Chemistry 142 Grossmont College 13–81EXPERIMENT 13 Cation Group III: The Basic Insoluble SulfidesBackgroundThe cations of this analytical group are those that form insoluble sulfides or hydroxides in a buff-ered ammonia solution saturated with H2S. The common cations we will look at from this group are nickel (II), iron (III), cobalt (II), chromium (III), and zinc (II).A combination of NH3, NH4C1, and thioacetamide is used to precipitate this group. The sulfides of this group have large enough solubilities that they do not precipitate under the 0.3 M HCl plus thio-acetamide used to precipitate Cation Group II. Under these conditions, the Fe (III) and Cr (III) pre-cipitate as hydroxides and the Co (II), Ni (II), and Zn (II) precipitate as sulfides. Table 13.1 on page 81 gives solubility product values for these salts.Precipitation of the GroupUnder the basic condition that H2S is being generated, it rapidly converts to sulfide ion by reacting with the ammonia present.2NH3 + H2S → 2NH4+ + S2–(EQ 13.1)It is sulfide ion that is the precipitating agent for the insoluble sulfides. For example,TABLE 13.1 Ksp Values for Group III PrecipitatesSaltKspCoS5.9×10–21NiS3.0×10–21ZnS1.1×10–21Cr(OH)36.7×10–31Fe(OH)36.3×10–38Cation Group III: The Basic Insoluble Sulfides13–82 Chemistry 142 Grossmont CollegeCo2+ + S2– → CoS(s)(EQ 13.2)The Fe (III) and Cr (III) react with the aqueous ammonia and precipitate as hydroxides. For exam-ple,Fe3+ + 3NH3 + 3H2O → Fe(OH)3(s) + 3NH4+(EQ 13.3)Separation of the Nickel and Iron SubgroupsSeparation of Ni and Co from Fe, Cr, and Zn is based on the fact that NiS and CoS are very slowly soluble in dilute HCl, whereas the ZnS, Cr(OH)3, and Fe(OH)3 dissolve rapidly. In doing this step, it is important that the NiS and CoS not remain in contact with the HCl very long. Typical reactions for dissolving the iron-subgroup are:ZnS + 2H+ → Zn2+ + H2S (EQ 13.4)Fe(OH)3(s) + 3H+ → Fe3+ + 3H2O (EQ 13.5)Analysis of the Nickel SubgroupCoS and NiS readily dissolve in aqua regia, a mixture of HCl and HNO3 that is a strong oxidizing agent. It readily oxidizes sulfide ion to elemental sulfur, causing the metal sulfides to dissolve.CoS(s) + 4H+ + Cl– + NO3– → Co2+ + S(s)+ NOCl + 2H2O (EQ 13.6)The tests for Co and Ni can be made in the presence of each other so further separation is unneces-sary. Simply divide the solution in half. To test for Ni, the solution is made basic with NH3, forming the ammine complex.Ni2+ + 6NH3 → Ni(NH3)62+(EQ 13.7)Dimethylglyoxime is an organic compound that forms a bright red insoluble complex with Ni(II).Ni(NH3)62+ + 2H2C4H6N2O2 → Ni(HC4H6N2O2)2(s) + 2NH4+ + 4NH3(EQ 13.8)Cobalt (II) also forms a complex with dimethylglyoxime but it is brown in color and is soluble, so it does not interfere with the formation of the red precipitate with Ni.The test for Co involves its reaction with thiocyanate, SCN–, to form a soluble complex-ion which is a distinctive blue-green in 1:1 H2O:acetone solution.Co2+ + 4SCN– → Co(SCN)42–(EQ 13.9)If the separation of the Ni-subgroup from the Fe-subgroup is incomplete, any Fe (III) ions present will interfere with this test. Iron (III) reacts with thiocyanate to form a deep-red soluble