Acid-base equilibria
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What colour does screened methyl orange indicator turn when in 1) alkali 2) acid and 3) neutral?
Screened methyl orange is ordinary methyl orange to which a blue dye, xylene cyanole, has been added.
The idea is to shift the colours into another part of the spectrum to tackle the problem some people have with partial colour-blindness affecting their perception of red and yellow. The acid colour becomes purple, the alkali colour becomes green, and the equivalence colour is a sort of muddy grey colour in between (this is not neutral because the pKa of methyl orange is 3.7). Personally, I have never thought this reasoning very satisfactory because the eye is still relying on the distinction between red and yellow. In any case I have yet to meet anyone who has trouble with methyl orange but finds screened methyl orange acceptable!
Ulex
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updated: 21 August 2003
