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I'm currently working on an electrolytic setting where a stainless steel (430 series, mainly made of iron and chromium, but no nickel) anode is dissolved in acidic environment (sulfuric and phosphoric acids at moderate concentrations).

My concern is the hypothetical creation of dichromate ions, since they are carcinogenic. But I can't confirm only color-wise because of the presence of other ions that mixes all the colors.

The experiment runs for long (3 hours). At first an orange color appears, which suggests dichromate ions (but could be Fe III). But after only a little while, the solution turns to a very dark and deep green, almost blue depending on the light. This suggests chromium III ions.

Several things could be happening, such as reduction of Cr VI to Cr III at the cathode, or just creation at the anode of Cr III whose color overwhelms that of Cr VI, or even Fe II or Fe III messing with the color.

Does anyone have an idea about a means to confirm or infirm the presence of dichromate ions despite not being able to judge by the color ? And if so, how could I determine the concentration of it ?

Thanks a lot in advance.

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The best reagent for detecting Cr(VI) in aqueous solution is the diphenylcarbazide, which produces a red color in presence of Cr(VI). The sensibility limit is as low as 0.2 ppm Cr(VI). This method has been developed one hundred years ago. See 1) M. A. Moulin, Dosage du chrome, Bull. Soc. chip. 31, 295 (1904). 2) H. Crossley, Diphenylcarbazide. An indicator for the titration of iron with dichromate. Analyst 61, 164 (1936).

The reaction with alcohols to produce aldehyde and acids does work. But it is not very sensitive. It needs at least some millimoles of chromate to be detected. The reaction with diphenylcarbazice is about 1'000'000 times more sensitive.

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  • $\begingroup$ Thanks a lot, I guess I'll order some and make the test. $\endgroup$ – MrFrederic Dec 26 '19 at 20:16
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I think, the other way to detect the presence of dichromate on a solution is when the primary alcohols are converted into aldehydes while the secondary alcohols into ketones.

On the other hand, the presence of aldehydes and ketones can be analyzed by various confirmatory tests.

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  • $\begingroup$ Oh thanks, indeed I remember this property ! So adding ethanol and heating should get me all the way to ethanoïc acid, which has a caracteristic vinegar smell... That should be enough to confirm. $\endgroup$ – MrFrederic Dec 26 '19 at 20:12
  • $\begingroup$ @MrFrederic. I think, you can determine the concentration of dichromate by means of titration. $\endgroup$ – HYDR0GEN Dec 27 '19 at 5:20

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