Complexes of the Silver Ion with Ammonia and 1,2=Diaminoethane Holger Meyer Marks Gymnasieskola, S-511 80 Kinna, Sweden
A fundamental, highly significant part of inorganic chemistry is the complex formation of metallic ions with water, ammonia, chloride ion, and other ligands. In order to ~rovidethe college students with elementary knowledge of this formation, &ring the past few years we have, a s a laboratory exercise, let them titrate silver nitrate with ammonia a i d 1,2-diaminoethane (from here on abbreviated "en") in an aqueous medium. Since en is a bidentate complexing agent, the exercise serves to introduce the students to chelating agents. The exercises also teach the students the use of simple analytical methods such as potentiometry, condudometry, and thermometry. They should be expected to be familiar with the pH-curve of an acid-base titration and with the concentration element of silver, having an electromotive force
where cl and cz are the concentrations of hydrated silver ion in the reference solution and titrated solution, respectively. Furthermore, students should be familiar with the law of mass action. They should be able to calculate the equilibrium constants for the reaction between hydrated silver ions and ammonia or 1,2-diaminoethane. In our treatment we consider the following simplified equilibrium reactions.
+ en [Ag(H20)21+ [Ag(en)lli +en
[Ag(en)llt+2Hz0 IAg(en)zli
constant k~ (3) constant kz (4)
Armeanu and Luca' have shown by potentiometric titration that in aqueous solution the complex ions [Ag(enhl+ and [Ag(en)%lt exist in equilibrium. In an ethanolic solution with a negligible protolysis of 1,2-diaminoethane, even the complex ion [Ag(en)31fexists. Thus, it has been confirmed that the silver ion has all three coordination numbers-2, 4, and 6. The most common is 2. In our experiments the coordination numbers 2 and 4 were found by simultaneous conductometric and thermometric titrations in addition to potentiometric titration of silver nitrate with 1,Pdiaminoethane. Experimental Potentiometric Titration with Ammonia
A porous clay cylinder was fixed over a magnetic stirring bar in a crystallization dish and placed on a magnetic stirrer. Then 75.0 mL of 0.1000 M AgN03 and 25.0 mL of the same were poured into the dish and the cylinder, respectively. Silver electrodes, connected to a digital voltmeter (range: 0-2000 mV), were dipped into the solutions. From
'
Amenau, V ; Luca, C. Zeitschrift fur Phys. Chemie (Leipzig)1961, 218, 149,72.
Figure 1. Potentiometric titration of 75.0 ml of 0.1000 M AgNO, with 1.52 M ammonia. a piston buret, 1.52 M ammonia was added into the dish space in volumes of 0.1-1.0 mL. From the inflection point of the plot of potential versus mL's of ammonia (Fig. 11,the coordination number 2 of the amminesilver ion is calculated within the margin of error. Potentiometric Titration with l,2-Diaminoethane With the same equipment as in the previous experiment, 75.0 mL of 0.1000 M AgN03 were titrated with 1.72 M 1,2-diaminoethane. The plot ofthe potential versus mEs is seen in Figure 2. The molar ratio en/Ag+ is 1.15. In other words, the average coordination number of the silver ion in this case is found to be 2.3. The matter will be discussed further in the result section. A Conductometric and Thermometric Titration with 1.72M 1,2-Diammoethane A couductometric and thermometric titration with 1.72 M l,2-diaminoethane was carried out simultaneously on a cold mixture of 50.0 mL of 0.1000 M &NO3 and 50.0 mL of distilled water, using a digital condudometer and a thermometer with a 0.1-degree scale. The break points in the plots in both Figures 3 and 4 indicate the same average coordination number, which is 2.3, as calculated from the potential curve in Figure 2. In Volume 69 Number 6 June 1992
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Figure 3. Conductometrictitration of 50.0 ml of 0.1 000 M AgN03with 1.72 M 1,2diaminoethane en. Figure 2. Potentiometric titration of 75.0 ml of 0.1000 M AgN03 with 1.72 M 1,2-diaminoethane en. Figure 3, the temperature change before the break point depends on the exothermicity of the reaction. After the break point, it depends on mixing solutions of different temperatures. The change of slopes in Figure 4 is a measure of differences in ionic mobilities and concentrations.
The complex formation with 1,2-diaminoethane and the calculationof the eqnilibrium constants kl and kz are more complicated to handle because we have to deal with two ionic species in solution, [Ag(en)J+and [Ag(enj21+.In a simplified attempt to solve the problem with the aid of the po-
Results and Discussion From the potential curve in Figure 1, which, of course, is similar to the well-known pHcurve of an acid-base titration, the coordinationnumber of the silver ion was found to be 2. This is equal to the molar ratio
a t the inflection point. This curve could also be used to calculate the equilibrium constant K of equilibrium reaction 2 above. When 14.7 mL of ammonia (50% in excess) is added to the dish space, we be sure that nearly all silver ions are complexed with ammonia. Consequently, it can be assumed that --
On the other hand the potential E of the [Ag(HzOjzl+/Ag element has changed from 8 to 300 mV at 22 T. The concentration of [Ag(HzOjzlt is cz in the titrated solution a t 22 'C (295.2 K).It is calculated with eq l above. Then cl is 0.1000 M, and cz is found to be 1.02 x 10" M. Finally, the concentration of uncomplexed ammonia will be
The equilibrium constant found will be
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Figure 4. Thermometric titration of 50.0 ml of 0.1000 M AgN03 with 1.72 M 1.2-diaminoethaneen.
tential curve in Figure 2, we find that after addition of 5.0 mL of 1.72 M 1,2-diaminoethane, the potential change is 144 mV. Using eq 1above, we consequently find that the concentration of [Ag(H~0)~1+ is 3.5 x lo4 M. The actual concentrations are I. [[Ag(H20),1+l= 3.5x 104 M 11. [[Ag7,1
= 0.00035 + [[Ag(en)ll'l
+ IIAg(en)zl+l
In. [en],, = [en1 + [[Ag(en),l'l + 2[[Ag(en)~l+l 50x.10 1.72 --= 75 - 80
Assuming that most of the added 1,2-diaminoethane has formed both the complex silver ions and that the concentration of en in sum 111is negligible, we find on combining the concentration sums I1 and 111that
Furthermore, as a result of relations 5 and 6,
M
The expressions ofthe equilibrium constants are We can, of course, assume k 1 to be of the same order of magnitude as K for the diamminesilver ion, 1O5-lo7 M-'. We can also assume that kz is 80-8000 M-'.
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