View source for Invisible ink

=== Inks developed by chemical reaction ===

In most cases, these substance changes color when mixed with an [[acid]] or [[base (chemistry)|base]].

* [[Phenolphthalein]], commonly used as a [[pH]] indicator, turns pink in the presence of a base such as [[ammonia|ammonia fumes]] or [[sodium carbonate]].
* [[Vinegar]], is revealed by red cabbage water<ref>[http://chemistry.about.com/library/weekly/aa012803a.htm Red cabbage water] {{Webarchive|url=https://web.archive.org/web/20090124193908/http://chemistry.about.com/library/weekly/aa012803a.htm |date=2009-01-24 }}</ref> Vinegar contains [[acetic acid]] that affects the pH indicator in red cabbage water.  Vinegar may also be developed by heat.
* [[Ammonia]], developed by red cabbage water.
* [[Semen]], developed {{By what|date=November 2024}}.<ref>{{Cite web |date=28 October 1999 |title=''The Independent'' review of ''The Quest for C: Mansfield Cumming and the founding of the British Secret Service'' by Alan Judd |url=https://www.independent.co.uk/arts-entertainment/books/reviews/the-spymaster-who-was-stranger-than-fiction-737707.html |url-status=live |archive-url=https://web.archive.org/web/20190831124549/https://www.independent.co.uk/arts-entertainment/books/reviews/the-spymaster-who-was-stranger-than-fiction-737707.html |archive-date=2019-08-31 |access-date=2017-09-11 |website=[[Independent.co.uk]]}}</ref>
* [[Copper sulfate]], developed by [[sodium iodide]], [[sodium carbonate]], [[ammonium hydroxide]] or [[potassium ferricyanide]].
* [[Lead(II) nitrate]], developed by [[sodium iodide]].
* [[Iron(II) sulfate]], developed by [[sodium carbonate]] or [[potassium ferricyanate]].
* [[Cobalt(II) chloride]], developed by [[potassium ferricyanide]].
* [[Iron(III) sulfate]], developed by [[sodium sulfide]].
* [[Starch]], developed by [[iodine]] solution which turns starch dark blue and the paper light blue.
* [[lemon|Lemon juice]], developed by [[iodine]] solution (ink turns white, paper turns light blue).
* [[Sodium chloride]] (common table salt), developed by [[silver nitrate]].
* [[Cerium oxalate]] developed by [[manganese sulfate]] and [[hydrogen peroxide]]<ref>{{cite web |url=https://www.sciencedaily.com/releases/2006/11/061108103815.htm |title=Cold War Invisible Ink Secrets Unlocked |website=ScienceDaily |access-date=2014-06-10 |url-status=live |archive-url=https://web.archive.org/web/20140320203148/https://www.sciencedaily.com/releases/2006/11/061108103815.htm |archive-date=2014-03-20 }}</ref><ref>{{cite journal |doi=10.1021/ed2003252 |title=Invisible Ink Revealed: Concept, Context, and Chemical Principles of "Cold War" Writing |journal=Journal of Chemical Education |volume=89 |issue=4 |pages=529–532 |year=2012 |last1=MacRakis |first1=Kristie |last2=Bell |first2=Elizabeth K. |last3=Perry |first3=Dale L. |last4=Sweeder |first4=Ryan D. |bibcode=2012JChEd..89..529M }}</ref>