complexFe3+ + SCN– → Fe(SCN)2+(EQ 13.10)This interference can be removed by addition of fluoride ions which form a more stable and color-less complex-ion with iron.Fe(SCN)2+ + 6F– → FeF63– + SCN–(EQ 13.11)Chemistry 142 Grossmont College 13–83BackgroundSeparation and Identification of IronThe separation of Fe (III) from the solution containing Fe (III), Cr (III), and Zn (II) is based on the amphoteric nature of the Cr (III) and Zn (II). Addition of excess NaOH results in precipitation of Fe(OH)3(s) and formation of hydroxo-complex ions with Cr (III) and Zn (II).Fe3+ + 3OH– → Fe(OH)3(s) (EQ 13.12)Cr3+ + 4OH– → Cr(OH)4–(EQ 13.13)Zn2+ + 4OH– → Zn(OH)42–(EQ 13.14)Hydrogen peroxide is also added at this step to oxidize the chromium complex to the yellow chro-mate ion, CrO42–2Cr(OH)4– + 3H2O2 + 2OH– → 2CrO42– + 8H2O (EQ 13.15)The Fe(OH)3 formed above is dissolved, using HCl.Fe(OH)3(s) + 3H+ → Fe3+ + 3H2O (EQ 13.16)The presence of the iron ion is confirmed by reaction with KSCN which forms a deep-red soluble iron-thiocyanate complex ion.Fe3+ + SCN– → Fe(SCN)2+(EQ 13.17)Identification of Chromium and ZincChromate and zinc can be tested for in the presence of each other, so the solution to be tested for these is divided in half.The solution to be tested for chromate is acidified with acetic acid and chromate is then precipitated as lead chromate, a yellow precipitate, by adding lead acetate solution. Lead acetate, Pb(C2H3O2)2, is one of the few covalent salts in solution.CrO42– + Pb(C2H3O2)2 → PbCrO4(s) + 2C2H3O2–(EQ 13.18)The solution to be tested for zinc is acidified with HNO3 which destroys the tetrahydroxozincate ion.Zn(OH)42– + 4H+ → Zn2+ + 4H2O (EQ 13.19)Addition of K4Fe(CN)6 then precipitates the zinc as a mixed complex salt, a pale yellow precipi-tate.Zn2+ + 2K+ + Fe(CN)64– → K2ZnFe(CN)(s) (EQ 13.20)Cation Group III: The Basic Insoluble Sulfides13–84 Chemistry 142 Grossmont CollegeProcedureFIGURE 13.1 Analytical Group III: The H2s Basic GroupTest a known solution simultaneously with your unknown. Do Steps 1, 2 and 6 in hood.STEP 1: Precipitation of Group III . To 10 drops of the solution to be analyzed, add 10 drops H2O, then 3 drops 1 M NH4Cl. Add 6 M NH3 until just basic to litmus. Add 2 drops 15 M NH3. Add 8 drops thioacetamide and heat 5 min in a water bath, stirring occasionally. Cfg and test the ppt (P7) as in step 2.NOTE: We are doing an abbreviated form of Group III. The complete Group III also includes A13+ and Mn2+. Time considerations require a shortened form of this groupNi2+, Co2+, Fe3+, Cr3+, Zn2+NH3, NH4Cl, H2SNiS, CoS, Fe(OH)3, Cr(OH)3, ZnSblack, black, brown, green, whiteHClNiS, CoSSNi(NH3)62+Ni2+, Co2+HCl, HNO3Co(SCN)2–blueNH3NH4SCN, acetoneDMGNi(C4H7N2O2)2redFe3+, Cr3+, Zn2+Fe(OH)3NaOH, H2O2Fe3+HClFeSCN2+redKSCNCrO42–, Zn(OH)42–PbCrO4yellowHC2H3O2, Pb(C2H3O2)2HNO3K4Fe(CN)6K2ZnFe(CN)6whiteChemistry 142 Grossmont College 13–85ProcedureSTEP 2: Separation of Subgroups. Wash ppt P7 with 15 drops H2O plus 1 drop 1 M NH4Cl. Cfg and discard the washings. To the ppt add 10 drops 1 M HCI, stir thoroughly. Cfg immediately and separate the solid (P8) from the solution (S8). Treat the ppt P8 with 4 more drops 1 M HCl, stir,