Expert: Dr. Henry Boyter, Jr. - 2/26/2006

Question
I was wondering if there was such a thing as reusable litmus paper. More specifically reusable litmus plastic. I am trying to design something that will change colors under acids and revert back to blank under neaturals.
thanks!

Answer
You have several things to consider.  

First, pH paper works by a reaction of the paper with acid or base to produce the indicated color.  Therefore, you need a reaction to undo the color change.  Then you need to know how to do the reverse reaction.  it will be much different when you are starting with paper exposed to pH=6 water and when you start with pH=1 water.  This is a big hurdle you have to overcome.

Second, the biggest hurdle in cost.  pH paper is darn cheap.  Just the cost of the person to recycle pH paper, would be much more than the cost of the paper itself.


One possibility is in your plastic idea.  You would need for the plastic to be a barrier for say a gel on the inside that would chenge color with a microscopic amount of exposure.  Then you might be able to wash out the color with a pH buffer.  I don't know how it would work, but the cost issue will kill you.  At best it would be a novelty.  In fact you can get a pH meter now for $30.