Ask A Scientist© Chemistry Archive

name        David
status      student
age         17

Question -  i'm looking for some ionic compounds or something that
can change color when exposed to air for a certain time. they also need
to be safe and non-toxic.
------------------------------------------------
Cobalt chloride will change in color from blue to pink as it absorbs water.
As a result, it is sprayed onto calcium sulfate to make indicating dessicant
(Drierite, tm of WA Hammond) This color change occurs as the dessicant
absorbs water from the air. (Color can be reversed by redrying the
dessicant in the oven - if I recall correctly, 3 hours at 160C)
Another ionic-chemistry trick is to expose a saturated solution of Ca(OH)2
to carbon dioxide in the air. The solution will change from clear/colorless
to hazy/white as the insoluble CaCO3 forms in solution.
A solution of FD&C Blue #1 will decolorize when exposed to sunlight (not
purely ionic, but gives a color change)
A solution of potassium iodide (colorless when freshly prepared) will
oxidize on exposure to light to form I(sub.3)(sup.-), or I2 complexed with
I-. This gives the solution a pale yellowish-brown tint. Alternatively,
addition of starch solution will make the I3- solution turn blue.
Particularly in small quantities, and if used in a safe, responsible manner,
these examples are all disposable or recyclable.
Tim  Spry
=========================================================