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Methyl orange, bromothymol blue and phenolphthalein indicators were mixed together to form a solution.

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Over what pH range would the solution be yellow?

I cannot understand the question. If two or more different indicators are mixed, do they still work as indicators? Because each their colours will blend with each other and result to formation of a new colour, how can we know what the color will be?

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  • $\begingroup$ The indicator molecules form their color by absorbing light, so they combine by the subtractive color mixing model. $\endgroup$ – R.M. Sep 29 '15 at 20:54
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In general, the chemistry of one indicator does not usually affect another as far as I know, but the colors will mix as you would expect. It gets tricky here because the question shows the colored ranges but leaves a blank in the transition pH ranges, but I would guess the answer they're looking for is from 3-7.5

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    $\begingroup$ They don't show the colors because in the transition region, because it's a mix of colors. The color of a solution of multiple chromophores is the (subtractive) combinations of their colors. For example, protonated bromothymol blue is yellow, and deprotonated is blue. At the pKa (~7.1), you have equal amounts of protonated and deprotonated forms, so the colors would mix just like you had equal amounts of unrelated blue and yellow chromophores/pigments. (i.e. green) $\endgroup$ – R.M. Sep 29 '15 at 20:52
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The mixture of indicators should still work because most indicators are weak acids, and the presence of other weak acids does not interfere with any given weak acid. If you had an indicator that happened to be a weak base, you would likely have a problem, but that is not the case for your solution.

Here's a nice video from the CHEM Study series on exactly this topic, including a thorough discussion on mixing the indicators.

Acid Base Indicators CHEM Study

In terms of the specific answer to your assigned question, I think @Gavin Kramar is spot on. I'm just adding this answer for some additional information.

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Looks to me like it would be: orange from 0 to 3, yellow from 3 to 7.5, green from 7.5 to 10, purple-ish? from 10 on up (what's green plus fuschia?). One could actually go into the lab and give it a try. . .just sayin. I have actually done bromothymol blue and phenolpthalein mixed, and it performed as expected. Several years ago I used methyl orange and phenolpthalein, and again it did as expected. Have not tried all 3.

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