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Experiment 10 Group III Cation Analysis Part II

Experiment 10 Group III Cation Analysis Part II. Valdosta State University. Important Announcements. There will be no laboratory lecture for CHEM 1212L on Wed Nov 7, Wed Nov 14, or Wed Nov 21 (Thanksgiving). The next laboratory lecture for Experiment #12 will meet on Wed, Nov 28.

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Experiment 10 Group III Cation Analysis Part II

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  1. Experiment 10Group III Cation AnalysisPart II Valdosta State University

  2. Important Announcements There will be no laboratory lecture for CHEM 1212L on Wed Nov 7, Wed Nov 14, or Wed Nov 21 (Thanksgiving). The next laboratory lecture for Experiment #12 will meet on Wed, Nov 28. The final exam for the lab will be at that same time, Wed Nov 28. Valdosta State University

  3. Purpose To determine which group III ions are present in an unknown solution. Valdosta State University

  4. Background Valdosta State University

  5. Background • For this experiment, the group III ions are Fe3+, Ni2+, Mn2+, Al3+ and Zn2+. • These ions initially precipitate as either metal sulfides (in an alkaline environment) or metal hydroxides. • This requires the chemist to generate a small quantity of sulfide ion to precipitate the metals. • A convenient source of S2- is thioacetamide, which decomposes when heated to give hydrogen sulfide (H2S) which yields S2- in chemical reactions. • A reagent that is made and consumed in the same flask is said to be produced in situ. Valdosta State University

  6. Background – Hydrogen Sulfide H2S(aq) + 2 H2O(l) D 2 H3O+(aq) + S2-(aq) • The addition of base to the second reaction consumes the hydronium ion and drives the reaction to the right, increasing the concentration of S2-(aq). Valdosta State University

  7. Background – Group III Separation Scheme HNO3 Valdosta State University

  8. Background – Group III Separation Scheme A – Preparation of Group III cations Group III unknown NH3, H2S, Heat • The group III ions are initially separated from the bulk solution by precipitation as either insoluble metal sulfides or hydroxides. NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS Group IV ions Ni2+(aq) + S2(aq) D NiS(s) (black) Fe2+(aq) + S2(aq) D FeS(s) (black) Zn2+(aq) + S2(aq) D ZnS(s) (white) Mn2+(aq) + S2(aq) D MnS(s) (pink) Al3+(aq) + 3 OH(aq) D Al(OH)3(s) (white, gel) Valdosta State University

  9. Background – Group III Separation Scheme A – Preparation of Group III cations Group III unknown NH3, H2S, Heat • Since iron has two common oxidation states, its chemistry in this step is more complex. • If iron(III) is present it is reduced to iron(II) and elemental sulfur in produced. NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS Group IV ions 2 Fe3+(aq) + H2S (aq)  2 Fe2+(aq) + S(s) + 2 H+(aq) Valdosta State University

  10. Background – Group III Separation Scheme A – Preparation of Group III cations Group III unknown NH3, H2S, Heat • Alternately, the iron(III) can combine with the hydroxide ion and precipitate as iron(III) hydroxide. NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS Group IV ions Fe3+(aq) + 3 OH-(aq) D Fe(OH)3 (rust color) Valdosta State University

  11. Background – Group III Separation Scheme B1 – Separation of Group III cations NiS, FeS, MnS, Fe(OH)3, Al(OH)3 • Following the precipitation, the metal ions are combined with acid to form the free (and soluble) metal ions. HCl, HNO3, Heat Waste Ni2+, Fe3+, Mn2+, Zn2+, Al3+ 3NiS(s) +8H+(aq) + 2NO3(aq)  3Ni2+(aq) + 2NO(g) + 3S(s) + 4H2O(l) FeS(s) + 2 H+ (aq)  Fe2+(aq) + H2S(aq) 3Fe2+(aq) + 4H+(aq) + NO3(aq)  3Fe3+(aq) + NO(g) + 2H2O(l) MnS(s) + 2 H+(aq)  Mn2+(aq) + H2S(aq) ZnS(s) + 2 H+(aq)  Zn2+(aq) + H2S(aq) Al(OH)3(s) + 3 H+(aq)  Al3+(aq) + H2O(l) Valdosta State University

