Qualitative salt analysis is the process of determining the number of constituents present in an unknown salt sample. The common ion effect is most commonly used in the salt analysis, i.e. qualitative analysis of basic radicals (in group separation). In qualitative salt analysis when a strong electrolyte with a common ion is added to a solution, dissociation of the weak electrolyte is suppressed to the required level. As a result, conditions are formed when one cation is preferentially precipitated while the other ions stay dissolved in the solution.
Any electrolyte may precipitate in specific circumstances where the ion concentration is greater than the solubility product (Ksp). This can only be obtained as the concentration of any one ion increases. Thus, the solubility product can be raised by the addition of a common ion to the level where precipitation results.
Metal ions are divided into groups based on the increasing solubility of related compounds such as chlorides, sulfides, carbonates, and hydroxides. Solubility product increases from group I to group V.
I. Precipitation of Group I metal ions
The metals i.e., Hg+, Ag+, and Pb 2+ are included in group I. Dil. HCl is used as the group reagent
The precipitating reagent of Group I is a dilute solution of hydrochloric acid, and the ions precipitated as their chlorides.
The net ionic equations are:
Ag+ + Cl– → AgCl(s) (white,)
2Hg2+ + 2 Cl– → Hg2Cl2(s) (white)
Pb2+ + 2 Cl– → PbCl2(s) (white)
When compared to the chloride of another group’s basic radicals, the solubility product of these metal chlorides is quite low. So, Ionic products quickly surpass this low-solubility product. As a result, these metal ions are readily precipitated when the HCl is added to the solution, whereas basic radicals of other groups are not.
II. Precipitation of Group II metal ions
Group II metal ions include Cu2+, Cd2+, Hg2+, Pb2+ , Bi3+, Sn2+ , Sn4+ , Sb3+, As3+ , and As5+ . Dilute HCl and H2S are used as the group reagent. When H2S gas is passed through the solution of these metal ions, these basic radicals precipitate as sulfides.
Pb 2+ + H2S ⇋ PbS(s) (black) + 2 H+
2 Bi 3+ + 3 H2S ⇋ Bi2S3 (s) (brown) + 6 H+
The sulfide precipitates of IInd group cations are having lower Solubility product (KSP) value while further group cation sulfides are having higher KSP values hence sulfides of II group get precipitated while sulfides of further group cations remain in solution.
If only H2S gas is supplied to the solution, the ionic product of the corresponding metal ions and sulfides not only exceeds the solubility product of group II metal sulfides but also exceeds the solubility product of group IIB metal sulfides. As a result, group IIIB metal ions as well as group IIA metals precipitate. To avoid the precipitation of group III B metal along with group II metal, H2S gas is passed through an acidified HCl solution.
Role of HCl
The concentration of sulfide ions in the solution has to be low enough so that the ionic product is only enough to exceed the solubility products of group II sulfide but not group III B metal sulfides to precipitate just group II metal ions but not group II B metal ions. Such a state should be produced by suppressing the dissociation of H2S with HCl. So, HCl suppresses the degree of ionization of H2S by operating the common ion effect.
II. Precipitation of group III A
Fe3+ , Al3+, and Cr3+ are included in group III A, for which NH4Cland NH4OH are used as group reagents. These basic radicals are precipitated in the form of their hydroxides when NH4OH is added to the solution.
I. Fe3+ + 3 NH4OH + NH4Cl → Fe(OH)3+3 NH4+
II. Al3+ +3 NH4OH + NH4Cl → Al(OH)3+3 NH4+
Since the hydroxide precipitates of III A group cations have lower Solubility product (KSP) values than the hydroxides of the following group cations, the hydroxides of the III A group precipitate while the hydroxides of subsequent group cations remain in solution.
Role of NH4Cl
If only NH4OH is added to the solution, the solubility product of group IIIA metal hydroxides as well as group IIIB metal hydroxides may also be exceeded by the ionic product of the corresponding metal ions hydroxides. As a result, group III B metal ions as well as group II A metal ions may precipitate. NH4OH is added in the presence of NH4Clto prevent the precipitation of group III B metal ions in addition to group IIIA metal ions.
