dissepearing ink EXPERIMENT
1.
Add 1 gram of thymolphthalein into 100 mL of ethyl alcohol. The
solution will require stirring to dissolve all of the powder.
2.
Add 900 mL of water to the solution and stir. Don't worry if the
solution looks white - that's because the thymolphthalein indicator is not
soluble in water. No worries, the next step will fix everything.
3.
Slowly add 10 mL of 3 molar sodium hydroxide to the solution to
turn the liquid a dark blue.
Caution: Sodium
hydroxide (commonly known as lye) is a caustic solution and must be handled by
an adult. Once the sodium hydroxide is diluted with water, the solution is safe
to use as disappearing ink, but care must be taken because the pH of the ink
solution is about 10 (basic). Be sure to wash your hands well with water after
handling the more concentrated solution.
Always test the
disappearing ink on a small piece of white fabric to make sure that it actually
disappears. In a few seconds, the ink stain will disappear. The color will
vanish more quickly if you apply a cotton ball dampened with vinegar or if you
blow on the spot (the carbon dioxide in your breath is the secret!). The pH of
the ink solution is 10-11, but after exposure to air the pH will drop to 5-6.
The damp spot will eventually dry. A white residue may be visible on some dark
fabrics. Be sure to store the disappearing ink solution in a sealed container.
All of the materials may be safely poured down the drain.
HOW DOES IT WORK?
The secret to making
the ink disappear is carbon dioxide in the air which reacts with the water in
the solution to form carbonic acid. The carbonic acid then reacts with the
sodium hydroxide in a neutralization reaction to form sodium carbonate. This
lowers the pH of the solution with the alcohol acting as an acid to turn the
indicator colorless and the ink stain magically disappears. The "fading
time" can be prolonged by adding a small amount (use drops to make these
adjustments) of sodium hydroxide. But care should be taken not to add too much
sodium hydroxide. Here's a fun fact... red disappearing ink can be made using
phenolphthalein (a very common acid-base indicator) in place of
thymolphthalein.
thymolphthalein
Type: HIn + H2O In- + H3O+
pK: 9.7
Approximate pH range for color change: 9.3-10.5
Color of acid form: colorless
Color of base form: blue
pK: 9.7
Approximate pH range for color change: 9.3-10.5
Color of acid form: colorless
Color of base form: blue
PREPRATION OF THYMOLPHTHALEIN
BY FREIDAL CRAFTS REACTION