Group 1 Insoluble Chlorides

In the second step, the acidic aqueous mixture containing the ranaining metal cations is treated with H2S, a weak diprotic acid that dissociates in two steps  

The concentration of ions in an H2S solntion is pH-dependent. At low pH (high H concentration) the equilibria shift left, minimizing the amount of available S . At high pH (low H+ concentration) the equilbria shift right, maximizing the amount of available S . 

Group 1 — Insoluble chlorides AgCl. Hg2Cl2, PbCl2 

At this stage, the solution is acidic (from the addition of HCl in the previous step), and the concentration of in solution is relatively low. Only the most insoluble metal sulfides (those with the smallest Ksp values) precipitate under these conditions. These include 

and Sb. If any of these metal cations are present, they precipitate out as sulfides. After the solid is separated from the liquid, the solution is ready for the next step. 

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Group 1. Insoluble chlorides Of the common metal ions, only Ag and... 

Group I consists of the only three common cations that form insoluble chlorides Ag+, Pb2+, and Hg22+. Addition of hydrochloric acid precipitates AgCI, PbCI2, and Hg2CI2. Cations in Groups II. III. and IV remain in solution, since their chlorides are soluble. 

Group II Pb2+, Cu2+, Hg2+, Cd2+, Bi3+, and Sn4+ After the insoluble chlorides have been removed, the solution is treated with H2S to precipitate the cations of group II as insoluble sulfides—PbS, CuS, HgS, CdS, Bi2S3, and SnS2. Because the solution is strongly acidic at this point ([H30 + ] 0.3 M), only the most insoluble sulfides precipitate. The acid-insoluble sulfides are then removed from the solution. 

The group I ions are predicted to precipitate as insoluble chlorides, as indicated in the following equations ... 

Cations of the first group form insoluble chlorides. Lead chloride, however, is slightly soluble in water and therefore lead is never completely precipitated when adding dilute hydrochloric acid to a sample the rest of the lead ions are quantitatively precipitated with hydrogen sulphide in acidic medium together with the cations of the second group. 

When dilute aqueous HC1 is added to a solution containing a mixture of the common cations, only Ag+, Pb2+, and Hg22+ will precipitate as insoluble chlorides. All other chlorides are soluble and remain in solution. The Group I precipitate is removed, leaving the other ions in solution for treatment with sulfide ion. 

Group 1 cations. When dilute HCl is added to the unknown solution, only the Ag, Hg2, and Pb ions precipitate as insoluble chlorides. The other ions, whose chlorides are soluble, remain in solution. 

You will study two groups of cations in this experiment. Group 1 cations form chloride salts that are insoluble in water. Be careful not to confuse these cations with the alkali metal cations from Group I of the periodic table which all form soluble salts. Group 1 includes Ag", Hg2 " or Hg(I), and Pb2+. They are separated from a mixture of cations because they precipitate white insoluble chlorides, for example, silver chloride, AgCl, when a small excess of Cl is added. 

Group I. Insoluble chlorides Of the common metal ions, only Ag, Hg2, and Pb form insoluble chlorides. When HCl is added to a mixture of cations, therefore, only AgCl, Hg2Cl2, and PbCl2 precipitate, leaving the other cations in solution. The absence of a precipitate indicates that the starting solution contains no Ag Hg2 +,orPb ... 

Group 2. Acid-insoluble sulfides After any insoluble chlorides have been removed, the remaining solution, now acidic, is treated with H2S. Only the most insoluble metal sulfides—CuS, 61283, CdS, PbS, HgS, AS2S3, Sb2S3, and SnS2—precipitate. (Note the very small values of for some of these sulfides in Appendix D.) Those metal ions whose sulfides are somewhat more soluble— for example, ZnS or NiS— remain in solution. 

In the scheme in Figure 18-7, about 25 common cations are divided into five groups, depending on differing solubilities of their compounds. The first cations separated, and Ag, are those with insoluble chlorides. The reagent... 

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