The amount of hydroxide ions required to precipitate these cations is relatively small since the hydroxide precipitate of III A group cations has low Solubility product (KSP) Values. Thus, NH4OH is ionized in the presence of NH4Cl, producing the common ion NH4+ in the solution. The ionization of NH4OH is suppressed as a result of the common ion action of the NH4+ ion from NH4Cl, and as a result, only a small amount of OH ions are formed, which is sufficient for the complete precipitation of III A group cations.
III. Precipitation of group III B metal ions
Metal ions i.e., Zn++ , Co++, Ni++, and Mn++ are placed under this group. H2S in presence of NH4OH and NH4Cl acts as the metal reagent. The basic radicals are precipitated as their sulphides when H2S is passed through them.
Zn2+ +H2S + NH4Cl+ NH4OH → ZnS + 2 H+
The group IIB metal sulfides have a very high solubility product. A high ionic product is needed to surpass high solubility, and this is only achievable when the solution contains a lot of S2- ions. NH4OH is added to the solution to produce a significant amount of sulfide ions.
Role of NH4OH
NH4OH helps to increase the dissociation of H2S and thereby increase the concentration of S2-ions in the solution.
H2S is ionized as
H2S ⇋ 2 H+ + S2-
Similarly, NH4OH ionizes in solution as,
NH4OH ⇋ NH4+ + OH–
When the III B group is precipitated with H2S in the presence of NH4Cl and NH4OH, the OH ion from NH4OH interacts with the H+ ion from H2S to form a un dissociated water molecule.
H+ + OH– ⇋ H2O
As H+ ions are removed from H2S, free ionization occurs, producing excess S2- ions in solution, which increases the ionic product of sulfides of group III B metal ions. As a result, the ionic product of sulfides of metal ions of group III B exceeds their solubility product and hence these sulfides get precipitated.
III. Precipitation of group IV metal ions
Metal cations Ca2+, Ba2+, and Sr2+ are placed under group IV. The group reagent for these basic radicals is (NH4)2CO3 in presence of NH4Cl and NH4OH. The metals ions are precipitated as carbonate when (NH4)2CO3 is added to the solution
1) Ca2+ +(NH4)2CO3 + NH4Cl+ NH4OH — ———–→ CaCO3 + 2 NH4+
The ionic product of ions may also be more than the solubility product of MgCO3 if just (NH4)2CO3 is added to the solution. Consequently, group IV metal ions and MgCO3 may precipitate together. Before the addition of (NH4)2CO3, NH4Cl is added to the solution to prevent the precipitation of MgCO3 along with group IV metal ions.
The group IV cation carbonate precipitate has low KSP values. Consequently, it is necessary to use limited CO32- ions in order to precipitate group IV cations. When group IV cations are precipitated in the presence of NH4OH and NH4Cl, the NH4 +common ions prevent the ionization of (NH4)2CO3, which results in a small amount of CO32-ions that are sufficient for group IV cation precipitation.
Role of NH4Cl
NH4Cl suppresses the ionization of (NH4)2CO3 by operating the common ion effect.
In order to precipitate metal of only group IV metal ions, not Mg++ concentration of CO32- ion in solution is just enough to exceed the solubility product of group IV but not MgCO3. The concentration of CO32- ion should be reduced in order to create this environment by preventing (NH4)2CO3 from dissociation.
NH4Cl is a strong electrolyte and produce the large number of NH4+ ion in the solution as:
NH4Cl → NH4+ + Cl–
(NH4)2CO3 being a weak electrolyte dissociates partially as,
(NH4)2CO3 ⇋ 2 NH4+ + CO32-
Due to the presence of the common ion NH4+, the common ion effect occurs when (NH4)2CO3is added to a solution containing NH4Cl. This causes the decrease in ionization of (NH4)2CO3. The ionic product of group IV metal carbonates is just enough to surpass their solubility product under these circumstances. Since MgCO3has a high solubility product, the ionic product does not surpass it. Consequently, only group IV metals precipitated as carbonates.
Role of NH4OH
It converts ammonium bicarbonate present in ammonium carbonate solution to the ammonium carbonate as
NH4HCO3 + NH4OH → (NH4)2CO3 + H2O
The formation of water-soluble bicarbonates of Ca2+, Ba2+, and Sr2+ occurs when bicarbonate ions are present in the solution. As a result, there was a significant reduction in precipitation.
- Cotton F., Wilkinson, G., Advanced Inorganic Chemistry, Fifth Edition, John Wiley, 1988.