  12. Background – Group III Separation Scheme B2 – Separation of Group III cations Ni2+, Fe3+, Mn2+, Zn2+, Al3+ • Aluminum and zinc ions are amphoteric. • This means that at high acid or base concentrations, these metals form soluble complexes, but precipitate at moderate pH. • Iron, manganese and nickel form insoluble hydroxides at high pH. NaOH Fe(OH)3, Ni(OH)2, Mn(OH)2 Al(OH)4-, Zn(OH)42- Fe3+(aq) + 3 OH(aq) D Fe(OH)3(s) (rust-color) Ni2+(aq) + 2 OH(aq) D Ni(OH)2(s) (green) Mn2+(aq) + 2 OH(aq) D Mn(OH)2(s) (light brown) Valdosta State University

  13. Background – Group III Separation Scheme B2 – Separation of Group III cations Ni2+, Fe3+, Mn2+, Zn2+, Al3+ • Aluminum and zinc ions are amphoteric. • This means that at high acid or base concentrations, these metals form soluble complexes, but precipitate at moderate pH. • Iron, manganese and nickel form insoluble hydroxides at high pH. NaOH Fe(OH)3, Ni(OH)2, Mn(OH)2 Al(OH)4-, Zn(OH)42- Al3+(aq) + 3 OH(aq) D Al(OH)3(s) (white, gelatinous) Zn2+(aq) + 2 OH(aq) D Zn(OH)2(s) (white) Valdosta State University

  14. Background – Group III Separation Scheme B2 – Separation of Group III cations Ni2+, Fe3+, Mn2+, Zn2+, Al3+ NaOH Fe(OH)3, Ni(OH)2, Mn(OH)2 Al(OH)4-, Zn(OH)42- Excess Acid Al(OH)3(s) + 3H+(aq) D Al3+ + 3 H2O(aq) Zn(OH)2(s) + 2H+(aq) D Zn2+ + 2 H2O(aq) Valdosta State University

  15. Background – Group III Separation Scheme B2 – Separation of Group III cations Ni2+, Fe3+, Mn2+, Zn2+, Al3+ NaOH Fe(OH)3, Ni(OH)2, Mn(OH)2 Al(OH)4-, Zn(OH)42- Excess Base Al(OH)3(s) + OH(aq) D Al(OH)4-(aq) Zn(OH)2(s) + 2OH(aq) D Zn(OH)42-(aq) Valdosta State University

  16. Background – Group III Separation Scheme C1 – Test for Mn2+, Fe3+, Ni2+ Fe(OH)3, Ni(OH)2, Mn(OH)2 • The precipitate is redissolved by adding acid to the precipitate. • The addition of nitric acid neutralizes the sodium hydroxide and regenerates the free cations. • There is no easy method which will allow Mn2+, Fe3+ and Ni2+ to be separated; therefore, the sample is divided. HNO3, KNO2 Fe3+, Ni2+, Mn2+ DivideSample Valdosta State University

  17. Background – Group III Separation Scheme C2 – Test for Mn2+ Fe3+, Ni2+, Mn2+ • If sodium bismuthate is added to a solution containing manganese(II), a redox reaction occurs resulting in the formation of the purple permanganate ion. DivideSample NaBiO3 MnO4-purple 14H+(aq) + 2Mn2+(aq) + 5BiO3-(s)  2 MnO4-(aq) + 5Bi3+(aq) + 7H2O(l) Valdosta State University

  18. Background – Group III Separation Scheme D1 – Separation of Fe3+ and Ni2+ Fe3+, Ni2+, Mn2+ • The nickel and iron ions can be separated by the addition of ammonia. • The increased pH causes the formation of the insoluble iron(III) hydroxide. • The nickel ion combines with ammonia to form a soluble complex ion, hexaamminenickel(II). DivideSample NH3 Fe(OH)3 Ni(NH3)62+ Fe3+(aq) + 3NH3(aq) + 3H2O(l)  3NH4+(aq) + Fe(OH)3(s) (brown) Ni2+(aq) + 6NH3(aq) D Ni(NH3)62+(aq) (blue) Valdosta State University

  19. Background – Group III Separation Scheme D2 – Test for Fe3+ Fe3+, Ni2+, Mn2+ • The presence of the iron(III) ion is confirmed by the addition of ammonium thiocyanate. • If iron(III) is present, a blood red solution forms. DivideSample NH3 Fe(OH)3 Ni(NH3)62+ HCl / NH4SCN Fe(SCN)63- blood red Fe3+(aq) + 6SCN-(aq) D Fe(SCN)63-(aq) blood red Valdosta State University

