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Question:

Cobalt glass colour of potassium as:

A) Crimson red
B) Lilac
C) Apple green
D) Brick red

I thought option B as true because on flame test, potassium gives lilac colour. But the given answer is option A. I can't understand, why they have given the answer as crimson red colour. Does cobalt glass changes the color of potassium flame?

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  • $\begingroup$ Hint: What wavelengths does potassium emit in flame? Find this out and modify your post. $\endgroup$
    – M. Farooq
    Sep 19 at 14:57
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    $\begingroup$ Cobalt glass has its Wikipedia page which is supposed to be already read by you. $\endgroup$
    – Poutnik
    Sep 19 at 15:08
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    $\begingroup$ I have searched in internet and there is no such type of question.And I have visited Wikipedia before posting this question but I couldn't able to get the answer.So that's why I have asked this question. $\endgroup$
    – Infinite
    Sep 19 at 16:23
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    $\begingroup$ Cobalt glass is used for removing the intense yellow lines of sodium that is present everywhere in small amount, and which prevents to see the color of potassium flames. Seen through a cobalt glass, the sodium flame color is invisible (entirely absorbed). And the potassium flame may be dim, but it is visible. $\endgroup$
    – Maurice
    Sep 19 at 16:45
  • $\begingroup$ ..and I would say it is lilac, not crimson red( even if who am I as a man to know colors? :-) ) // Search also youtube for potassium flame cobalt glass videos. $\endgroup$
    – Poutnik
    Sep 19 at 16:51
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Color is somewhat subjective, that is why people study wavelengths with atomic spectra rather than color. For elementary classes, this is fine. Certainly A is a wrong choice because I checked the transmission characteristics of cobalt glass,* the blue glass transmits both the violet and the deep red doublets of potassium. So, the flame color appears like lilac as you can see in various Youtube videos of flame tests with cobalt glass.

In low temperature flames like that of the Bunsen burner, K atoms emit 404.4, 404.7 (pure violet), and 766, 769 nm (really deep red, pretty hard to see in a pocket spectroscope). The resulting mix of emission color looks like the classical lilac potassium flame shown in textbooks.

*Ref: Frederick C. Strong III, Improving the Potassium Flame, J. Chem. Educ. 1969, 46, 3, 178 (behind paywall)

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