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

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

In our most recent article,¹ we reported a surprisingly simple method for preparing small samples of tetraiodotin(IV) in discarded food cans. We found that the interior tinplate surface of food cans was sufficiently reactive to form SnI₄ on exposure to iodine dissolved in toluene. This observation led us to consider other reactions that could take place on the tin coating of old food cans. In this short article, we describe a simple and fast demonstration of the reduction of Au³⁺ to colloidal Au⁰ using discarded tinplate cans that had been used for soup, baked beans and certain fruits.

Background

From the 17^th century, it was known that ruby red glass or “Cranberry glass” (see figure 1 for an example) could be prepared using a gold compound known as “Purple of Cassius”.^2,3 The discovery of this material has often been attributed to Andreas Cassius, however later scholarship has found that the use of gold to colour glass items was established prior to his work.

cranberry_glass.png

Figure 1: A picture of an early 19^th century cranberry glass jug that contains colloidal gold as a pigment.

In the mid-17^th century, Glauber described methods for preparing colloidal gold form acidic solutions using tin salts.^2,4 The following short passage, attributed to Glauber,² describes a method for preparing colloidal gold:

"The tin should be pure, and the whiter and harder, and the better its ring, so much is it better for this work or composition, and you can make use of these weights: Take 1 loth of fine gold powder, dissolve it with 3 or 4 loth of strong rectified Spiritu Salis; to the solution add 12 or 15 loth of pure water and put in a small piece of tin weighing 2 loth. Put the vessel on a warm sand bath and let it stay warm for 1 or 2 hours but do not boil it, and the gold will precipitate from the solution in the form of a brilliant purple-coloured powder."

Note - The term “loth” is a historical unit of mass that roughly equates to 15-18 grams (depending on country or region).

The modern-day interpretation of the process is outlined in the equations below. Tin metal is reacted with concentrated hydrochloric acid (“strong rectified Spritu Salis”) to form tin(II) chloride (equation 1).

Sn (s) + 2 HCl (aq) → SnCl₂ (aq) + H₂ (g)          Equation 1

The “Spiritu Salis” is unlikely to have been pure aqueous HCl (this alone would not be capable of reacting with gold metal), it was almost certainly contaminated with a species capable of oxidizing Au⁰ to Au³⁺. It is not possible to determine the true composition of the mixture described in Glauber’s text. However, Fe³⁺ and Cu²⁺ salts are likely impurities since “Spiritu Salis” was prepared by distillation in earthenware vessels. More recent studies have shown that iron(III) and copper(II) chlorides can facilitate gold leaching,^5,6 so it is possible these were among the contaminants in Glauber’s mixture. The formation of colloidal gold is controlled by a redox reaction between Sn²⁺ and Au³⁺ in solution, in this case the tin ions are oxidized and the gold ions are reduced (equations 2 and 3).

Sn²⁺ → Sn⁴⁺ + 2e-                                Equation 2

Au³⁺ + 3e- → Au                                   Equation 3

Combination and balancing of equations 2 and 3, leads to equation 4 for the overall redox process.³

3Sn²⁺ + 2Au³⁺ → 3Sn⁴⁺ + 2Au          Equation 3

The gold obtained from such reactions is commonly observed as a red or purple colloidal suspension that settles out over time as a fine powder. The presence of tin-containing by-products is important, since gold nanoparticles are believed to be stabilised by the presence of insoluble grains of SnO₂ (and associated hydrates).³ Colloidal suspensions of gold vary in colour, this observation is due to the formation of gold nanoparticles of different shapes and sizes.⁷ In our study, the gold-containing solutions were either red or purple, in both cases they co-precipitated very fine particles when allowed to stand for 1-2 hours (see figure 2 for examples).

