SOLUBILITY PRODUCT A
Demonstration Introducing the Common Ion Eflect and Formation of Complex Ions
SAVER10 ZUFFANTI Northeastern University, Boston, Massachusetts
T
H E following equilibrium between solid silver acetate and its ions may be used to demonstrate the solubility product principle, the common ion effect, and the formation of complex ions. AgAc (Solid)
* Ag+ + Ac-
The use of AgAc for this demonstration is not original. Although the solubility product of AgAc, a t room temperature, is about 4 X 10WSits use in this demonstration is justified, from a pedagogical viewpoint, by its effectiveness. A saturated solution of AgAc is prepared by beating distilled water with an excess of AgAc. On heating, some metallic Ag will be formed causing the solution to darken. Filter while hot and allow the clear filtrate to cool. On cooling, some white crystals of AgAc will settle out to the bottom of the bottle. These may be left in the bottle to show the students that the solution is saturated. + The application of the solubility product principle is best understood in connection with the common ion effect. Pour about 250 cc. of the satukated AgAc solution into a flask or beaker and, with this before the class, discuss the relation : Ks. P. = [Ag+I LAC-.I
The common ion effect is demonstrated by adding 10 cc. of a saturated AgNOa solution to about 250 cc. of the saturated AgAc solution and obsening the precipitation of AgAc caused by the addition of the common ion Ag+. I t may be pointed out to the student that the same effect may be produced by increasing the concentration of the Ac- ions. This may be demonstrated in another flask or beaker by adding 10 cc. of a saturated NaAc solution to about 250 cc. of the saturated AgAc solution. In both cases about 20-30 seconds elapse before a fairly heavy precipitation of AgAc occurs. To demonstrate the fact that common ions are necessary for the precipitation, 10 cc. of a saturated NaNOz
solution are added to about 250 cc. of the saturated AgAc solution. No precipitation occurs. These changes may be explained to the student as follows : N a * AcN a + AeNa+ A e NI+ A * -
I t is obvious that the numbers here used as concentrations of the Ag+ ions and Ac- ions do not represent gram ion concentrations. These npnbers are merely used to increase the effectiveness of the discussion and to enable the student to obtain a more thorough understanding of the principle discussed. The relationship between activities (effective concentrations) and concentrations may also be pointed out. Having caused the precipitation of the AgAc by the addition of common ions, i. e., by increasing the concentration of either the Ag+ ions or the Ac- ions, it may be demonstrated that the precipitated AgAc can he caused to go back into solution by decreasing the Ag+ ions or the Ac- ions. This can be shown by adding to the precipitated AgAc some concentrated HNOI. to form the slightly dissociated HAc, and, to another portion, some concentrated NHaOH, to form the silverammonia complex ions.
