Determining the Solubility Rules of Ionic Compounds

Trends in the solubility properties of ionic compounds can be used for the qualitative analysis of ionic solutions. When a compound is added to a mixture of ionic solutions, many products can form, each with different solubility properties. If only one product is insoluble, then it alone will leave the solution. By performing sequential reactions, ions in a solution can be systematically identified and isolated.

While a variety of analytical instruments exist for elemental analysis, the techniques are often time-consuming or require transporting samples between laboratories. Qualitative analytical techniques such as examining solubility properties are fast, accessible pre-screening methods for analysis.

This video will introduce the solubility properties of ionic compounds, demonstrate procedures for selectively precipitating ionic compounds, and introduce a few applications of qualitative analysis using solubility trends in industrial settings.

Ionic compounds are composed of a cation and an anion. When a reaction occurs between two different ionic compounds, the cation of one compound is electrostatically attracted to the anion of another, forming a new compound. The ions that do not participate in the reaction are called spectator ions, and are omitted from the net ionic reaction. When an ionic compound dissolves, they reversibly interact with solvent molecules, and the ions dissociate. If the interaction between an ion and the new counter-ion is stronger than between the ion and the solvent molecules, it will be more favorable for the product to be in the solid phase. The formation of solid product from solution is known as precipitation, and the solid is called the precipitate.

Ions can be selectively isolated from solution by inducing reactions with insoluble precipitates. To design these reactions, cations and anions are assigned to broad categories based on solubility trends. Cations are grouped by identifying the anion common to their insoluble reaction products, and anions are likewise grouped by common cations. Solutions of these common ions are used to test for these groups.

When separation is desired for ions belonging to the same group, specialized reagents or concentrated solutions can be used to induce selective reactions once the ions in that group have been isolated. These specialized reagents can also be used to confirm the identity of an isolated ion. Now that you understand the principles behind the qualitative analysis of ions, let's go through a technique for analyzing a solution for phosphate, followed by a procedure for separating a mixture of metal cations.

To analyze a solution for phosphate, first prepare dilute test solutions of aqueous calcium, ammonium orthomolybdate, and concentrated nitric acid. Then, place 5 mL of the unknown solution in a test tube.

Add the calcium solution dropwise to the unknown solution. The formation of a white precipitate could indicate the presence of calcium phosphate, or calcium carbonate. To verify the presence of phosphate, slowly add nitric acid to the test tube. Dissolution of the precipitate indicates that hydrogen phosphate has formed. The lack of gas bubbles indicates that no carbonate is present, as carbonate would have reacted with the acid to form carbon dioxide and water.

Finally, slowly add the ammonium orthomolybdate to the test tube. Ammonium phosphomolybdate forms as a yellow precipitate, confirming the presence of phosphate in the solution.

First, prepare dilute test solutions as listed in the text protocol. Obtain four test tubes and caps suitable for use in a centrifuge. Place a mixture of aqueous zinc, nickel, silver, and iron nitrates in one test tube. To begin separation, first slowly add dilute hydrochloric acid to the mixture, swirling gently. The white precipitate that forms is silver chloride. Continue adding chloride solution until no more precipitate forms.

Separate the liquid, or supernatant, and the solid silver chloride by centrifugation. Decant the supernatant into the second test tube. Wash the silver chloride three times with water and decant each wash into the second test tube. Next, add the sodium hydroxide solution dropwise to the second test tube. Three precipitates will form: white zinc hydroxide, yellow iron hydroxide, and green nickel hydroxide. Continue adding sodium hydroxide until the solid white zinc compound dissolves, forming the soluble zincate ion. Separate the zinc solution and the solid nickel and iron compounds by centrifugation, and then decant the solution into the third test tube. Wash the solids with water three times and decant each into the zinc solution.

Slowly add hydrochloric acid to the zinc solution in the third test tube until zinc hydroxide precipitates and then dissolves.

Then, add potassium hexacyanoferrate dropwise to the zinc solution to form potassium zinc hexacyanoferrate as a white precipitate. Now, to the test tube containing solid nickel hydroxide and iron hydroxide, slowly add ammonia to form the soluble blue nickel hexammine ion. Separate the nickel solution from the solid iron hydroxide by centrifugation and decant the nickel solution into the fourth test tube. Wash the iron hydroxide three times with water and decant the washes into the nickel solution. Then, slowly add dimethylglyoxime to the nickel solution to form nickel dimethylglyoxime as a red precipitate. To the solid iron hydroxide, carefully add concentrated hydrochloric acid to form a solution of ferric chloride. To confirm the presence of iron, add thiocyanate to form the deep red thiocyanatoiron cation.

The simplicity and speed of performing qualitative analysis of ions in solution makes this technique widely used in environmental chemistry and industry.

When water contains a high concentration of metal cations such as calcium or magnesium, it is called hard water. These metal cations can react with anions in the water such as carbonate to form chalky deposits that clog pipes or hot water heaters. Water hardness can be assessed by adding a carbonate solution to a water sample. White precipitate indicates high levels of calcium.

Phosphate is an important nutrient for many forms of life and is therefore used in both industrial and garden fertilizers, but an excess of phosphate can be detrimental, particularly in freshwater environments. Wastewater in residential and commercial areas can be tested for phosphates by adding nitric acid and ammonium orthomolybdate. Yellow precipitate indicates high levels of phosphates.

You've just watched JoVE's introduction to solubility rules for ions. You should now be familiar with the principles of ionic reactions, a few procedures for qualitative analysis of solutions, and some applications of qualitative analysis using solubility.

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