SCH4C                                     Introduction to Qualitative Analysis

Initial Groupings of the Ions

The 21 cations broken down into five distinct groups.
Group 1.  The Silver Group
-  all these cations will form insoluble chlorides.
Ag1+
Hg22+
Pb2+
Group 2.  The Hydrogen Sulphide Group
- all these cations will form insoluble sulphides under acidic conditions.
Hg2+
Pb2+
Bi3+
Cu2+
Cd2+
Group 3.  The Ammonium Sulphide Group
- all these cations will form insoluble sulphides under neutral or basic conditions.
Al3+
Cr3+
Fe2+
Fe3+
Mn2+
Zn2+
Co2+
Ni2+
Group 4.  The alkali Earth Group
- all these cations will form insoluble carbonates.
Ba2+
Sr2+
Ca2+
Mg2+
Grup 5.  The Alkali Group
-all these cations are completely soluble in water.
Na1+
K1+

 
     Study the division in the above table and notice that they concur with the solubility rules.  With the above table in front of you, consider what would happen if hydrochloric acid were added to the solution containing all 21 cations.  The addition of this acid would add the Cl1-, chloride ion, to the solution and this would cause the cations with insoluble chlorides to precipitate out.  This precipitate could then be filtered out and examined for the presence of silver, mercury(I) and lead(II) ions.   These three cations constitute Group 1 and the group reagent is dilute hydrochloric acid.
      Another glance at the table will show that after having removed the Group 1 cations with hydrochloric acid, we adjust the acidity of the remaining solution to about pH = 2 and add hydrogen sulphide gas, to provide the sulphide ion, only the cations that react with sulphide ions in acid will precipitate out.  The precipitae will be the  Hg2+, Pb2+, Bi3+, Cu2+and Cd2+ ions. These ions constitute the Group 2 and the group reagent is hydrogen sulphide gas in an acidic environment.  (Lead(II) chloride)) is found in both groups because even though lead(II) chloride is insoluble in very cold water it is soluble enough at room temperature for some of the Pb2+ ions do filter through into the filtrate.  This means that there will be enough Pb2+ ion in solution to react with the sulphide ions and a precipitate of PbS will result.)
    The table also shows that the Group 3 will consist of Al3+, Cr3+, Fe2+, Fe3+, Mn2+, Zn2+, Co2+ and Ni2+  and the group reagent will again be hydrogen sulphide but this time the solution will first be made alkaline (basic) with ammonia before the addition of the reagent.
     After having filtered out the Group 3 precipitate it can be seen that the addition of a soluble carbonate, to provide the carbonate ion, (we use ammonium carbonate) will now precipitate the only ions left in solution which have insoluble carbonates; namely Ba2+, Sr2+, Ca2+and Mg2+ .  These ions constitute Group 4 and the group reagent is ammonium carbonate.
     The removal of the Group 4 precipitates leaves us with a solution which may contain Na1+and K1+  ions.  These two ions constitute Group 5.  There is no group reagent for this group as all of the common salts of the this group are soluble.

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Copyright 1997, 2002, 2003  Tom Stretton (stretton@ripnet.com) Updated August, 2002 ts.