SCH4C                                     Introduction to Qualitative Analysis

Qualitative Cation Analysis
Lab Notes

     It is the intention of this unit to study the separation and identification of 21 of the most common cations and 8 of the most common anions.
Common Cations:  Ag1+, Hg22+, Pb2+, Hg2+, Bi3+, Cu2+, Cd2+, Al3+, Cr3+, Fe2+, Fe3+, Mn2+, Zn2+, Co2+, Ni2+, Ba2+, Sr2+, Ca2+, Mg2+, Na1+, K1+
Common Anions:  SO42-,  CO32-,  PO43-,  Cl1-,  Br1-,  I1-,  S2-,  NO31-
     Ions are usually identified by the characteristic reaction they show on the addition of a certain reagent.   This reaction may result in the formation of a precipitate or a coloured solution.  If each ion in the above list had a specific reagent with which it, an it alone, would react to form a precipitate or coloured solution then all that would be required for quantitative analysis would be a complete set of these reagents.  Each one would be added in turn to samples of the solution and it  would be noted for which reagents a positive reaction was observed.
      No such set of reagents exist.  Any reagent which gives a characteristic reaction with one cation or anion usually gives one with other cations or anions or its action is interferred with by the presence of other ions in solution.
     Before you can identify an ion it must first be separated from the other ions in solution.   The first step in the separation is to divide the ions in the above list into smaller groups by adding a reagent which will precipitate certain of the ions as insoluble salts of that reagent.   The precipitation of mixed salts is then filtered out and to the filtrate is added another reagent which will precipitate out another samll group of ions, and so on until all the ions have been separated into samll groups.  These small groups are now taken in turn and by appropriate manipulations each ion in the small group is separated and filtered.
     The choice of reagents to use for the precipitation of these small groups may be made afteeeer a study of the general solubility rules.  This has already been done for you but these should be memorized.
     The separation and identification of ions in a soluiton  provide an interesting illustration and application of acid-base solubility and complex-ion equilibriua.  It also provides us with an opportunity to learn a great deal about the descriptive chemistry of the elements.  There are a number of rther extensive schemes designed for laboratory work in qualitative analysis.  All of them involve the systems and principles you have studied up to this point.  One method of separating and identifying ions in solution is based on the differences in the solubilities o f their compounds.  The wide range of Ksp value is evidence of the differneces in the solubilities of the salts.
      By controlling he concentration of the H3O1+ ions and other reagents, it is possible to add a reagent which precipitates specific ions and leaves others in solution.

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Copyright 1997, 2002, 2003  Tom Stretton ( Updated August, 2002 ts.