lect7_exp25c - Separation of Group III Cations The group...

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Unformatted text preview: Separation of Group III Cations The group III cations form insoluble sulfide or hydroxide salts in basic solution. They are distinguished from group II cations because these salts dissolve in acid. Al3+ Cr3+ Fe2+, Fe3+ Ni2+ Co2+ Zn2+ Chemistry 123 – Dr. Woodward Al3+, Cr3+, Fe2+, Fe3+, Ni2+, Co2+, Zn2+ H2O2 + HCl Al3+, Cr3+, Fe3+, Ni2+, Co2+, Zn2+ Evaporate to a paste NH4Cl acts as a buffer to keep the solution from becoming too basic H2O2 oxidizes any Fe2+ to Fe3+. The solution is acidic to prevent oxidation of Cr3+ and Co2+. Removes excess H2O2, be careful not to overdo it. Dissolve with H2O/HCl NH4Cl + NH3 Al(OH)3, Fe(OH)3, Cr(OH)3 Ni(NH3)62+, Co(NH3)62+, Zn(NH3)62+ Al3+(aq) + 3OH−(aq) ↔ Al(OH)3(s) white ppt white Ni2+(aq) + 6NH3(aq) ↔ Ni(NH3)62+(aq) blue blue Cr3+(aq) + 3OH−(aq) ↔ Cr(OH)3(s) gray-green ppt gray- Co2+(aq) + 6NH3(aq) ↔ Co(NH3)62+(aq) pink pink Fe3+(aq) + 3OH−(aq) ↔ Fe(OH)3(s) rust ppt Zn2+(aq) + 4NH3(aq) ↔ Zn(NH3)42+(aq) Chemistry 123 – Dr. Woodward colorless colorless 1 Why is the H2O2 pretreatment needed? Later on we will separate iron from chromium and aluminum based on the insolubility of its hydroxide salt. Consider the differences in solubility of Fe(OH)3 and Fe(OH)2: Fe(OH)2 Cr(OH)3 Al(OH)3 Fe(OH)3 Ksp = Ksp = Ksp = Ksp = 7.9 6.7 4.6 6.3 × × × × 10−15 10−31 10−33 10−38 This separation will not work if the iron is Fe2+. The hydrogen peroxide oxidizes any Fe2+ to Fe3+: 2Fe2+(aq) + H2O2(aq) + 2H+(aq) → 2Fe3+(aq) + 2H2O(aq) Chemistry 123 – Dr. Woodward Aluminum Subgroup Al(OH)3, Fe(OH)3, Cr(OH)3 NaOH + H2O2 Don’t leave solution in test tube overnight or Al3+ will leach from the glass Al(OH)4− Fe(OH)3 CrO42− colorless sol’n rust ppt yellow sol’n Cr(OH)3(s) + 3H2O2 + 4OH− ↔ 2CrO42−(aq) + 8H2O Al(OH)3(s) + OH− ↔ Al(OH)4−(aq) 1. Acidify 2. NH3 + NH4Cl Al(OH)3 white ppt CrO42− yellow sol’n Ppt should be white. If it is not clean it up or you may get a false test for Al3+ Chemistry 123 – Dr. Woodward 2 Nickel Subgroup Ni(NH3)62+, Co(NH3)62+, Zn(NH3)62+ Make sure to heat as directed. Don’t cut short heating time or separation of Zn may not work. NH3, CH3CSNH2 + heat NiS, CoS, ZnS The separations in this subgroup are not very clean. Pay attention to colors to get clues. Follow directions closely. Ni(NH3)62+ + HS− ↔ NH4+ + 5NH3 + NiS(s) black ppt NiS(s) Co(NH3)62+ + HS− ↔ NH4+ + 5NH3 + CoS(s) black ppt CoS(s) Zn(NH3)42+ + HS− ↔ NH4+ + 3NH3 + ZnS(s) white ppt ZnS(s) Cold 1 M HCl Zn2+ NH3, CH3CSNH2 + heat ZnS white ppt Make sure the ppt is white. If not you have Co and/or Ni contamination NiS, CoS Chemistry 123 – Dr. Woodward Nickel and Cobalt NiS (black ppt), CoS (black ppt) Dissolve in aqua regia (HCl + HNO3) NiCl42− (blue soln), CoCl42− (blue soln) NH3, HDMG NH4SCN, acetone Ni(DMG)2 Co(NCS)42− brilliant red ppt Blue soln Co contamination will form a brown ion. If observed add more HDMG Chemistry 123 – Dr. Woodward 3 Qualitative Analysis of a Simple Salt Experiment 26 Ag+, Pb2+, Bi3+, Cu2+ Al3+, Cr3+, Ni2+, Co2+ Zn2+, Sb3+/Sb5+ Sn2+/Sn4+, Fe2+/Fe3+ Cl−, Br−, I−, Br SO42−, SO32−, S2− PO43−, NO3−, CO32− Examples: Bi(OH)3, ZnCO3, Ni(NO3)2, CoS, AgCl, Cu3(PO4)3 CoS, AgCl, Your assignment is to identify both the cation and the anion Chemistry 123 – Dr. Woodward Initial Characterization • Note the color and physical appearance (lime green powder, shiny black crystals, wet transparent solid, etc.) • Test the solubility in water • Test the solubility in HCl, HNO3, NaOH & NH3 • Note the color of the solution when you test solubility – Some ions will be different colors than their salts (i.e. NiS is black, Ni2+ ions in aqueous solution are green, Ni(NH3)62+ is blue) – The appearance of a color in solution can be a clue that the salt is slightly soluble (even though not all of the solid dissolves) Chemistry 123 – Dr. Woodward 4 Solubility and your Anion Anion Solubility Exceptions Chloride, Cl− Soluble AgCl, PbCl2 Bromide, Br− Soluble AgBr, PbBr2 Iodide, I− Soluble AgI, PbI2, BiI3 Sulfate, SO42− Soluble PbSO4, Ag2SO4 Sulfite, SO32− Insoluble None Carbonate, CO32− Insoluble None Phosphate, PO43− Insoluble None Chemistry 123 – Dr. Woodward Colors of Ions in Solution Color Ions Color Ions Green Pink Co2+ Blue Cu2+, Fe2+, Cr3+, Ni2+ Cu2+, Co2+, Ni2+ Orange Cr2O72−, NO2, I2 Purple Cr3+, Co2+ Yellow Colorless Ag+, Pb2+, Bi3+ Sb3+, Sb5+ Colorless CrO42−, Fe3+, I2, Sxn− Sn2+, Sn4+, Al3+, Zn2+ Chemistry 123 – Dr. Woodward 5 ...
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