How/why does using the cobalt blue glass work to help you identify potassium ions?
When a compound containing potassium is burned, potassium ions go to a higher energy state. When they "cool down", energy is given off in the form of a visible light that has a characteristic color. See the source that I listed below.
What the cobalt glass does is filter out yellow light, so that you will be able to see the violet color of the potassium ions.
(Here's the same question asked by someone else. Take a look at those answers: http://answers.yahoo.com/question/index?qid=20080127045723AABAKJK)
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