Unit 5: Principles in Qualitative Analysis

Course Code: CHM-DSC-252

Paper Name: Inorganic Chemistry - III (Organometallic and Analytical Chemistry)

1. Solubility Product and Analysis Principles
2. The Common Ion Effect in Analysis
3. Separation of Cations into Groups
4. Choice of Group Reagents
5. Handling Interfering Anions
6. Exam Focus: Tips and FAQs

1. Solubility Product and Analysis Principles

Qualitative inorganic analysis is based on the systematic identification of cations and anions present in a sample by utilizing differences in their solubility.

Solubility Product (Ksp)

For a sparingly soluble salt like AgCl, an equilibrium exists between the solid salt and its ions in a saturated solution.

AgCl (s) ⇌ Ag+ (aq) + Cl- (aq)
Ksp = [Ag+][Cl-]

Precipitation Rule: A precipitate forms only when the ionic product (IP) exceeds the solubility product (Ksp).

2. The Common Ion Effect in Analysis

The common ion effect is the suppression of the degree of dissociation of a weak electrolyte by the addition of a strong electrolyte containing a common ion.

Application in Analysis

Group II Precipitation: In the presence of HCl, the ionization of H2S is suppressed. This lowers the S²⁻ concentration so that only the very insoluble sulphides of Group II cations precipitate.
Group III Precipitation: NH4Cl is added to NH4OH to suppress the concentration of OH⁻ ions. This ensures that only the hydroxides of Group III (like Al³⁺, Fe³⁺) precipitate, leaving higher group cations in solution.

3. Separation of Cations into Groups

Cations are classified into groups based on their behavior toward specific reagents, allowing for systematic separation.

Group	Group Reagent	Cations Included	Precipitated As
Group I	Dilute HCl	Pb²⁺, Ag⁺, Hg₂²⁺	Chlorides
Group II	H2S gas in presence of HCl	Cu²⁺, Cd²⁺, As³⁺, Sn²⁺, Pb²⁺	Sulphides
Group III	NH4OH in presence of NH4Cl	Fe³⁺, Al³⁺, Cr³⁺	Hydroxides
Group IV	H2S gas in presence of NH4OH	Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺	Sulphides
Group V	(NH4)2CO3 in presence of NH4Cl	Ba²⁺, Sr²⁺, Ca²⁺	Carbonates

4. Choice of Group Reagents

The choice of group reagent depends on the solubility product of the compounds formed.

Specificity: Reagents are chosen so that they precipitate all members of the group while keeping members of subsequent groups in solution.
Order: Analysis must follow the numerical order (I to VI) because the reagents of later groups may also precipitate cations from earlier groups.

5. Handling Interfering Anions

Certain anions like fluoride, borate, oxalate, and phosphate are called interfering anions because they form insoluble salts with cations of later groups (IV, V, VI) in the alkaline medium of Group III.

Removal of Interfering Anions

Need for Removal: These must be removed after Group II and before Group III to prevent premature or incorrect precipitation.
Common Removal Method: Boiling the solution with concentrated HNO3 and subsequent chemical treatments (e.g., adding ammonium molybdate for phosphate) is used to eliminate their interference.

6. Exam Focus: Tips and FAQs

Exam Tips

Ksp and IP: Always state that IP > Ksp for precipitation to occur.
Reasoning Questions: Be ready to explain why NH4Cl is added before NH4OH in Group III (Common Ion Effect).
Interfering Anions: Memorize the list: fluoride, borate, oxalate, and phosphate.

Frequently Asked Questions

Q: Why is HCl added before H2S in Group II?
A: To decrease the sulphide ion concentration via the common ion effect, ensuring only Group II sulphides precipitate.

Q: What happens if interfering anions are not removed?
A: They will cause Group IV and V cations to precipitate along with Group III, leading to a failed analysis.

Q: What is the group reagent for Group V?
A: Ammonium carbonate [(NH4)2CO3] in the presence of NH4Cl and NH4OH.