required to titrate the sample to the first end point. In other words, the volume required to reach the second end point, V ep2 , will be twice that required to reach the first end point. ep1 ep2 2V V = ( 2 b ) The concentration of weak acid in the sample titrated will be given by:
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2 s b ep1 0 BH2 V C V C = ( 2 c ) If the original sample was diluted before the titration step, it will be necessary to take the dilution step into account. 2. Diprotic acid with conjugate base . If the sample contains initial concentrations of both the diprotic acid, C BH2 0 , and the conjugate base, C BH - 0 , then, in addition to Reactions 1a and 1b, the conjugate base contained in the sample initially, C BH - 0 , will also react with titrant as follows: O H B Na NaOH BH 2 2 + + + + (Conj. base in sample initially) (3a) In this case, a larger volume of titrant will be needed to titrate the sample from the first end point to the second than from the start to the first end point. The volume needed to reach the first end point will be the same as in Eq. 2a. The volume needed to reach the second end point, V ep2 , is related to the other volumes and concentrations as follows: ) C C 2 ( V ) C C ( V C V C V 0
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Experiment5_plus flow chart - EXPERIMENT 5 ACID/BASE...

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