samples_of_colloidal_gold.png

Figure 2: Samples of colloidal gold and co-precipitated with particles of SnO₂

Demonstrations

The 17th century procedure described above is ambiguous and is conducted on a large and very expensive scale! However, we reasoned that exposure of a dilute solution of gold(III) chloride, (most commonly encountered as chloroauric acid, HAuCl₄) in hydrochloric acid might react with the tinplate surface of a discarded food can. Before trying this reaction, we performed a test with SnCl₂ and a solution of chloroauric acid. Pleasingly, addition of a few drops of SnCl₂ dissolved in glycerol immediately resulted in the Au³⁺ solution changing from a very pale-yellow colour to red or violet (see figure 3).

reduction_of_haucl4.png

Figure 3: Reduction of HAuCl₄ (in aqueous HCl) with SnCl₂. Panel A shows the initial solution of HAuCl₄ in aqueous HCl. Panel B shows the HAuCl₄ in aqueous HCl mixture after dilution in water. Panel C shows the HAuCl₄ in aqueous HCl mixture after addition of a few drops of SnCl₂ dissolved in glycerol.

The initial test (figure 3) showed that Sn²⁺ was reactive toward the gold-containing solution, so a food can was then used as the tin source. A discarded tin can was rinsed with 5 M HCl for a minute or two, then the liquid was poured out. A dilute solution of chloroauric acid was then added to the pre-treated can, and the contents were gently swirled to ensure good contact with the internal tinplate surface. The initial yellow solution usually faded within a few minutes and a purple or violet colour became visible instead. Pictures of the colloidal gold solutions prepared by reaction with food cans are shown in figure 4. See video 1 to see the reaction in a tinplate can.

reaction_of_haucl4_with_tinplate.png

Figure 4: Reaction of a tinplate can with HAuCl₄ in aqueous HCl. Panel A shows the initial mixture, Panel B shows the reaction mixture after swirling the can contents for a few minutes.   

Video 1: Gold Colloid in Tin Can, ChemEd X Vimeo Channel, August 2023.

Summary

We have devised a simple, quick and colourful demonstration of the preparation of colloidal gold that is inspired by the historical glass pigment “Purple of Cassius”. In this case we have found that discarded tinplate cans are a convenient source of tin species required to reduce Au³⁺. The gold source in this case is costly, however we have limited the quantities required to be as low as possible. We have also provided a video and photographs that may be useful if the required materials are too expensive to perform a live demonstration.

References

1. I. Smellie, I. Patterson, Colourful Chemistry of Canning – Part 3 ChemEd X, April 2023. (Accessed 7thJuly 2023).
2. Hunt, L. B. “The true story of Purple of Cassius”. Gold Bull., 1976, 9, 134-139.
3. Habashi, F. “Purple of Cassius: Nano Gold or Colloidal Gold?”, Eur. Chem. Bull., 2016, 5, 416-419.
4. Wentrup, C. “Chemistry, Medicine, and Gold-Making: Tycho Brahe, Helwig Dieterich, Otto Tachenius, and Johann Glauber”, ChemPlusChem., 2023, 88, e202200289.
5. Seisko, S.; Lampinen, M.; Aromaa, J.; Laari, A.; Koiranen, T.; Lundström, M.  "Kinetics and Mechanisms of Gold Dissolution by Ferric Chloride Leaching", Miner. Eng., 2018, 115, 131-141.
6. Seisko, S.; Aromaa, J.; Lundström, M.  "Effect of Redox Potential and OCP in Ferric and Cupric Chloride Leaching of Gold", Hydrometallurgy, 2020, 195, 105374.
7. Long, N. N.; Vu, L. V.; Kiem, C. D.; Doanh, S. C.; Nguyet, C. T.; Hang, P. T.; Thien, N. D.; Quynh, L. M.  "Synthesis and optical properties of colloidal gold nanoparticles", J. Phys.; Conf. Ser., 2008, 187, 012026.

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

Supporting Information

Experimental procedures (and safety information) are provided for experiments conducted in discarded food cans. (Log into your ChemEd X account to access. Don't have an account? Register here for free!)