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GEORGE L. GILBERT - Denison University Granville. Ohio 43023
Solubility and Complex Ion Equilibria of Silver(1) Species in Aqueous Solution Bassam Z. Shakhashiri, Glen E. Dirreen and Fred Jueraens University of isc cons inMadison, W153706
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Reagents are added in a specified order to a large beaker containing an aqueous solution of sodium carbonate. AgzC03, AgzOW3 or AgOH, AgC1, Ag(NHd:, AgBr, Ag(S203)z3-, AgI, Ag(CN); and AgzS are prepared. The sequential formation of each precipitate and complex ion is used to demonstrate how solubility of silver(1) compounds is related to K.,,values and the formation of soluble complex ions. An alternate procedure may be used. Materials Procedure A 600-ml beaker magnetic stirrer with stirring bar graduated cylinders, 10-ml,50-ml and 250-ml 200 ml distilled water 1mlO.l M Na2C03 10 ml0.l M AgN03 10 mlO.l M NaOH 30 mlO.1 M NaCl
Procedure B 11000-mlbeaker, preferably with approximate graduations 9 150-mlbeakers graduated cylinders, 10-ml,25-ml, 50-ml and 100-ml 10 glass stirring rods 800-ml distilled water 10 beakers of sufficient size to hold the followingsolutions (100-ml beakers are convenient): 4 mlO.1 M Na2C03 40 mlO.l M AgN03 35 mlO.l M NaOH 60 mlO.l M NaCl 100 mll.O M NH3 25m10.1 M NaBr 100 ml0.l M Na&03 15 mlO.l M KI 30 mlO.1 M KCN 10 mlO.1 M NazS or (NH32S Procedure A Place 200 ml of distilled water in the large beaker along with the stirring bar. Place the beaker on the magnetic stirrer and turn the power switch to "on". Add 1 ml of 0.1 M Na2C03 solution and allow for thorough mixing. Add 10 ml of 0.1 M AgN03 and note the formation of the white insoluble AgzC03 (the precipitate may be gray or brownish in color due to the
coprecipitation of hydrated silver oxide). Add 10 ml of 0.1 M NaOH and note the darkening of the precipitate color due to the conversion of AgzC03 to the less soluble AgzO.H20. Continue adding the specified reagents in the following order and note the changes: NaC1, NHa, NaBr, Na2S203,KI, KCN and NazS. Do not use HC1 solution as the source of C1- ion, or any other acidic reagents (see "Hazards"). Prolonged exposure of the insoluble silver(1)compounds to light will cause photochemical decomposition. This procedure is a modification of the procedure developed by Schwenck (I). Alternate Procedure B Place the nine 150-ml beakers in a row. Place 800-011 of distilled water in the 1000-ml beaker. Place a stirring rod in each beaker. The reagents should be premeasured into graduated cylinders or 100-mlbeakers. Pour 4 ml of 0.1 M NazC03 solution into the distilled water and stir. Pour 40 ml of 0.1M AgN03 solution into this and stir; note the change. (Note: be sure to add AnNOn . . solution to the NaqCO? . .. solution. not the reverse). P o w 100 rnl of this suspension into the first 150 ml beaker, using iherraduationson the beaker asaauide.'ro the suspension Cn thclOOO-ml beaker add 35 ml of 6.1 M NaOH solution, stir, and note the change. Pour 100 ml of this suspension into the second 150-ml beaker. T o the suspension in the large beaker add 60 ml of 0.1 M NaCl solution. Continue in this fashion, pouring off 100 ml of each suspension or solution into the next 150-ml beaker. This procedure aliows the d i s ~ l a vof ao~roximatelv100 ml of each susoension and so1ut;onprod;ced in the demonstration. Hazards Silver compounds when absorbed by skin will be reduced by tissues to produce dark stains. Contact with soluble silver comoounds should be avoided. (Stains of silver metal -mav - - -, ~ he -removed from skin by successive treatment, first with 1%iodine in 10%aqueous KI, and then with 10%sodium thiosulfate.) Ammonium hydroxide solutions may cause skin irritation. Be cautious in handling cyanide solutions, i.e., keep the solution basic and avoid skin contact. HCN isan extremely poisonous gas which is formed when cyanide containing solutions a r e acidified. ~
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Disposal At the end of the demonstration dispose of all the watersoluble chemicals by pouring down the sink and washing with a large volume of water. Before pouring cyanide solutions down the sinks make sure to run water down the sink to remove any acid solutions which may have been poured previously. The insoluble silver salts including Ag2S should be dissolved in 1.0 M cyanide solution. Large quantities of silver may be recovered by the procedure described by Bush and Diehl(2) and reused. Discussion
