Analytical Chemistry PHCMp 101

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Presentation transcript:

Analytical Chemistry PHCMp 101 Lab 4 Identification of Group IV Cations Group IV Cations

І ΙΙ ΙΙΙ ΙV V VІ Group reagent Ions Formulae of precipitates Grp Group reagent Ions Formulae of precipitates Distinguishing features І Cold dil HCl Ag+, Pb++,Hg2++ AgCl, PbCl2, Hg2Cl2 Chlorides insoluble in cold dil HCl ΙΙ H2S in presence of HCl (0.2-0.3 M) Subgroup IIA Cu++,Cd++,Hg++,Bi++, Pb++ Subgroup IIB Sn++, Sn4+, Sb3+, As3+, As5+ CuS, CdS, PbS, HgS, Bi2S3, SnS, SnS2, Sb2S3, Sb2S5, Precipitated as Sulfides in acidic medium (0.2-0.3 M HCl) ΙΙΙ NH4OH in Presence of NH4Cl Al3+, Cr3+, Fe3+ Al(OH)3, Cr(OH)3, Fe(OH)3 Precipitated as Hydroxides by NH4OH in presence of excess NH4Cl ΙV H2S in Presence of NH4OH Ni++, Co++, Mn++, Zn++ NiS, CoS, MnS, ZnS Precipitated as Sulfides by H2S from ammoniacal solution in presence of NH4Cl V (NH)2CO3 in presence of NH4OH &NH4Cl Ba++, Ca++, Sr++ BaCO3, SrCO3, CaCO3 Precipitated as Carbonates in alkaline medium by NH4OH in presence of NH4Cl VІ No particular reagent Na+, K+, NH+4 , Mg++ Different precipitated forms Ions not precipitated in previous groups

Group IV cations are: Zn2+ / Mn2+ / Co2+ / Ni2+ Zinc/Manganese/Cobalt/Nickel They are precipitated as sulphides: Zn2+ + H2S ZnS (s) White ppt. Mn2+ + H2S MnS (s) Buff ppt. Co2+ + H2S CoS (s) Black ppt. Ni2+ + H2S NiS (s) Black ppt. Group IV Cations

In the presence of NH4Cl & NH4OH Group Reagent is: H2S In the presence of NH4Cl & NH4OH forming insoluble sulphide salts Group IV Cations

The concentration of the [S-2] should be kept high! WHY? The solubility products of group IV sulphides are higher than those of group II sulphides i.e. group IV sulphides require higher conc. of S-2 for their precipitation. Group IV Cations

HOW? H2S 2H+ + S2- NH4OH OH- + NH4+ NH4OH is added to enhance the ionization of H2S as follows: H2S 2H+ + S2- NH4OH OH- + NH4+ NH4Cl is added to decrease the [OH-] by common ion effect to prevent the precipitation of the Mg(OH)2 (group VI). NH4OH OH- + NH4+ NH4Cl Cl- + NH4+ Group IV Cations

NH4Cl + xss conc NH4OH + K3Fe(CN)6 If Not, take a new portion sample + xss NH4Cl + xss NH4OH + xss H2S If black ppt. appears CoS or NiS New portion of the sample If White ppt. appears ZnS New portion of the sample White ppt: Zn2+ If Buff ppt. appears MnS New portion of the sample NH4Cl + xss conc NH4OH + K3Fe(CN)6 xss NaOH + 3 drops of H2O2 + Boil CH3COOH + K4Fe(CN)6 If Not, take a new portion Reddish brown ppt : Co2+ Brown ppt: Mn2+ NH4Cl + Conc NH4OH + Drops of DMG Pink ppt : Ni2+ Group IV Cations