III. Qualitative chemical analysis of cations and anions

Never will the power and grandeur of nature be comprehended, should the spirit grasp only its parts, but not the whole.

Pliny the Elder (23-79), an ancient Roman scholar

Chapter 11

ANALYTICAL CLASSIFICATION OF CATIONS BY GROUPS. HYDROGEN SULFIDE (SULFIDIC)

ANALYSIS OF CATIONS

11.1. INTRODUCTION

The main goal of qualitative chemical analysis of cations and anions is identification (detection) of a particular ion, i.e., evidence of its presence or absence in an analyte. The best way to achieve that goal would be if a specific reaction was known for each ion, allowing to detect it in the presence of other ions, i.e., to use the fractional method of analysis. However, few such reactions are known. Usually, several ions, not an individual one, interact with the analytical reagent forming various reaction products, which complicate the unique identification of each individual ion.

Group reagents, which are used in a systematic analysis of cations and anions, are widely used in analytical chemistry. A group reagent reacts in a similar way with a whole group of ions. Thus, for example, chloride ions Cl^- are precipitated from aqueous solutions comprising silver Ag⁺, mercury(I) Hg₂²⁺, lead Рb²⁺, cations, and also a mixture of chloride precipitates AgCl, Hg₂Cl₂ and РbСl₂, i.e., using reactions with chloride ions, it is possible to remove the

whole group of cations from aqueous solution (lead cations are not completely removed, since lead chloride is noticeably soluble in water). Similarly, sulfate ions - SO₄^2- precipitate a group of cations from solutions - calcium Са²⁺, strontium Sr²⁺, barium Ва²⁺, lead Рb²⁺ as a mixture of sulfates, which are slightly soluble in water (sulfates CaSO₄, SrSO₄, BaSO₄ and PbSO₄.