| Introduction to Qualitative Analysis | |
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Identifying Anions and Cations
Qualitative analysis is used to separate and detect cations and anions in a sample substance. In an educational setting, it is generally true that the concentrations of the ions to be identified are all approximately 0.01 M in an aqueous solution. The 'semimicro' level of qualitative analysis employs methods used to detect 1-2 mg of an ion in 5 mL of solution.
First, ions are removed in groups from the initial aqueous solution. After each group has been separated, then testing is conducted for the individual ions in each group. Here is a common grouping of cations:
Group I: Ag+, Hg22+, Pb2+
Precipitated in 1 M HCl
Group II: Bi3+, Cd2+, Cu2+, Hg2+,
(Pb2+), Sb3+ and Sb5+, Sn2+ and Sn4+
Precipitated in 0.1 M H2S solution at pH 0.5
Group III: Al3+, (Cd2+), Co2+, Cr3+,
Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+
Precipitated in 0.1 M H2S solution at pH 9
Group IV: Ba2+, Ca2+, K+, Mg2+,
Na+, NH4+
Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M
(NH4)2CO3 solution at pH 10; the other ions are
soluble
Many reagents are used in qualitative analysis, but only a few are involved in nearly every group procedure. The four most commonly used reagents are 6M HCl, 6M HNO3, 6M NaOH, 6M NH3. Understanding the uses of the reagents is helpful when planning an analysis.
Common Qualitative Analysis Reagents
| Reagent | Effects |
|---|---|
| 6M HCl |
Increases [H+] Increases [Cl-] Decreases [OH-] Dissolves insoluble carbonates, chromates, hydroxides, some sulfates Destroys hydroxo and NH3 complexes Precipitates insoluble chlorides |
| 6M HNO3 |
Increases [H+] Decreases [OH-] Dissolves insoluble carbonates, chromates, and hydroxides Dissolves insoluble sulfides by oxidizing sulfide ion Destroys hydroxo and ammonia complexes Good oxidizing agent when hot |
| 6 M NaOH |
Increases [OH-] Decreases [H+] Forms hydroxo complexes Precipitates insoluble hydroxides |
| 6M NH3 |
Increases [NH3] Increases [OH-] Decreases [H+] Precipitates insoluble hydroxides Forms NH3 complexes Forms a basic buffer with NH4+ |
Among the most common reactions in qualitative analysis are those involving the formation or decomposition of complex ions and precipitation reactions. These reactions may be performed directly by adding the appropriate anion, or a reagent such as H2S or NH3 may dissociate in water to furnish the anion. Strong acid may be used to dissolve precipitates containing a basic anion. Ammonia or sodium hydroxide may be used to bring a solid into solution if the cation in the precipitate forms a stable complex with NH3 or OH-.
Complexes of Cations with
NH3 and OH-
| Cation | NH3 Complex | OH- Complex |
|---|---|---|
| Ag+ | Ag(NH3)2+ | -- |
| Al3+ | -- | Al(OH)4- |
| Cd2+ | Cd(NH3)42+ | -- |
| Cu2+ | Cu(NH3)42+ (blue) | -- |
| Ni2+ | Ni(NH3)62+ (blue) | -- |
| Pb2+ | -- | Pb(OH)3- |
| Sb3+ | -- | Sb(OH)4- |
| Sn4+ | -- | Sn(OH)62- |
| Zn2+ | Zn(NH3)42+ | Zn(OH)42- |
A cation is usually present as a single principal species, which may be a complex ion, free ion, or precipitate. If the reaction goes to completion the principal species is a complex ion. The precipitate is the principal species if most of the precipitate remains undissolved. If a cation forms a stable complex, addition of a complexing agent at 1 M or greater generally will convert the free ion to complex ion.
The dissociation constant Kd can be used to determine the extent to which a cation is converted to a complex ion. The solubility product constant Ksp can be used to determine the fraction of cation remaining in a solution after precipitation. Kd and Ksp are both required to calculate the equilibrium constant for dissolving a precipitate in a complexing agent.