JOURNAL OF CHEMICAL EDUCATION
SOLUBILITY PRODUCT AND THE SILVER-AMMONIA HALIDES KELSO 8. MORRIS Howard University, Washington, D. C.
AN UNDERSTANDING
of equilibrium considerations, involving application of the mass law to various phenomeua, such as ionization of weakelectrolytes, common-ion effect,and solubility product principle, ia of very great importance to the student of chemistry. Of the three phenomena to which reference has just heen p d e , the interpretation of the solubility product principle, partitularly with regard to solubility relations for the insilver and aqueous 'Omem times a d a c u l t y for the student. During the past severa1 years only a few papers U , g , 3) appearing in b u r nals have been concerned with an introductory treatment of the principle of solubility product. Among such papers, Mortimer and Joster offer, respectively, a geometric method and a method making use of the old lever principle. A general and more advmced discussion of the theory of the solubility product based on thermodynamics is given by Denbigh (4). The present paper suggests a simple approach to the problem with a the difficulty for the view toward at least specific case of the insoluble silver halides and aqueous ammonia. For the insoluble silver halides, the following are the usual generalized equilibrium equations:
+
~gx(a# ) Agf XK h x = CA.+.CX-
The soluhility product constants are 1.2 x 10-10 (for AgCl), 3.5 X 10-la (for AgBr), and 1.7 X 10-16 (for AgI). According to the solubility product principle. the following cases arepossible: case1: cx- Khx, A prec'ipitate results, ion at the moment of mi-g ~ h initial , may be identical in value or they may be different in value, and it is only necessary that their be p a t e , than the solubility product constant for the ,tance. ~f precipitation is complete and the reagent ion in excess, the solution in contact with A ~ X is is satocratedwith respect to AgX,
cAg+.
,
Case 11: C A . + . ~ X - = K A ~ XNO precipitation takes place. The ion concentrations may be identical in value or they may be diierent in value a t the moment of mixing. If the concentrations are identical, the solution is saturated with respect to AgX. Of course, addition of either ion as reagent to such a solution causes precipitation because Case I then exists. Case 111: C A g +C. x - < KAgx No precipitationtakes place. At the moment of mixing, ion concentrations may be identical in value or they may be different in value. The solution is unsaturated with respect to AgX. In this case, addition of either ion as reagent to the solution will bring about Case I1 or Case I, according to
a37
MAY. 1947
whether the product of the ion concentrations becomes equal to or greater than K A , ~ . In the presence of aqueous ammonia, silver ions will unite with neutral ammonia molecules to form the complex ion, Ag(NH&+. The complex ion dissociates in the following manner,
A precipitate forms in either of the two cases involved. Further calculations would show that in solutions 0.05 M with respect to the complex ion and for any concentration of ammonia less than approximately 13 M, silver chloride will precipitate and so will the other two silver halides. Here, decrease in CN=,resulta in an increase in CA,+ through ionization of the silver-amAg(NH&+ @ Ag+ 2NHr monia complex ion. The silver ion concentration will, Cwl C'NH, K = for any ammonia concentration 13 M and smaller, be C*B(NH~)~+ great enough so that its product with that of the halide for which the instability constant, K, accordmg to the ion concentration always exceeds KA,x, the solubility classical data1 of Bodlander, has the value 6.8 X lo-%. product constant for the silver halide. In the absence of interfering ions or molecules, asTwo facts are evident also from the foregoing dissume a solution in which the concentrations of the com- cussion, i. e., either (1) increasing Ccl- to values explex ion and the ammonia are definitely and respectively ceeding the numerical value of eight (perhaps through 0.05 M and 15 M. addition of NHdC1) or (2) increasing CA,+ by actual addition of AgNOs would bring about precipitation of AgCl even i n the presence of 16 M a m o n i a and 0.06 M Ag (NHa),+! Another approach to the problem of solubility conTABLE 1 siderations is that of calculating the maximum quantity Molarity Values for Saturated Solutions o f the Sodium of halide ion that remains unprecipitated, i. e., in soluHalides tion, in the presence of 15 M ammonia and where the Solubility at %"C.' Approz. concentration of the complex ion is 0.05 M. Accord(g. saltllW g. mo2arity ingly, for chloride, SaU sald. s o h ) (caw.)
+
NaCl NaBr .2H20 NaI . 2 B O
26.47 48.6 64.75
The solubilities at 25T. are taken from S E ~ E L and L ATHERTON,"Solutilitie~of Inorganic and \letal Organic Compounds," 3rded.. 1). Vnn Sostmnd Comoanv. h e . . Sew Yurk. Vd. 1. 1940.
M and CI- = 1.13 In like manner, C B ~= - 2.33 X X M. One observes indirectly from these values that for the conditions specified, AgCl is extremely Now, consider a hypothetical experiment: If a solu- soluble, AgBr moderately soluble, and AgI practically tion saturated with NaCl a t 25'C. (the solution is insoluble. ' Ammonia concentrations of lower values 6.23 M according to Table 1) were made 0.05 M with would decrease the solubility of each of the silver respect to the silver-ammonia complex and 15 M with halides. respect to the NH8,would silver chloride precipitate? For greater simplicity no consideration has been It is well to emphasize here that equilibria involving made either of the solubility of the silver halides in the both the complex ion and th'e insoluble halide must be correspondmg sodium halid&. or of the common-ion considered. A previous calculatiaa reveals the concen- effect existing in each instance. The effects of the two tration of silver ion in the presence of our 15 M am- factors are, however, opposite in character, i. e., solumonia to be 1.5 X 10-". Assuming the effective con- bility increase of the silver halides from the former centration of chloride ion to be the same as the molarity factor versus decrease in solubility of the silver halides in of the salt, then water from the latter factor. Also, mention has not been made of the concepts of activity and of ionic C&+.Ccl-= 1.5 X lo-" X 6.23 = 9.35 X lo-" strength. It is the writer's belief that comprehension of which is slightly smaller than K A ~ nevertheless, ; factors and concepts, when included in subsequent AgCl is not precipitated even though saturated sodium courses, is probably easiest for the student if the inchloride was specified! structor's introductory approach has been of a nature Similarly, for a case involving saturated NaBr or suchasthatemployedhere. NaI, CA.+.CB.-= 1.5 X 10-1' X 6.85 = 1.09 X 10-lo LITERATURE CITED CAz+.Cl-= 1.5 X X 9.93 = 1.49 X 10-lo (1) ZUFFANTI,S., J. CXEM.Eke., 17, 433 (1940). (2) Monnmu, F. S., ibid., 7 , 2119 (1930). A more recent value dculated from solubility data, 6.05 X G. W.,%bid.,17, 345 (1940). 10-**0.07 X 10-8 is reported by P. F. Derr, R. M. Stockdale, (3) JOSTER, and W . C . Vosburgh, J . Am. Chm. Soc., 63,2670 (1941).
(4) DENBIGA, K . G., ibid., 18, 126-30 (1941).
Wise men are instructed b y reason; m e n of k s s u d e r s t a n d i n g , by ezperienee; the most ignorant, by necessity; and beasts, by nature.
