At,
the level of freshman chemistry laboratory, th? number of ligands associated with the central metallic ion can be measured titrimetrically, with photometric and potentiometric endpoints in the two cases of Cu(NH&++ and Ag(NH,),+. In titrating the Cu++ (aq) with a standard solution of NH,, an excess of NH,+ is needed to repress the hydmxyl concent,ration and prevent t,he precipitation of CU(OH)~. In our procedure, 10 ml 0.1 M CuSOn, 10 g NH-NO8, and about 75 ml HzO are combined in a 150-ml beaker and placed in a simple photometer.' The photometer is adjusted to give a full scale reading, aud then portions of a standard solution of anlmonia are added and the optical density observed with each stepwise addition of ammonia. Results of this type of determination are shown in Figure 1, which is from a student's report on this experiment. The number of moles of NH, equivalent to a mole of Cu++ in the coordirlation sphere is derived from the endpoint data. A variation in this experiment that introduces the
unknown factor may he achieved by supplying a fictitious concentration for either the Cu++ (aq) or NH,(aq) solutions. I n such cases the student is advised to beware of a coordination number other than 4, but to report the number derived from calculations based on the data obtained experimentally. The potentiometric titration of the Ag+ NH, reaction is performed by measuring the emf of the silver electrode in the titration mixture by means of an ordinary pH meter. The glass electrode is replaced with a silver electrode (silver foil or wire) and the millivolt scale of the meter is observed. I n our procedure, 10 mlO.l ill AgNOa, 10 g NHINOI, and 25 ml HzO are placed in a beaker, a magnetic stirrerZsupplies uniform agitation, and the silver and calomel electrodes are inserted. Standard amnonia solution is added stepwise and the corresponding emf of the electrode system is observed. Typical student results, from which the ratio of Ag+ to NH, can be calculated, are shown in Figure 2.
+
'SLABIIUGH, W. H., THIS JOCTRNAI., 40,641 (1963).
0
LO
MiIllliters
Figure 1.
6,
90
0.1 M
70
=a
NU,
Photometric titrotion of copperllll ion with ammonia.
470 / Journal of Chemical Education
i
2
10
30
-
Mlihiitrrs
Figure 2.
0.l
M NH,
Potentiometris titrotion of silver ion with ammonia.
