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In school, we had an experiment to find out the biomolecules contained by the different food-stuffs. After doing Tollens' test which turned out to be positive, our teacher instructed us to clean the test tube by using crystals of $\ce{K2Cr2O7}$ and concentrated $\ce{H2SO4}$. Then the solution in the test-tube turned green-yellow with an odour similar to that of Arlin oil (Ayurveda product, I really don't know what adjective I should describe it as).

When I asked my teacher about it, she told that it helps in cleaning the test tube from the silver mirror but did not know the chemistry behind it. Can anyone please explain to me how it works, with chemical equations supporting the same?

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This mixture, often called "chromic acid," is a common way to clean glassware. It is a strongly oxidizing acid which reacts with most organic material, so it is able to remove most types of organic sludge. You are likely smelling some mixture of the oxidation products.

It is very important to clean up Tollen's reagent, as it produces an explosive upon standing. More info here, including an accident report. However, chromates and dichromates are carcinogens and need to be disposed of as hazardous chromium waste. Making the spent (or unused) reagent safe only requires a dilute acid, so in most school labs, the spent Tollens reagent is disposed of with dilute acid and lots of water, producing fewer hazardous byproducts. This does not remove the silver mirror, which could be removed with nitric acid... or you can just dispose of the test tube. Chromic acid is overkill and most people would not risk exposure to chromium(VI) to save a test tube.

This paper [1] is very old, but it describes the reaction between silver, sulfuric acid, and potassium dichromate. I haven't looked beyond the first page (I don't want to pay for a paper from 1837!), but it tells you want to expect. Perhaps the ability to react with the silver mirror is why your teacher uses chromic acid despite the hassle and risks.

[1] R. Wazington, Esq., "On the action of chromic acid upon silver, and its combinations with the oxide of that metal," The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 1837 (first page).

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  • $\begingroup$ Further from the paper that you've shared, '...and is changed to a dark crimson ; this salt is the bichromate of silver... During this operation the colour of the supernatant liquid becomes much deeper, until the orange red tint of the original solution has become of a dark mahogany hue, and it then passes gradually into a deep green." So did I notice an extremely fast reaction? As I could see only green and not all these details, given the lack of slo-mo cameras in the 19th century... $\endgroup$ Nov 25, 2023 at 3:09
  • $\begingroup$ Plus it would also be nice if you could include the equations, given that glucose solution was the sample for the Tollen's test. $\endgroup$ Nov 25, 2023 at 3:12
  • $\begingroup$ @HarikrishnanM regarding your first comment - that is a possibility along with other dichromate salts it is generating. All of them are orange. $\endgroup$ Nov 25, 2023 at 7:17
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This answer will focus on the equations involved in the reactions of the questions. For the explanation please refer anon's answer.

Equations:

  1. Tollen's test of formaldehyde (works same for glucose, only difference being that instead of formic acid, gluconic acid is formed)

$\ce{HCHO + [Ag(NH3)2+]OH- -> HCOO- + Ag \downarrow + NH4+}$ Here Ag forms the silver mirror.

  1. Removal of Silver mirror with acidified $\ce{K2Cr2O7}$

$\ce{Ag + H2CrO4 -> Ag2CrO4 + Cr^3+}$

$\ce{Ag2CrO4}$ is a crimson red insoluble compound. $\ce{H2}$ gas is evolved.

  1. Oxidation of HCOOH formed to $\ce{H2CO3}$ followed by further decomposition into water and $\ce{CO2}$ gas along with reduction of Cr(VI) to Cr(III)

  2. Formation of Ammonium Chromate - yellow

From all this, the chromates formed are precipitated by the formation of chromic acid in the reaction $\ce{K2Cr2O7 + H2SO4 -> H2CrO4 + other products}$.

The smell may be explained by the ammonium ion formation in Tollen's test and the colour changes may be explained by the different precipitates that are formed throughout.

SOURCES: Wikipedia, ChemicalAid, ACS Publications, NCERT.

Answer updated in accordance to inputs from comments.

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    $\begingroup$ Chromic acid is a tremendous oxidizing agent and whatever organics the Tollen's test generates will react with it along with the residing silver and in that process, the chromium which is initially in +6 O.S. will be reduced to +3 O.S. which defines the gradual change in color to green. This is a necessary steps as untouched test tubes after performing Tollen's test can be dangerous.. $\endgroup$ Nov 25, 2023 at 7:05
  • $\begingroup$ @NilayGhosh post has been verified and edited accordingly. Any other suggestions are welcome. $\endgroup$ Nov 25, 2023 at 8:03
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    $\begingroup$ Ag dissolution in K2Cr2O7/H2SO4 would lead to Cr(III), not H2. Otherwise, Ag would get dissolved in HCl too, what is not the case. $\endgroup$
    – Poutnik
    Nov 25, 2023 at 8:54
  • $\begingroup$ @Poutnik inputs have been incorporated. $\endgroup$ Nov 25, 2023 at 8:57
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    $\begingroup$ Well, not really. The reaction is not enumerated (You have just replaced H2 by Cr^3+) and the comment remains wrong. $\endgroup$
    – Poutnik
    Dec 17, 2023 at 7:33

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