Determining the Solubility Rules of Ionic Compounds

Overview

Source: Laboratory of Dr. Neal Abrams — SUNY College of Environmental Science and Forestry

An ionic compound's solubility can be determined via qualitative analysis. Qualitative analysis is a branch of analytical chemistry that uses chemical properties and reactions to identify the cation or anion present in a chemical compound. While the chemical reactions rely on known solubility rules, those same rules can be determined by identifying the products that form. Qualitative analysis is not typically done in modern industrial chemistry labs, but it can be used easily in the field without the need of sophisticated instrumentation. Qualitative analysis also focuses on understanding ionic and net ionic reactions as well as organizing data into a flow chart to explain observations and make definitive conclusions.

Many cations have similar chemical properties, as do the anion counterparts. Correct identification requires careful separation and analysis to systematically identify the ions present in a solution. It is important to understand acid/base properties, ionic equilibria, redox reactions, and pH properties to identify ions successfully.

While there is a qualitative test for virtually every elemental and polyatomic ion, the identification process typically begins with knowing a "class" of ions being analyzed; cations or anions, elemental or polyatomic, groups or periods, transition or main group. In this experiment, both types of ions, cations and anions, are identified. The cations include polyatomic ions as well.

Procedure

1. General Methods

Preparing for Qualitative Analysis
1. Reactions are generally done in small test tubes with volume of 5 mL or less.
2. Solutions need to be fully soluble and should be relatively dilute, typically ~0.1 M.
3. Reagents should be slowly added drop-wise and observed carefully.
4. Several common "test solutions" are required to establish solubility rules or identify an unknown ion. These contain ions known to react specifically with certain chemical speci

Application and Summary

The reactions shown here can be used to identify the presence of a class of cations or anions or be used very specifically for a certain ion. Because two reagents are used in the analyses, either reagent can be typically detected using the other. For example, instead of analyzing for the presence of chloride using silver ion, silver ion can be identified using chloride. A combination of common rules of precipitation followed by specific colorimetric or precipitation tests can be used to p

References

Eaton, A. Standard Methods for the Examination of Water & Wastewater. Centennial ed. Washington, DC: American Public Health Association (2005).