By Tom Kuntzleman, Tori Talaski, and Quincy Banini
Want a great experiment to have your students perform during the Halloween Season? Try this one that involves an orange to black color change:
To learn how to carry out this reaction on your own, keep reading, and/or see the video further on below!
The orange-to-black color change seen above is reminiscent of the Old Nassau reaction, which requires the use of mercury (II) chloride. Unfortunately, the use of this reagent generally prohibits students from being able to conduct this experiment on their own. Also, some of the reagents in the Old Nassau reaction are difficult to obtain for folks that aren’t science teachers. However, in the experiment seen in the video above, the orange-to-black color change is accomplished using materials that are appropriate for students to use and also quite simple to obtain.
The initial orange color change is made possible by simply mixing red and yellow food dyes together in the reaction mixture to make orange. The dramatic color change to black is made possible by a series of reactions. The first of these is the reaction between iodide ion, (I-), and hydrogen peroxide, (H2O2), to form iodine (I2):
2 H+ (aq) + 2 I- (aq) + H2O2 (aq) --> I2 (aq) + 2 H2O (l) (Equation 1)
Starch reacts with I2 to form a black complex:
I2 (aq) + starch --> I2-starch complex (blue-black) (Equation 2)
Thus, if I- and H2O2 are mixed in the prescence of starch, a black colored mixture is immediately observed. To delay the appearance of the black color, ascorbic acid (C6H8O6) can be added to the initial reaction mixture. Ascorbic acid (also known as vitamin C) reacts with I2 to form I-:
C6H8O6 (aq) + I2 (aq) --> 2I- (aq) + C6H6O6 (aq) + 2 H+ (aq) (Equation 3)
By adding small amounts of ascorbic acid to the reaction mixture, the appearance of the black color can be delayed because the ascorbic acid consumes any I2 formed. The rapid removal of I2 by ascorbic acid (Equation 3) prohibits the formation of the black color (Equation 2). Once all of the ascorbic acid has been used up through Equation 3, any I2 formed will not be consumed and the black color appears. By adding varying amounts of ascorbic acid to otherwise identically prepared reaction mixtures, the time until the black color appears can be delayed to varying extents. A large amount of ascorbic acid produces a long delay, while a small amount yields a short delay.
What is interesting about this reaction is it can be made entirely from items found in the grocery store and pharmacy. Tincture of iodides (also known as decolorized iodine) is used as the source of I-, and can easily found in any pharmacy (Note: don’t use tincture of iodine in this experiment). Also, 3% hydrogen peroxide is used as the source of H2O2, vinegar is used as the source of acid (H+ in Equation 1), Fruit Fresh is used as the source of ascorbic acid, and laundry starch is used as the source of starch. All these items are readily available in most stores.
The video below describes how to carry out this reaction using these items.
In addition to setting up and carrying out the experiment, I find it useful to have students calculate the final concentration of many of the reagents in the mixture that changes color from orange to black. To calculate these concentrations, students must know that decolorized iodine comes at a concentration of 0.43 M I-, vinegar is usually 0.83 M acetic acid (in a 5% solution), and 3% hydrogen peroxide is about 0.88 M H2O2. The 100 mL of ascorbic acid solution made with Fruit Fresh is about 0.10 M ascorbic acid.
Once students calculate the concentrations of these reagents in the initial experiment, they can explore how the reaction rate changes upon varying the concentration of I-, acetic acid, and/or H2.
Shakhashiri, Chemical Demonstrations, volume 4, pp. 37-43.