If you have not already done so, put on your goggles, gloves, and protective clothing.

Transfer 2.0 mL of water to each of two clean test tubes, and place those tubes in the rack.

Add 10 drops of Gram’s iodine to each of the two test tubes and swirl to mix the contents.

Working as quickly as possible, draw up a full pipette of the standard vitamin C solution and recap the centrifuge tube.

Add the vitamin C one drop at a time to the first test tube while swirling the tube, keeping track of the number of drops you’ve added. As you continue adding the vitamin C solution, the color of the contents of the tube gradually fades from brownish to a pale yellow.

Once the tube contents appear pale yellow, add a few drops of starch water to the tube. The solution should immediately turn dark, indicating that some free iodine is still present.