【題組】

15. Assume that the conjugate base, E', of the monoprotic acid can be used as complexing agent for determination of

using a direct titration method. An experiment is planned in which standard aqucous solutions of

are prepared and Eriochrome Black T (EBT) is employed as the indicator. In general, there are two types of direct titration

 (non-potentiometric) methods for quantifying metal ions using a complexing agent, depending on the availability of suitable indicators. (a) Using the titration of 

 as an cxample, describe the chemistry and the conditions of use for the two different  direct titration methods when a complexing agent E" is used. (b) Sketch and properly label a titration apparatus that can be used in a direct potentiometric titration of 

 with E-. (c) Why should an auxiliary complexing agent be used in this titration?

(d) What is the condition of use for the auxiliary complexing agent? (e) The electrode potential of the apparatus you sketched in

(b) is expected to change during the titration. (i) Based on the Nernst expression of the electrode potential, which often involves  both the reduced and the oxidized states, is formality, species molarity, or activity obtained from the titration? Define these three  terms before explanation. (ii) Will a color change be observed if a general redox indicator is also present in the solution of your  apparatus? Explain. (f [Cu(NH3)4H2O]SO4 (FM 245.75) can be prepared by adding concentrated solution of ammonia to a  saturated aqueous solution of copper sulfate followed by precipitation of the product with ethanol. What is the number of  reacting units (eq/mol) for NH3 in the reaction of the preparation? (g) 0.983 g [Cu(NH3)4H2O]SO4 is dissolved in 100.0 mL aqueous solution that contains 0.200 M in NH3. Calculate the potential of a copper electrode immersed in the solution. (β4 for

0.337 V)(h) Assume that both 

  and Et solutions are 0.100 M in NH3 and  0.175 M in NH4CI to provide a constant pH of 9.0. Without actually going through the calculation, show the calculation   procedure for determining pCu at the equivalent point when the complexing agent E- is employed to titrate 

 . In the  procedure, use your defined symbols to show variables whose values are not provided in the question. (27%)