We use this demonstration in introductory college chemistry courses when solubility and complex ion equilibria concepts are discussed. Typically, this demonstration and Volume 57, Number 11. November 1980 1 813
discussion requires 20-35 min (or longer) of class time. The selection of procedure depends on the visual effect desired. The sequence of adding reagents is based on the relative solubility and stability of different compounds of silver(1).Each student is provided with a handout containing the following data ( 3 , 4 , 5 ) Substance AgCOz AgOH AeCl A~B=
Ad AgzS
Ag(NH3): Ag(S2O3)iAg(CN)i
Color white hrownblack white pale yellow yellow black Kf, 25'C 2.5 X 107 4.3 X 1012
1.0 X 1021
K, 25OC 6.2 x 10-12 6.8 X
Solubility, mole11 1.2 x 10-~ 8.2 X
1.6 X 10-lo 7.7 X 10-l3
1.3 - - x lo-5 -~ 8.8 X
1.5 X 10-I6
1.2 X
3.3 X
HA% HCO,'
4.4 X 10-18 K., 25'C 4.3 x lo-" 5.6 x lo-"
I t is pedagogically useful to point out that direct comparison of K., values for Ag2C03and AgCl does not show that Ag~C03 is more soluble that AgC1; students are reminded that K., values can be used as relative measures of molar soluhility only if the equilibrium constant expressions are of the same form. We place emphasis on learning the chemical formulas, color, and soluhility of the substances involved in this sequence. Students are asked to write net ionic equations for the changes observed (all in aqueous solution):
the precipitate may appear dark in color due to: C0;- + H20 e HCO,' + OH-
we note that
and
K = (K.,)(Kr) = (1.6 X 10-W2.5 X 107) = 4.0 X Students are shown how to calculate the molar solubility of AgCl in 1.0 M NH3 solution from the above data. Let x be the solubility of AgCl (molesfliter)
Take the square root of both sides (otherwise, expand and use the quadratic equation)
Hence, 0.056 moles of AgCl will dissolve in 1.0 l of 1.0 M NH3. Several problems can be given to students for homework. For example, calculate the minimum concentration of NH3 solution needed to dissolve 0.1 moles of AgCl per liter at 2 5 T ; or, what is the molar solubility of AgI in a 0.1 M KCN solution; or, show by calculation that AgBr is not soluble in 15 M NH3 solution. Other problems may be devised by students and instructors. Students in analytical chemistry courses may be asked to perform more difficult calculations such as calculating the molar solubility of Ag2S in 1.OM CN-; the procedure is the same as above, but the algebra is more complicated. Other variations can be developed by the reader. Acknowledgement
The equilibrium constant for each reaction can he calculated from the K.!, and Kr data shown above. For example, consider the reaction
We thank Derek Davenport of Purdue University for calling our attention to Schwenck's sequence. Also, we thank George Gilbert of Denison University for reviewing our original manuscript and for suggesting we modify other alternate procedures into what is now Procedure B. Both were visiting professors a t Wisconsin in 1977 and 1979, respectively. Literature Cited (1) khwenek,
to obtain the numerical value of K, multiply both numerator and denominator by [Ag+]
J.R., J.CHEM.EDUC.,36.45(1959).
(2) L3ush.K. J.,andH.Diehl,J.CHEM.EDUC.,56,5411919). (31 Weast, R. C.. "Handbmk of Chemistry and Physics." 54th Ed., Chemieal Rubber Company. Cleveland. Ohio. 1973, p. 8232. (41 Link*, W . F., "Solubility of lnorgsnic and Metal-Oqanic Compounds': 4th Edition. American Chemical Society, We~hingLon,1915. (5) Sillon. L. G.and Msrtoll. A. E.,"StabilityConstan~ofMetal-lonCompler8s,"Sp~ial Publication No. 25, Supplement No. L. The Chemical Society (London), 1911.
Catalogue of Curriculum Resources The "Catalogue of Curriculum Resources," published by The Science Teachers' Association of Ontario, lists 110 curriculum units for elementary through secondaryschool teachers and students in most areas of science. Each unit is briefly described. Grade level and cost are included. The catalogue is available free from the Science Resource Centre, Faculty of Zducation, Queen's University, Kingston, Ontario, Canada, K7L 3N6. 814 1 Jwrnalof Chemical Education
