Using potassium dichromate and sulfuric acid to clean test tubes after Tollens’ test

Question

We had an experiment at school to find out the biomolecules contained in the different foodstuffs. After doing Tollens’ test which turned out to be positive, our teacher instructed us to clean the test tube by using crystalline potassium dichromate and concentrated sulfuric acid.

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).

The teacher said that it helps in cleaning the test tube from the silver mirror, but did not know the chemistry behind it. Can anyone please explain how it works, preferably with chemical equations?

Answer 1

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).

Answer 2

For the explanation please refer anon's answer. This answer focuses on the reactions related to the question (adapted from Wikipedia, ChemicalAid, ACS Publications, NCERT):

1. Tollens’ test of formaldehyde, or the silver mirror-making reaction. Works similarly for glucose with the only difference is that gluconic acid is formed instead of formic acid.

   $$\ce{HCHO + [Ag(NH3)2+]OH- -> HCOO- + Ag(s) + NH4+}\tag{R1}$$

2. Removal of the silver mirror with acidified potassium dichromate:

   $$\ce{Ag(s) + H2CrO4(aq) -> Ag2CrO4(s) + Cr^3+(aq) + 2H+(aq)}\tag{R2}$$

   Silver chromate is a crimson-red and poorly soluble compound. $\ce{Cr^3+}$ shows the green colour.

3. Oxidation of $\ce{HCOOH}$ formed to $\ce{H2CO3}$ followed by further decomposition into water and $\ce{CO2}$ gas along with reduction of $\ce{Cr^6+}$ to $\ce{Cr^3+}.$

4. Formation of yellow ammonium chromate.

From all this, the chromates formed are precipitated by the formation of chromic acid in the reaction

$$\ce{K2Cr2O7 + H2SO4 -> H2CrO4 + other products}.\tag{R3}$$

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