Qualitative Salt Analysis: Common Ion Effect

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.

Explanation

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 S^2- 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 S^2-ions in the solution.

H₂S is ionized as

H₂S ⇋ 2 H⁺  + S^2-

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 S^2- 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 CO₃^2- 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 CO₃^2-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.

Explanation:

In order to precipitate metal of only group IV metal ions, not Mg⁺⁺ concentration of CO₃^2- ion in solution is just enough to exceed the solubility product of group IV but not MgCO₃. The concentration of CO₃^2- 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⁺ + CO₃^2-

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.

References

1. Cotton F., Wilkinson, G., Advanced Inorganic Chemistry, Fifth Edition, John Wiley, 1988.
2. https://byjus.com/chemistry/common-ion-effect/#:~:text=The%20common%20ion%20effect%20is,(or%20the%20Equilibrium%20Law).
3. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Qualitative_Analysis.
4. https://www.researchgate.net/publication/346316520_Principle_of_Common-ion_Effect_and_its_Application_in_Chemistry_a_Review
5. http://www.jchemlett.com/article_113421_dda787767fd8d21427b4d5e1c32e8a90.pdf
6. https://resources.saylor.org/wwwresources/archived/site/wp-content/uploads/2012/07/Common-Ion-Effect.pdf
7. https://www.911metallurgist.com/blog/wp-content/uploads/2016/10/Common-Ion-Effect-and-solubility.pdf