Burning paper towels soaked in various metal nitrates

A classic chemistry experiment¹ involves using a concentrated solution of KNO₃ to “paint” a design in a continuous line on a piece of paper, allowing it to dry, and then igniting one end of the paper (Video 1).²

Video 1: Chemistry is Love, Tommy Technetium YouTube Channel, Sept 26, 2022.

One way to explain this result is that the ignition of the paper causes the KNO₃ to thermally decompose, producing oxygen gas:¹

2 KNO₃ (s) → 2 KNO₂ (s) + O₂ (g)                        Equation 1

The oxygen gas released supports the combustion of the paper, allowing it to turn more vigorously than it does in air.

I wondered what would happen if I soaked paper towels in potassium nitrate and ignited them after allowing them to dry. In addition to the paper towels burning quite vigorously, I observed a red-orange colored flame, which seemed a bit different than the lavender-orange flame I’m used to seeing in potassium flame tests (Video 2).³

Video 2: My Heart Burns for Chemistry, Tommy Technetium YouTube Channel, Sept 24, 2022.

The white smoke observed when these paper towels burn could perhaps be comprised in part of solid particulate KNO₂ (Equation 1). I noted that paper towels treated in this manner burn in a fashion reminiscent of the burning of guncotton.⁴ Given that the preparation of guncotton requires concentrated acids, this procedure eliminates hazards as well as streamlines the time for preparation.  

I next wondered if I could get different colored flames by burning towels after soaking them in concentrated NaNO₃ or Cu(NO₃)₂. The sodium-infused paper towel behaved as I expected, giving a yellow flame (Video 3).⁵ But I was in for a big surprise with the paper towel⁶ soaked in copper (II) nitrate (Video 3).

Video 3: Burning copper nitrate paper, Tommy Technetium YouTube Channel, Oct 24, 2022.

Isn’t that remarkable? The copper ions infused into the paper towel get reduced to copper metal upon burning! When I initially saw this result, I wondered if the orange residue might have been a copper (I) compound, because such compounds tend to be orange. To test for this possibility, I investigated whether the orange residue conducted electricity or not. Indeed, it did conduct electricity, indicating the residue it is copper metal.

Of course, I next had to check to see if a paper towel soaked in silver nitrate, allowed to dry, and ignited might form silver metal (Video 4).⁷

Video 4: Burning nitrate soaked paper, Tommy Technetium YouTube Channel, Oct 31, 2022.

Interestingly, it appeared as though very fine particles of metallic silver dust were produced. These results are consistent with reports of the formation of metallic copper, silver, and other metals being formed when organic material is infused with metal nitrates and combusted.^8-11 Such processes are referred to as carbothermal or carbothermic reduction. The formation of copper metal observed in this experiment (Video 3) could have formed via a series of processes similar to those outlined in these previous reports such as:

Cu₂(OH)₃NO₃ → 2 CuO + H₂O + HNO₃       Equation 2

CuO (s) + CO (g) → Cu (s) + CO₂ (g)               Equation 3

2 CuO (s) + CO (g) → Cu₂O (s) + CO₂ (g)        Equation 4

2 Cu₂O (s) + C (s) → 4 Cu (s) + CO₂ (g)              Equation 5

Where the CO(g) and C(s) are formed by the combustion of the carbon containing material. Processes related to the formation of silver was not explicitly mentioned in these articles.^7-10

Nevertheless, the following (highly speculative) reactions could account for the formation of the silver dust:

Ag₄(OH)₃NO₃ → 2 Ag₂O + H₂O + HNO₃                 Equation 6

Ag₂O (s) + CO (g) → 2 Ag (s) + CO₂ (g)                      Equation 7

It is interesting to view Equations 2-7 in light of the physical form of the metals produced in these experiments. That is, the silver metal appeared to form as silver dust, while the copper metal formed in something more akin to a continuous, flake-like solid. The physical phase of the reducing agent in each case could account for this difference. For example, reduction of copper by carbon char (Equation 5) on the solid paper surface seems consistent with its formation of a continuous solid. In contrast, reduction of silver by gas phase carbon monoxide (Equation 7) seems more consistent with the formation of tiny, diffuse particles of silver dust.

In any event, I’d love to hear your thoughts on this experiment and my musings of what might be going on. I ESPECIALLY want to hear from you if you try this experiment by igniting paper towels that have been soaked in gold nitrate…

Happy Experimenting!

References:

1. https://www.flinnsci.ca/api/library/Download/114c566507544ace8b51e4dc658aff3b
2. Video 1: Chemistry is Love, Tommy Technetium YouTube Channel, Sept 26, 2022.
3. Video 2: My Heart Burns for Chemistry, Tommy Technetium YouTube Channel, Sept 24, 2022.
4. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers, vol. 1, 1983 University of Wisconsin Press, pp. 43-45.
5. Video 3: Burning copper nitrate paper, Tommy Technetium YouTube Channel, Oct 24, 2022.
6. Note that the towel soaked in a concentrated solution of copper (II) nitrate had to be dried in a dessicator.
7. Video 4: Burning nitrate soaked paper, Tommy Technetium YouTube Channel, Oct 31, 2022. (No, drying these paper towels in the microwave oven does not cause a big explosion. However, I would often observe them to spontaneously burst into flame. I also observed this when heating these paper towels with a hair dryer. Don’t expose the paper towel to heat until ready to ignite.)
8. Yudha, S. S. et. al. International Journal of Technology 2023, 14, 290-299.
9. Hoekstra, J. et. al. J. Phys. Chem. C 2015, 119, 10653−10661.
10. Ren, J. et. al. J Therm Anal Calorim 2015, 120, 1929–1939
11. Karunaratne, T.N., et. al. Biochar 2022, 4, 70.