Sciencemadness Discussion Board

Cobalt Chloride - Making Invisible Ink??

miraglia - 12-8-2006 at 12:22

I would like to make an invisible ink that turns a deep pink/magenta when water is applied to the solution/page. Trying to figure this out as a fun lesson for my summer camp kids. This would essentially be a moisture indicator.

Does ayone know how to do this? From what I've gathered online, cobalt chloride is typically blue and turns pink when water is applied. But blue won't work for invisible ink.

Does anyone know how to make it colorless in it's dry state?

Thank you in advance...

guy - 12-8-2006 at 12:28

From what I know the dilute version is pale pink, so you are supposed to concentrate by warming it and it turns blue.

miraglia - 12-8-2006 at 12:39

Quote:
Originally posted by guy
...so you are supposed to concentrate by warming it and it turns blue.



Not sure what you mean. Could you explain a little more. I wanted to see how to start with colorless and end up pink... From your response, it looks like you've explained how to make it blue...

pantone159 - 12-8-2006 at 12:56

This isn't cobalt chloride, but there are some other possible 'invisible' inks that you develop by applying ferric ion solution.

Namely,
thiocyanate - turns blood-red upon development
ferrocyanide - turns deep blue
salicylate - turns red-purple (you can make this from aspirin)

guy - 12-8-2006 at 15:03

Quote:
Originally posted by miraglia
Quote:
Originally posted by guy
...so you are supposed to concentrate by warming it and it turns blue.



Not sure what you mean. Could you explain a little more. I wanted to see how to start with colorless and end up pink... From your response, it looks like you've explained how to make it blue...


No its supposed to be pink (which is kind of hard to see on white) and when you heat it it turns blue. Thats how it becomes visible.

not_important - 12-8-2006 at 20:55

And the pink of cobalt salts is generally not real intense, so it doesn't stand out.

phenolphthalein is colourless when neutral acidic, pink-red when alkaline. You can write with an alcohol solution of it, develop by spritzing it with sodium carbonate (washing soda), or household ammonia solution. With ammonia as the paper dries the writing fades away again.

Thymolphthalein work similarly, but gives a blue instead of red.

Methyl red plus citric acid is a fairly pale yellow, any base including sodium hydrogen carbonate (sodium bicarbonate, baking soda) or borax will turn it red.

indicators list
http://en.wikipedia.org/wiki/PH_indicator

list of pH vales
http://www.engineeringtoolbox.com/bases-ph-d_402.html

neutrino - 13-8-2006 at 08:14

Why not just use colored paper in the first place? Some purists would consider it cheating but it would work.

On that note, remember that many of the dyes used in colored paper are themselves pH-sensitive. I'm sure there is some way you could use that.