Colourful Chemistry of Canning – Part 3

Co-Authored by Iain A. Smellie*, Iain L. J. Patterson*, Brian A. Chalmers*

*University of St Andrews, School of Chemistry, North Haugh, St Andrews KY16 9ST, United Kingdom

In our previous two articles,^1,2 we have reported the results we have obtained from experiments involving reactions of anthocyanins (derived from plants) with the tinplate interior of discarded food cans. In this short article, we describe some further reactions that can be accomplished using the tin coating in food cans as a reagent. To date, our attention has been directed toward anthocyanin molecules binding to Sn²⁺ ions under acidic conditions. The tinplate surface of discarded food cans has been found to be convenient to use as a reagent, and the cans are easily disposed of once experiments have been completed. We anticipated that the tin metal present in food cans would react with materials other than plant-derived substances. Our most recent studies have focused on the reaction of tinplate cans with iodine and these are outlined below.  

 

Figure 1: Reaction of tin metal with molecular iodine to form SnI₄.

 

Reaction of tinplate with iodine

The reaction of tin metal with iodine to form tetraiodotin(IV) will likely be familiar to many undergraduate or advanced high school students.^3-5 This reaction is a classic demonstration of the formation of a metal halide from the parent elements (figure 1) and the vivid orange crystalline product is straightforward to isolate. The most familiar routes involve dissolving iodine in carbon tetrachloride,³ naphtha⁴ or toluene,⁵ the resulting mixture is then boiled in the presence of excess of tin metal (see Video 1).

 

Video 1: Demonstration of the formation of tin IV iodide. ChemEd X Vimeo Channel, April 2023.

 

The reaction of tin metal granules with iodine in boiling solvent usually takes 30-60 minutes to go to completion. However, in this study, we decided to try a small-scale version of the reaction at room temperature, using the tinplate interior of a food can as the source of metallic tin. A small quantity of iodine was dissolved in toluene and the mixture was exposed to the tinplate surface for 24 hours (the full procedure is outlined in the supporting information file). Pleasingly, we found that the initial red-brown solution of iodine in toluene (figure 2, panel A) had become pale orange overnight (figure 2, panel B), suggesting the iodine had been consumed and that SnI₄ had become dissolved in the solvent.

 

Figure 2: Reaction of a tinplate plate can with molecular iodine to form SnI₄. Panel A shows the initial mixture (iodine dissolved in toluene), Panel B shows the reaction mixture after 24 hours.

 

The solvent was allowed to evaporate off slowly in a fumehood (at room temperature), either directly from the can, or after transferring the solution to a glass beaker. The interior of the can was examined and an orange solid was seen to have formed where the solvent had evaporated. In addition, the tinplate was seen to have corroded where dissolved iodine had contacted the metal surface (figure 3, panel A). When the solvent was transferred to a beaker and allowed to evaporate, the residue was found to contain an orange-coloured crystalline material (figure 3, panel B). 

 

Figure 3: SnI₄ residues on the tin plate interior of an empty tin can (Panel A) and a glass beaker (Panel B).

 

SnI₄ is reported to be an orange solid with a melting point of 144 °C,⁶ the orange material that originated from the treated food cans was found to have a melting point of 145-146 °C. The melting point results were encouraging, but further testing was conducted to confirm the material isolated was SnI₄. It is known that tin(IV) halides are hydrolysed in aqueous solution to release halide ions,⁷ so simple tests for iodide were attempted (experimental details are provided in the supporting information file). Small samples of the orange solid recovered from discarded food cans were dissolved in cold water (figure 3, panel A) and then treated with silver nitrate solution. In each case, a pale-yellow precipitate of silver iodide was readily observed as soon as the solutions were mixed (figure 3, panel B). A Beilstein test⁸ for iodide ions was also conducted, which involved dipping a hot copper wire (heated in a Bunsen flame) into the solution of hydrolysed product. The copper wire was then heated in a Bunsen burner flame whereupon a bright green emission was observed (figure 3, panel C). The colour of the flame was very distinctive and characteristic for copper halides⁹ (see Video 2).

 

Video 2: Beilstein Flame Test (from iodide obtained from a Heinz Tomato Soup can.) ChemEd X Vimeo Channel, April 2023.

 

Figure 4: Tests conducted on SnI₄ residues collected from the tin plate interior of an empty tin can. Panel A shows a solution obtained from mixing a sample of SnI₄ in water. Panel B shows AgI precipitation from addition of AgNO₃ solution to hydrolysed SnI₄. Panel C shows the green flame observed in a positive Beilstein test for halide ions.

