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A couple of weeks ago we did a lab where we had a bunch of standard solutions of alcohol, dichromate, and sulfuric acid. We also had one unknown concentration.

We heated them so they would all react, and the initial orange dichromate turned slightly green (the ion $\ce{Cr^{3+}}$ formed). Then we placed each reacted soln in a calorimeter which can emit light of certain frequencies. I believe we chose 635, which should have emitted orange light through the solution. the absorbance/transmittence were calculated for each standard soln until we had a line of best fit. Then the absorbance of the unknown concentration was given, so we used to line to match it to it's concentration (of alcohol).

I'm confused as to why we had to use the color orange in our emission and why a reaction was even needed in the first place.

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Since green is the absorption of orange light, you need to supply the orange light to measure how much is absorbed. As the alcohols are oxidized to the acids, the dichromate is reduced, so you can measure how much acid has been formed. BTW, what alcohol was used? Butyl alcohol oxidation has a distinct aroma that can be detected without spectroscopy.

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