  20. Background – Group III Separation Scheme E – Test for Ni2+ Fe3+, Ni2+, Mn2+ • The presence of the nickel ion is confirmed by the addition of dimethylglyoxime. • Dimethylglyoxime combines with the nickel ion to form a complex which forms a strawberry red precipiate. DivideSample NH3 Fe(OH)3 Ni(NH3)62+ HCl / NH4SCN H2DMG Fe(SCN)63- blood red Ni(DMG)2strawberry red ppt. Ni(NH3)62+(aq) + 2 HC4H7N2O2(aq)  4NH3(aq) + 2NH4+(aq) + Ni(C4H7N2O2)2(s) (red) Valdosta State University

  21. Background – Group III Separation Scheme F1 – Separation of Al3+ and Zn2+ Al(OH)4-, Zn(OH)42- • Careful control of pH allows for the separation of aluminum and zinc ions. • The solution is made very slightly basic. • At these conditions, the aluminum ion precipitates as aluminum hydroxide. • The zinc ion remains in solution. HNO3 NH3 Al(OH)3 Zn(NH3)42+ Al3+(aq) + 3NH3(aq) + 3H2O(l) D NH4+(aq) + Al(OH)3(s) Zn2+(aq) + 4NH3(aq) D Zn(NH3)42+(aq) Valdosta State University

  22. Background – Group III Separation Scheme F2 – Test for Al3+ • A successful test for aluminum requires that the previous reactions and their pH control were properly performed. • If not, false positive tests result. • The test for aluminum requires the free aluminum ion to react with ammonia in the presence of a reagent called aluminon and form a red precipitate. • Be careful, if there is iron or zinc left in the sample, a red precipitate will form resulting in a false positive. Al(OH)4-, Zn(OH)42- NH3 HNO3 Al(OH)3 Zn(NH3)42+ NH3, aluminon Al(OH)3 aluminoncherry red ppt. Al3+(aq) + NH3(aq) + H2O + aluminon(aq) D NH4+(aq) + Al(OH)3aluminon(s) (red) Valdosta State University

  23. Background – Group III Separation Scheme F2 – Test for Al3+ Al(OH)4-, Zn(OH)42- • To confirm that the red precipitate is the aluminum complex, ammonium carbonate is added. • If the red color does not fade, aluminum is present. NH3 HNO3 Al(OH)3 Zn(NH3)42+ NH3, aluminon Al(OH)3 aluminoncherry red ppt. Valdosta State University

  24. Background – Group III Separation Scheme G – Test for Zn2+ Al(OH)4-, Zn(OH)42- • To test for the zinc ion, a solution of potassium hexacyanoferrate(II) is added to the test solution. • If zinc is present a white precipitate forms. • The exact color of the precipitate can vary depending on the presence of other ions. • If iron is present the color can change to yellow, green or blue. NH3 HNO3 Al(OH)3 Zn(NH3)42+ NH3, aluminon K4Fe(CN)6 Al(OH)3 aluminoncherry red ppt. K2Zn3[Fe(CN)6]2white ppt. Zn2+(aq) + K+(aq) + Fe(CN)64 (aq) D K2Zn3[Fe(CN)6]2 (s) Valdosta State University

  25. Background – Group III Separation Scheme Zn2+ Al3+ Ni2+ Fe3+ Mn2+ Valdosta State University

  26. Safety • Concentrated HNO3 causes severe burns to the skin which can take weeks to completely heal and also can cause serious eye damage. This chemical also destroys books and clothing. If you get any on you, wash the affected area(s) with copious quantities of water for ten minutes. • Sodium hydroxide causes severe eye damage! If you spill some in your eyes, wash the eyes for at least 15 minutes. Get immediate medical attention. • Concentrated ammonia (NH3) is corrosive. If you spill some on your skin, wash the affected area for five minutes. If the acids get in the eyes, rinse the eyes out at the eyewash station for fifteen minutes and get prompt medical attention. In addition, the vapors of concentrated ammonia are very irritating. • Other 6 M acids can cause burns. If you spill some on your skin, wash the affected area with water for five minutes. • Thioacetamide should always be handled in solution. Carefully wash any of the spilled reagent from your skin. Avoid breathing the toxic fumes of H2S which are evolved from this reagent. Valdosta State University

  27. Waste Disposal Because the solutions used in this experiment contain ions that can hazardous to the environment and to human health, all solutions and precipitates used in this experiment must be placed in the container marked "Recovered Metals and Metal Ions". Valdosta State University

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