Redox Qualitative Analysis Puzzle

In this qualitative analysis applet there are four bottles labeled A, B, C, and D. All contain colorless solutions. The first part of the lab is to mix the reagents one after another in a certain order to produce an colorful sequence: the colorless solutions turn yellow when two are mixed, blue when a third solution is added, and back to colorless when the fourth solution is added. The second part of the lab is to identify the contents of each bottle. Each bottle contains one of the following reagents: potassium iodide (KI), sodium thiosulfate (Na2S2O3), hydrogen peroxide (H2O2), and soluble starch. Here are some hints:

1. Iodine (I2), in the presence of iodide at these concentrations, will appear yellow.

2. Starch makes a blue complex with iodine (I2).

3. Hydrogen peroxide is an oxidizing agent. It could, for example, oxidize iodide (I-) to iodine (I2).

4. Sodium thiosulfate is a reducing agent. It could reduce iodine (I2) to iodide (I-).

Exercise. First use the applet to determine the order of addition so that you can produce the colorless-yellow-blue-colorless pattern. Then devise a procedure that will always allow you to identify which reagent is in which bottle.

When using the Lab Bench, drag the droppers to the test tubes to add reagents. A tube will become red when it is ready to receive reagent. Test tubes can be dragged to the waste can to empty them.