Qualitative Analysis for First year Lab folder 2

FOLDER 2
Please see the complete lecture

II & III-1
Experiment 2-3
Qualitative Analysis of Metal Ions in Solution
Introduction
It is extremely useful to know how to detect the presence of specific ions in an aqueous
solution. This type of analysis falls under the general category of analytical chemistry called
qualitative analysis, which addresses the question "What is in a sample?" The basis for a
qualitative analysis is the fact that ions will undergo specific chemical reactions with certain
reagents to yield observable products. For example, silver ion can be precipitated with
hydrochloric acid to yield solid silver chloride. Because many cations will not react with
hydrochloric acid in this way, this simple reaction can be used to separate ions that form
insoluble chlorides from those that do not. In fact, the qualitative detection of ions in a mixture
is commonly accomplished by a system of analysis in which precipitation reactions play a major
role. In addition, acid-base reactions, oxidation-reduction reactions, and the formation of
complex ions are often used in a systematic way for either separating ions or for determining the
presence of specific ions.
The qualitative analysis of ions in a mixture must follow a scheme that can be
summarized as follows: (1) Add reagents that exploit the more general properties of ions to
separate major groups of ions; (2) separate major groups into subgroups with reactions that will
distinguish less general properties; and (3) add reagents that will specifically confirm the
presence of individual ions. Following this general three-step plan, several different analytical
schemes have been devised that are based on various ways of defining major groups. In this
course, you will follow a scheme which separates cations into five major groups according to the
solubilities of their compounds:
Group I (Ag+, Pb2+, Hg2+) cations produce insoluble chlorides so they can be precipitated with
dilute HCl, while all other cations remain in solution.
Group II (Cu2+, Bi3+, Cd2+, Hg2+, As3+, Sb3+, Sn4+) cations produce very insoluble sulfides
(Ksp values less than 10-30) so they can be precipitated by low amounts of sulfide ion; this can be
achieved by adding an acidic solution of H2S.
Group III (Al3+, Cr3+, Fe3+, Zn2+, Ni2+, Co2+, Mn2+) cations produce slightly soluble sulfides
(Ksp values more than 10-20) so they can be precipitated by relatively high amounts of sulfide ion;
this can be achieved by adding a basic solution of H2S.
Group IV (Mg2+, Ca2+, Sr2+, Ba2+) cations, as well as all of the above groups, produce
insoluble carbonates so they can be precipitated by the addition of carbonate once the ions of the
first three groups have been removed.
Group V (Na+, K+, NH4+) cations do not precipitate with any of the above reagents.
For a successful qualitative analysis based on the above five groups, the order of reagent
addition is crucial and should follow the order listed above (that is, group I first, group II second,
and so on). For example, addition of carbonate first would precipitate everything but the group
V ions or addition of basic H2S first would precipitate all group II and group III ions.
Because much of qualitative analysis involves selective precipitation and then separation of
the two phases, it is convenient to represent an analytical scheme in the form of a flow chart. For
example, a simple flow chart is shown below for separating a known mixture of Ag+(group I),
Cu2+(group II), and Ca2+ (group IV) ions. The addition of each reagent (presented in order from
top to bottom) produces a solid phase and an aqueous phase that can be separated by centrifugation
or filtration.