Using Sodium Hydroxide Solution to Identify Metal Ions

Instructor: Matthew Bergstresser
Sodium hydroxide is a base, and reacts with metallic ions. The results of these reactions can be used to identify the metal in the ionic compound. In this lesson, we will learn how to conduct these tests and how to interpret the results.

Testing For Metallic Ions

Often times when people visit the doctor, their blood is drawn to run tests. The doctor might be able to make some inferences from the color of the blood drawn, but a wealth of information is obtained when it is tested. These results can help the doctor diagnose any potential health issues in the patient. The same premise applies when testing for metal ions in a solution. Sometimes the color of the solution can give a hint at what metal ions are present, but executing a simple test can allow a chemist to positively identify which metallic element is present in a solution. Let's learn how to conduct this test, and how to interpret the results.

Light purple precipitate of cobalt(II) hydroxide

Sodium Hydroxide

Sodium hydroxide (NaOH) is a base that reacts with aqueous solutions containing metallic ions. Let's assume that all of our solutions are metallic sulfates. The sulfate ion (SO4 -2) is a spectator ion because it isn't involved in the reaction with sodium hydroxide, which means we can ignore it. The sodium ion in sodium hydroxide is also a spectator ion so we won't include it the equations of the reactions either.

The metallic ions bond with the hydroxide ion (OH-) and a precipitate is formed, which is a solid. Precipitates look like milk or milk dyed with food coloring when it is dropped into water. The type of precipitate formed helps identify the metallic ion present. Let's look at a few examples of how this works.

Aluminum, Calcium or Magnesium Ions?

A white precipitate is formed when sodium hydroxide is added to solutions containing aluminum ions (Al+3), calcium ions (Ca+2) or magnesium ions (Mg+2). The net ionic reactions, ignoring the spectator ions are:

  • Al+3 (aq) + 3OH-1 (aq) → Al(OH)3 (s)
  • Ca+2 (aq) + 2OH-1 (aq) → Ca(OH)2 (s)
  • Mg+2 (aq) + 2OH-1 (aq) → Mg(OH)2 (s)

Each metallic hydroxide has (s) after its formula, which indicates it is a solid. Since all of these precipitates are white, we have to do a second test to narrow down which metallic ion is in our original solution. All we have to do is add more sodium hydroxide, and if the precipitate dissolves, it is aluminum hydroxide. Calcium hydroxide and magnesium hydroxide won't dissolve upon the addition of more sodium hydroxide.

If the white precipitate doesn't dissolve upon the addition of more sodium hydroxide, we can do an additional test to determine if the metallic ion in the precipitate is calcium or magnesium. If we add ammonia (NH3) to the original solution and a white precipitate is formed, the metallic ion is magnesium. Ammonia will not cause any significant precipitate to form with calcium ions. A chart will help clarify this information.

Metallic Ion Color of Precipitate When NaOH is added Precipitate Dissolves with Additional NaOH? Precipitate Generated with Ammonia?
Aluminum (Al+3) White Yes Not Required For Identification.
Magnesium (Mg+2) White No Yes, a white precipitate that dissolves upon addition of more ammonia.
Calcium (Ca+2) White No Very slight, if any.

Copper(II) Ions?

A light blue solution hints that it is an aqueous copper(II) sulfate solution. To be sure it contains copper(II) ions (Cu+2), we can add sodium hydroxide.

Cu+2 (aq) + 2OH-1 (aq) → Cu(OH)2 (s)

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