Precipitation in Qualitative Analysis of Solutions — Overview

Imagine your teacher hands you a solution. They ask you to remove all the metal solutes. What would you do? You might panic. But, it's okay! Maybe, you're considering selective precipitation. You're on the right track! But, what's the problem? You don't know what ions are in the solution. How could you design an experimental procedure for unknown metals? You need to manipulate the differing solubilities.

We've been studying precipitation reactions. So far, we've been doing a lot of quantitative calculations. We used the solubility constant to calculate ion concentrations and the precipitate's mass. But, that doesn't help if we have unknowns. We need to use the qualitative solubility rules. By taking advantage of exceptions to the rules, we can precipitate many common metals.

Procedure for a Qualitative Solubility Analysis

1. Add 6.0 molar hydrochloric acid. Most halogen-metal compounds are soluble. But, there are a few exceptions. Our first goal is to remove insoluble chlorides and lower the pH. Adding hydrochloric acid will precipitate silver (Ag), lead (Pb), and mercury (Hg). We distinguish between them based on their physical and chemical properties. Anything that precipitates, we filter from the solution.

2. Now, our solution is acidic. So, we can take advantage of acid-insoluble sulfides. Add 0.2 molar hydrogen sulfide. The insoluble sulfides are copper (Cu), bismuth (Bi), cadmium (Cd), lead (Pb), mercury (Hg), arsenic (As), tin (Sn), and antimony (Sb).

3. Third, we want to remove any basic insolubles. So, we add a strong Arrhenius base to raise the pH to 8. The base-insoluble hydroxides are aluminum (Al) and iron (Fe). Then, we add di-ammonium sulfide, $(NH_{4})_{2}S$. The base insoluble sulfides are chromium (Cr), zinc (Zn), nickel (Ni), and manganese (Mn).

4. By now, we've removed the common transition metals and metalloids. So, our solution should only contain alkali and alkaline earth metals. To remove the alkaline earth metals, we add di-ammonium hydrogen phosphate, $(NH_{4})_{2}HPO_{4}$. The insoluble phosphates are calcium (Ca), magnesium (Mg), and barium (Ba).

5. Finally, we should have only alkali metals. Almost all the alkali salts are soluble. So, we typically have to test for each individual metal. Previously, we learned some qualitative tests. For example, we can use a flame test. The flames test excites the valence electrons. As the electrons relax, they release visible light. For example, sodium ions are bright yellow, and potassium ions are violet. There are also quantitative tests we can do. For example, an analytical chemist could use spectroscopy.

(Video) Solubility and Qualitative Analysis

By Tamara Allen

This video is a step-by-step walkthrough of qualitative solubility analysis. Tamara Allen starts by showing you how we think through the various steps. Then she performs an experimental investigation. So, you get to see both the theoretical and practical parts of this technique. This is an excellent video for learning qualitative solubility analysis.