|MadSci Network: Chemistry|
Thanks for the question. I cannot get specific about how the indicator paper is made because each manufacturer will do it differently, but I will give you some insight on the basic aspects of the process.
The paper is just standard cellulose-based paper without many additives. The paper is then soaked in the actual indicator chemicals and then dried. The indicators, I assume, are the same that are used in the typical chemistry laboratory to indicate the stopping point on titrations (i.e. Phenolphthalein and Methyl orange).
Although the indicators are in solid form on the paper, they must be wet to react and work, so it really is the same as doing a titration until the color of the solution changes.
Indicators are weak acids and bases whose ionized forms differ in color from the unionized form. This means that the chemicals are a different color after they react.
I found a site that uses vegetables to make homemade indicator paper. It looks like it works pretty well. Natural pH indicators. You can also try the Cabbage Juice pH Inidcator demo on this site.
I hope this helps you out.
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