[429] It is also frequently called the ‹dissociation constant› of the complex ion, indicating the tendency of the complex ion to dissociate into its components.
[430] Two independent experimental methods were used and gave concordant results—one having as its basis the solubility of silver salts (chloride, bromide), the other the electrolytic potentials of silver against ammoniacal silver solutions (see Chap. XV).
[431] ‹Bull. de la Soc. Chim. de Paris›, (3), «13», 386 (1895).
[432] We may consider the salt to be ionized to about the same extent as ammonium or potassium nitrate in 0.05 molar solutions, or, approximately, 87%. If we call ‹x› the concentration of silver-ion, formed by the decomposition of the silver-ammonium-ion, then 2 ‹x› is the concentration of the free ammonia, and (0.05 × 0.87 − ‹x›) is the concentration of the complex ion. Since ‹x› is a small number in comparison with 0.0435, we may write, with sufficient accuracy for our purposes,
[NH_{ 3}]^2 × [Ag^{ +}] / [(NH_{ 3})_{ 2}Ag^{ +}] = (2 ‹x›)^2 × ‹x› / 0.0435 = 6.8E−8.
[433] Kohlrausch and Holborn, ‹loc. cit.›, p. 202.
[434] The dilution of the silver-ammonium nitrate (10 c.c. to 11 c.c.) and the decrease in ionization due to the added salt reduce the concentration of silver-ion from 0.0009 to 0.00085. [Ag(NH_{ 3})_{ 2}^{ +}] = (0.05 × 10 / 11) × 0.8 = 0.0364 and 4 ‹x›^3 = 6.8E−8 × 0.0364 (see footnote, p. 220). Then ‹x› = [Ag^{ +}] = 0.00085.
[435] Thiel (‹cf.› Bodländer and Fittig, ‹loc. cit.›). The solubility given in the table at the end of the laboratory manual refers to 18°. The constant for the complex ion was determined at 25°.
[436] The combined concentration of the salts is 0.055 and their degree of ionization may be taken as 87%, the same as the degree of ionization of 0.05 to 0.06 molar KNO_{ 3}. Then [Cl^{ −}] = (0.1 × 1 / 11) × 0.87 = 0.008. [Ag(NH_{ 3})_{ 2}^{ +}] = (0.05 × 10 / 11) × 0.87 = 0.04 and ‹x› = [Ag^{ +}] = 0.00089 (see the method of calculation in the footnote, p. 220).
[437] The strong solution of ammonia is used in order to avoid unnecessary dilution, and in the experiment, described below, the dilution of the liquids by the added ammonia is considered negligible.
[438] The following solubilities have been determined at 25°: