what is the purpose of using cobalt glass in identifying a mixture of Na and K salts?

Answer 1

cobalt filters the yellow light corresponding to sodium, so the purple light of K can be observed (if not using the cobalt glass, the light would be yellow - orange)

Answer 2

Cobalt glass filters yellow light from sodium impurities

Answer 3

when you do the flame test the cobalt glass filters out the overwhelming sodium signal.