 

Attempts were also made to test for the presence of tin-containing compounds. The chemistry of tin(IV) halides in aqueous solution is complicated, indeed this was subject to lively debate in the early 20^th century.^7,10 Equations 1 and 2 show a simplified series of reactions relating to the behaviour of tin(IV) halides in water.

Equation 1:  SnX₄ + 4H₂O → Sn(OH)₄ + 4HX (X = Cl, Br or I)

Equation 2:  Sn(OH)₄ → SnO₂ + 2H₂O

The tin(IV) oxide obtained (equation 2) from the initial hydrolysis product can exist in hydrated forms (such as SnO₂•H₂O). Once hydrolysis has taken place, this material is visible as a gelatinous precipitate and as a colloidal suspension. The colloidal particles were readily visualised by shining the beam from a laser pen through the solution (figure 5).

 

Figure 5: Colloidal SnO₂ visualised with a laser pen.

 

It is well known that SnCl₂ can be reduced with metallic zinc to form tin metal,¹² so an attempt was made to reduce SnI₄ with zinc. A sample of SnI₄ was mixed with water to form a slurry (initially a mixture of SnI₄ and hydrolysis products) and a strip of zinc metal was placed in the hydrolysis mixture. Within a few minutes, bubbles of hydrogen gas were observed, this is most likely due to reaction of HI with zinc metal (equation 3).

Equation 3:  Zn + 2HI → H₂ + ZnI₂

As the reaction proceeded, particles of tin metal were observed to deposit (figure 6, panel A). The reaction mixture was stored for 24 hours, after this period all of the orange material had been consumed and a mixture of tin metal and hydrated tin oxide remained (figure 6, panel B). The strip of zinc metal was retrieved and cleaned, the area of metal that had been in contact with the reaction mixture was found to be significantly corroded (figure 6, panel C). As indicated above, the reactions of tin(IV) halides in water are complicated, however, equation 4 represents a useful overview of the reaction taking place.

Equation 4:  SnI₄ + 2Zn → Sn + 2ZnI₂

 

Figure 6: Reaction of tin(IV) iodide with zinc metal (in water).

 

It is also possible to confirm the metallic material is tin by conducting simple tests. Zinc metal reacts with 5M hydrochloric acid at room temperature, in contrast tin metal will only dissolve after immersion in hot hydrochloric acid. The colourless solution obtained from dissolving zinc in aqueous HCl becomes brown when dilute aqueous iodine solution is added. However, the same treatment of the solution containing dissolved tin metal results in the iodine solution being decolourised. These observations can be explained in terms of the metal ions present. Zn²⁺ is incapable of further oxidation by iodine, in contrast, Sn²⁺ is oxidised by iodine to Sn⁴⁺. The procedures for these tests are described in the accompanying supporting information document.

It is possible to detect tin compounds using ¹¹⁹Sn NMR spectroscopy, so a sample of material recovered from a discarded soup can was collected and analysed. A solution of the substance that was believed to be SnI₄ was prepared in deuterated dimethyl sulfoxide (d6-DMSO). The resulting ¹¹⁹Sn NMR spectrum showed a diagnostic signal for SnI₄ at -2027 ppm, this chemical shift value compares well with an authentic sample and with the results of NMR studies in the literature¹¹ (a copy of the NMR spectrum has been provided in the supporting information).

 

Summary

We have continued to investigate the reactivity of the tin layer inside many food cans. Previous investigations have focused on the reactivity of anthocyanin-containing plant extracts. This short study shows that tinplate surfaces react with iodine to form tin(IV) iodide. This method can be attempted on a small scale and in contrast to more established methods, the reaction does not require any heating.

 

Supporting Information

Experimental procedures (and safety information) are provided for experiments conducted in discarded food cans. Videos of an example of tin iodide being prepared in refluxing solvent and a Beilstein flame test have been provided. (Log into your ChemEd X account to access. Don't have an account? Register here for free!) 

 

Acknowledgements

We would like to express our sincere thanks to Dr Federico Grillo (University of St Andrews) for very helpful discussions during the course of this study.

 

References

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4. Hickling, G. C. “Gravimetric analysis – The synthesis of tin iodide”, J. Chem. Educ., 1990, 67, 702-703.
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10. Posnjak, E. “The Nature of Stannic acids”, J. Phys. Chem. 1926, 30, 1073-1077.
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