Articles

    The Chemistry Behind "Magic" Pens

    Kids, let¹s look at the cool chemistry that makes color-changing markers work. These special colored markers are used to make colorful masterpieces, and when they are drawn over with a special white marker the colors change. Let¹s be investigative about this and look at the science of this rather than the magic. You¹ll need a marker set, Q-tips, a 4 x 6" index card, white vinegar, a baking soda solution (dissolve 3 tsp of baking soda in a cup of distilled water), distilled water, 3 plastic cups, and a red cabbage leaf.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Place your index card the long way (vertically) on a table with the blank side up. With each of three magic pens, draw parallel, horizontal lines about 2 inches long and about 1/4 inch wide, separated by about half an inch or so. Add a fourth line by bunching up the red cabbage leaf and rubbing it until you see a streak of color on the card. Using the white color-change wand, draw a straight, vertical line (perpendicular), that crosses all four color lines. Label it "wand". Now pour a little bit of water, vinegar, and baking soda solution into three separate plastic cups. Using new cotton swabs for each solution, dip in and "draw" the wet swabs across all four color lines as before, labeling each one.

    Describe and compare the effects that the wand, water, acid (vinegar) and base (baking soda) solutions have on all of the colored lines. Are the pens sensitive to acids or bases? Here is one example of using the pens: draw a line with the color-change wand first, then draw across it perpendicularly with a blue marker. It¹ll be blue on either side of the wand path, but a different color (turquoise), where it crosses the path. The low pH color (blue) is observed where there is no base (ink from wand), and the high pH color of turquoise appears on the base¹s path. (Remember, high pH means basic and low pH means acidic).

    Here is the chemistry behind the "magic". Like many pens, the ink used here is composed of many parts. The magic pen ink contains deionized water, glycol as a humectant (substance that retains moisture), a non-sudsy detergent (to help spread liquid on a surface), citric acid (the acid in lemons and orange juice) to create a low pH, and a dye. The color-change wand contains water, glycol, and detergent, as well as a base (such as sodium hydroxide), and a reducing agent (such as sodium sulfite). Because of the base, it¹s pH is about 10-12 depending on the brand.

    Each colored marker actually has two separate dyes. One dye becomes colorless at high pH in the presence of a reducing agent. The other dye is not affected by either. When you draw with a colored marker, you see a particular color. When the color change wand is drawn over that color, one of the dyes disappears (turns colorless), leaving the other color for you to see. Some clever chemists have used simple chemical reactions based on acid-base and oxidation-reduction chemistry to make these wonderful pens work. 

    --------------

    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 2000

    ----------------

    References: C. Anderson and D. Katz, ChemMatters, American Chemical Society, 10/98, p. 4.