August 19 2012 1 19 /08 /August /2012 02:28
There are several ways to make invisible ink, but I decided to try a dye that responds to changes in pH. Phenolphthalein is a white crystalline substance which is soluble in alcohol. Solutions of it are colorless at a pH below 8.2 and magenta or pink at a higher pH. (Wikipedia says that it turns colorless above pH 13 as well.) Sodium carbonate solutions have a pH of around 11.6 for a dilute solution, making it the perfect trigger for phenolphthalein. (It is easily made by heating baking soda or sodium bicarbonate in an oven at 300 degrees F.)
To make the invisible ink, I was originally planning to paint the phenolphthalein solution on with an eyedropper. Revealing it would be done by a spray mist bottle filled with sodium carbonate solution. However, my eyedroppers were dirty and we had no excess spray bottles, so I decided to use paint brushes. I used a fine tipped paint brush for the phenolphthalein solution and a 1 inch brush for the sodium carbonate solution. Phenolphthalein dissolved in isopropyl alcohol is delicately painted onto paper in the form of writing. It dries in about 30 seconds. Then sodium carbonate solution is swiped over the paper until the invisible writing shows itself. The writing stays visible after drying.
Here is a video demonstration:
Only tiny amounts of chemicals are used, making this a good invisible ink for those already possessing phenolphthalein. Those who do not possess it should look to other compositions, some of which may be discussed here at a later date.