Experiment 5 Determining the Ksp of AgCl using Potentiometric Titrations

Adding together redox reactions Ag + + e- ↔ Ag (s) E 0 1 AgCl (s) + e- ↔ Ag (s) + Cl - E 0 2

Ag + + e- ↔ Ag (s) E 0 + Ag (s) + Cl - ↔ AgCl (s) + e- E 0 - __________________________________ Ag + + Cl - ↔ AgCl (s) E 0 cell = E E 0 -

 G 0 = -nFE 0 cell = -RT ln(K sp ) K sp = e -  G0/RT = e (nF/RT)E0cell = e E0cell/59.16

Adding together redox reactions Ag + + e- ↔ Ag (s) E 0 1 AgCl (s) + e- ↔ Ag (s) + Cl - E 0 2

Experimental Setup Voltmeter Double junction Ag/AgCl Reference Electrode Ag bullet Electrode [Ag +] = M KCl in buret

Potentiometric titration Use very low current electrochemical cell to monitor the course of a redox tiratration The reactions at the cathode do not change the bulk concentrations of reagents

Experiment: Part 1 Add M AgNO 3 to 100 mL of M KNO 3. Titration reaction; none. This is not a titration Cathode reaction: Ag + + e- ↔ Ag (s) Cell Voltage increases because the Ag + ion concentration increases Ecell = E 0 (Ag + /Ag) – 59.16log(1/[Ag + ]) - E ref

Part 2 Add a diluted solution of KCl Titration reaction; Ag + + Cl - → AgCl(s) Equivalence point reached when just enough Cl - reacts with all the Ag + Cathode reaction before Equiv. point: Ag + + e- ↔ Ag (s) Cell Voltage decreases because the Ag + ion concentration decreases Ecell = E 0 (Ag + /Ag) – 59.16log(1/[Ag + ]) - E ref

At the Equivalence point A sharp drop in cell voltage indicates when the equivalence point is reached The equivalence point is used to determine the concentration of the diluted KCl solution

After the Equivalence Point In bulk: Adding excess Cl - Reaction at cathode; AgCl (s) + e- ↔ Ag (s) + Cl - Cell Voltage decreases because the Cl - ion concentration increases Ecell = E 0 (AgCl/Ag) – 59.16log([Cl - ]) - E ref

Recap Before Equivalence point: Ecell = E 0 (Ag + /Ag) – 59.16log(1/[Ag + ]) – E ref Plot Ecell vs. log(1/[Ag + ]) and intercept = E 0 (Ag + /Ag) - E ref

Recap After Equivalence point: Ecell = E 0 (Ag/AgCl) – 59.16log[Cl - ] – E ref Plot Ecell vs. log [Cl - ] and intercept = E 0 (Ag/AgCl) - E ref

intercept 1 = E 0 (Ag + /Ag) - E ref intercept 2 = E 0 (AgCl/Ag) – E ref Int 1 – Int 2 = E 0 (Ag + /Ag) - E 0 (AgCl/Ag) K sp = e (Int1 –Int 2)/59.16