E730: Complex Ions – Solubility and Complex Ion Equilibria

Introduction

A series of precipitations and complex-ion-formations are performed in a single reaction vessel.

1.       AgCl ⇔ Ag⁺ + Cl^-                                                 Ksp = 1.6 x 10^-10
2.       Ag⁺ + 2NH3 ⇔ Ag(NH₃)₂⁺                                   Kf = 1.7 x 10⁷
3.       NH₃ + H⁺ ⇔ NH₄⁺                                                 K = 1.8 x 10⁹                                                        
4.       NH₄⁺ + OH^- ⇔ NH₃ + H₂O                                   K = 1/Kb = 5.6 x 10⁴
5.       AgBr ⇔ Ag⁺ + Br^-                                                 Ksp = 3.3 x 10^-13
6.       Ag⁺ + 2S₂O₃^2- ⇔ Ag(S₂O₃)₂^3-                             Kf = 2.0 x 10¹³
7.       AgI ⇔ Ag⁺ + I^-                                                        Ksp = 1.5 x 10^-16

To Conduct Demonstration:

Add a minimum amount of reagent until precipitation or complexation is complete.

1.                Place a small amount of AgNO₃ in a 1000-mL short form beaker on a stir plate and begin stirring.  (In a  tall form beaker, the solution will not stir well at the top.)
2.                Add NaCl solution.  AgCl precipitates.
3.                Add NH₃ solution.  Ag(NH₃)₂⁺ forms and the precipitate disappears.
4.                Add HNO₃.  AgCl reappears as Ag(NH₃)₂⁺ is reacted away.
5.                Add NaOH solution.  Ag(NH₃)₂⁺ is re-formed and the precipitate disappears.
6.                Add KBr solution.  AgBr precipitates
7.                Add Na₂S₂O₃ solution.  Ag(S₂O₃)₂^3- forms as AgBr dissolves.
8.                Add KI solution.  AgI precipitates.
9.                Pour solution into disposal container and cap tightly.

Safety 

Goggles should be worn.  Silver solutions will stain skin black on contact.  Gloves should be worn. 

Reference:

Stewart Strickler, Spring 1987.

Edited 3/2016 KAB