Invisible Ink

The CIA has declassified a number of documents from the first World War, some detailing the nature of invisible inks used at that time. These documents are available in the Freedom of Information Act Electronic Reading Room at cia.gov

One recipe for invisible ink was as follows
"A solution of nitrate of soda and starch in water may be carried for example in handkerchiefs or starched collars, starched shorts or anything else starched. These things being laid in this solution and then ironed. The article thus treated is later on again put in water and a solution obtained which can be used for invisible writing. The best means for developing are iodate of potassium."

And the recipe for developing the ink using potassium iodate
"Iodate of potassium, 5 grams with 100 grams of water, 2 g of tartaric acid added."

The beauty of this recipe for invisible ink is that the ingredients would be quite readily available to the spy.
Nitrate of soda (sodium nitrate) could be found in lawn fertilizers.
Iodate of potassium (potassium iodate) could be found in disinfectants.

As all science students know,
starch + iodine solution → blue-black iodine-starch complexBut a person carrying around a bottle of iodine solution may have been a bit suspicious during the war.
Potassium iodate can react with tartaric acid in aqueous solution to produce potassium iodide solution. The iodide ions released can react further with the iodate ions to produce aqueous iodine solution (I_2(aq)):
5I^-_(aq) + IO₃^-_(aq) + 6H⁺_(aq) → 3I_2(aq + 3H₂O_(l)


Reference
http://www.washingtonpost.com/world/cia-recipe-for-invisible-ink-among-newly-released-wwi-era-documents/2011/04/19/AFn5Ej7D_story.html
http://foia.cia.gov/CIAsOldest/Secret-writing-document-one.pdf
http://foia.cia.gov/CIAsOldest/Secret-writing-document-two.pdf
http://foia.cia.gov/CIAsOldest/Secret-writing-document-three.pdf
http://foia.cia.gov/CIAsOldest/Secret-writing-document-four.pdf
http://foia.cia.gov/CIAsOldest/Secret-writing-document-five.pdf
http://foia.cia.gov/CIAsOldest/Secret-writing-document-six.pdf

Further Reading
Naming Ionic Compounds
Writing Ionic Formula
Molecular Mass (Formula Weight)
Percentage Composition
Oxidation and Reduction
Oxidation States (numbers)

Study Questions:

1. Write the formula for each of the following
   
   • sodium nitrate
   • potassium iodide
     
   • potassium iodate
2. Calculate the molecular mass (formula weight) for each of the following:
   
   • sodium nitrate
   • potassium iodide
     
   • potassium iodate
3. Calculate the percentage composition of each of the following:
   
   • sodium nitrate
   • potassium iodide
     
   • potassium iodate
4. What is the oxidation state (oxidation number) of iodine in each of the following:
   
   • potassium iodide
     
   • potassium iodate
   • molecular iodine
5. In the reaction between iodide ions and iodate ions, which species are being
   • oxidized
   • reduced
6. Write half-reaction equations for each of the reactions below:
   
   • iodide → iodine
   • acidified iodate → iodine