12 Inorganic Qualitative Analysis (QUAL) Because the equilibrium constants are close to offsetting each other, the combined reaction: Cu(OH)2 (s)  +  4NH3(aq) Cu(NH3)42+(aq)  + 2OH–(aq) will dissolve the insoluble Cu(OH)2 with a small excess of NH3.  Similarly acid will dissolve the precipitate: Cu(OH)2 (s)  +  2H+(aq) Cu2+(aq)  +  2H2O(l) 4.    Co(OH)2 can exist as either a blue or a pink precipitate. The blue form is believed to be more finely dispersed. It changes to the coarser pink form that is more stable. Air will oxidize Co(OH)2 to Co(OH)3, which is a black/brown precipitate. If students do their cobalt ion tests during the same period, using fresh precipitates, the presence of Co(OH)3 can be ignored. As with copper ions, it is possible to form the amine complex of cobalt ions. The equations follow: Co(OH)2 (s) Co2+(aq)  +  2OH–(aq) Ksp   =  12.5x10–16 Co2+(aq)  +  4NH3(aq) Co(NH3)42+(aq) Kf   =  8 x 104 Because the relative sizes of the equilibrium constants favor the formation of insoluble Co(OH)2 over Co(NH3)42+, the precipitate will not dissolve unless largeexcessesofammoniaarepresent.Thus,incontrastwithCu(OH)2,Co(OH)2 does not dissolve. This gives a basis for separating and identifying Cu2+ and Co2+. 5.    Adjusting the pH of solution is not much of a problem in doing these tests. However, if an unknown or a combination of ions used to try out separations is used, pH control becomes more critical. If the 0.3 M solutions are too concentrated  to  make  the  pH  adjustments,  suggest  that  students  make dilute solutions for this purpose. Answers to Implications and Applications 1.    Add NaOH to the solution, and, if Ca2+ is present, a white precipitate will form. Wash the precipitate, save the solution liquid, and add Na3PO4 to the solution liquid. The formation of another white precipitate confirms the presence of Ca2+. 2.    Add potassium ferricyanide and nitric acid solutions to the solution. A yellow precipitate confirms the presence of Zn2+. Centrifuge and decant the solution liquid into another test-tube, and add Na3PO4 solution. If a white precipitate forms,Al3+isconfirmed.Ifbothofthesetestsproducetheexpectedprecipitate, then both ions are present. 3.    Addition of NaOH to this solution will result in the formation of precipitates of both Co(OH)2 and Cu(OH)2. Cu(OH)2 is soluble in concentrated NH3, and theCu(NH3)42+ionformsadarkbluesolution.Ifalltheprecipitatedissolves, then Co2+ was not present. If the Cu(OH)2 dissolves and there is a dark precipitateremaining,thenthepresenceofCo2+isconfirmedbythatCo(OH)2 precipitate. 4.    See the following flow chart. Alternatives: Can dissolve Co(OH)2 in acid and confirm with K3Fe(CN)6 or Na3PO4. Can use a portion of the unwashed NaOH precipitate to look for Ca2+ by dissolving in acid, precipitating Cu2+ and Co2+ with K3Fe(CN)6 and then using Na3PO4 to look for Ca2+.