Homemade Bismuth Plating by Galvanic Displacement

Chemistry Teacher International 2021; aop

Good practice report

Jose Luis Aguilar-Charfen, Ines Castro-Sayago, Jimena Turnbull-Agraz and Jorge G. Ibanez* Homemade bismuth plating by galvanic displacement from bismuth subsalicylate tablets: a chemistry experiment for distance learning https://doi.org/10.1515/cti-2021-0002 Received January 23, 2021; accepted June 8, 2021; published online June 30, 2021

Abstract: This paper presents a student-designed one-pot electroless deposition of Bi extracted from a Pepto Bismol® tablet by galvanic displacement of the Zn coating of a galvanized iron nail. This experiment relies on a readily accessible and reasonably safe method and materials and it has been used during the present COVID pandemic as a hands-on activity with higher education students (i.e., Junior and Senior Chemical Engineering students). Its simplicity should allow its use with High School students as well. The entire procedure can be completed in 30–45 min.

Keywords: Bismuth; distance teaching; electroless; galvanic displacement; Pepto Bismol®; remote laboratories.

Introduction

Because of its industrial importance, the thorough understanding of the principles of metal plating is fundamental in electrochemistry and material science courses. Although the theoretical concepts needed to distinguish electroless depositions are simple, the experimental application is quite more challenging espe- cially for distance learning settings or poorly equipped laboratories. Active metals such as Fe, Zn, or Al are common in household products. However, the more noble metals needed for the spontaneous deposition in reaction with the former are either expensive (e.g., Ag, Au, or Pt) or toxic (e.g., Hg). Plating is used since ancient times to allow for versatility, functionality (in reducing corrosion, for example), and better designs of many common materials. In particular, bismuth is widely used in low-melting point alloy manufacturing, security seals, and electrode materials (Atiﬁ, Boyce, DiMeglio, & Rosenthal, 2018). Interest has increased in recent years because of its applications in electrochemical catalysis (Zhang, Qiu, Yao, Li, & Zhang, 2019) and as a versatile sealed weld when found in speciﬁc alloys (Carragher, 2018). Also, its use as heat insulator-based on Bi’s low thermal conductivity compared to that of Zn (i.e., 8 vs. 116 W m−1 K−1) (Ho, Powell, & Liley, 1974) – is gaining relevance. In addition, because Bi is the most diamagnetic metal, it can be used for superconductor testing without the need for large amounts of material (Li, 2013). It represents a very low risk for the environment (Wang, Li, & Sun, 2019) and is considered nontoxic (Miller, 2018). In addition to its solid-state applications,

*Corresponding author: Jorge G. Ibanez, Chemical, Industrial, and Food Engineering Department, Mexican Center for Green and Microscale Chemistry, Universidad Iberoamericana, Prol. Reforma 880, 01219 Mexico City, Mexico, E-mail: [email protected]. https://orcid.org/0000-0003-3247-6751 Jose Luis Aguilar-Charfen, Ines Castro-Sayago and Jimena Turnbull-Agraz, Chemical, Industrial, and Food Engineering Department, Mexican Center for Green and Microscale Chemistry, Universidad Iberoamericana, Prol. Reforma 880, 01219 Mexico City, Mexico

Open Access. © 2021 Jose Luis Aguilar-Charfen et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. 2 J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement

organic compounds like bismuth subsalicylate possess anti-acid and anti-diarrheal properties (NCBI Bismuth Subsalicylate, 2007). In the present paper, a simple student-designed method for the homemade, low-cost, low environmental impact, microscale extraction of Bi from a Bi-subsalicylate commercial tablet is proposed to demonstrate the redox processes that occur during metal extraction and the generation of surface platings via galvanic displacement. This method allows the coating of conductive and active metals submerged in a solution by the electroless deposition of less reactive metal ions. Such a spontaneous process obviates the need for an electrolytic setup. This is a microscale adaptation of a published procedure aimed at producing pure Bi pellets (Gray, 2012). As demonstrated in the experiment below, the rate of the plating exchange on the galvanized nail is remarkable. This differs from many deposition processes where reactions are not spontaneous and require an electricity supply to compensate for the potential difference, which, together with the opportunity to make it a one-pot process, makes the proposed method relatively simpler compared to other techniques.

Theoretical framework

Electroless plating methods rely on interfacial reactions and can be grouped into autocatalytic deposition and galvanic displacement. In the former, deposition of a film is accomplished by the oxidation of a compound in a solution, which acts as a reducing agent by providing the electrons for the reaction to occur (Chu, 2015). In the galvanic displacement, the electrode or substrate acts as a reducing agent. The material is displaced by the metallic ions in solution that have a lower standard potential, according to the following general reaction (Papaderakis, Mintsouli, Georgieva, & Sotiropoulos, 2017): n+ + n/m → + n/m +m M noble Mactive Mnoble M active (1) This is exemplified by the thermodynamically spontaneous displacement of Zn by Bi(III) ions shown in Figure 1. A modified version will be performed in the experiment described below. Transmetallations of inorganic complexes involve similar processes (Papaderakis et al., 2017). Both techniques are widely used for the generation of catalytic surfaces and the creation of ultrafine crystal films at low energy expenditures (daRosa, Maboudian, & Iglesiaz, 2008). Further background readings on the following subjects could be useful: metal plating (Formlabs, 2021), electroplating (Home Science Tools, 2021), galvanic replacement (Jenkins, Gohman, Miller, & Chen, 2015), and electrolytic plating (Osella, Ravera, Soave, & Scorza, 2002).

Experimental

To extract Bi from Pepto Bismol®, the latter needs to be hydrolyzed first through an acid-base reaction in the presence of HCl to yield bismuth − trichloride (Gray, 2012) that, in turn, becomes BiCl4 in excess chloride, as shown in the Pourbaix diagram of Figure 2, depicting the diagrams of aqueous Bi and Bi species in chloride media (Nam & Choi, 2017). To achieve this, pulverize approximately one quarter of a 250-mg chewable Pepto Bismol® pill either in a mortar (if available) or in any other suitable container. Next, place six drops of ∼3MHClina10-mL beaker (Muriatic acid – a commercial form of hydrochloric acid – can be obtained in hardware stores and then diluted accordingly). Transfer ® to this beaker, the amount corresponding to the tip of a spatula of the pulverized Pepto Bismol and stir gently. CO2 bubbles are generated (see reactions below). Once the production of gas stops, place the nail in the solution for ∼2 min. If the volume of liquid were insufficient to cover the nail, add some drops of water. This step should not change the pH of the solution significantly as evidenced by the color of the pH-sensitive dye in the tablet; if the pH rises, it will turn purple or pink, thus, concealing the desired phenomenon. In addition, insoluble species might form (see below). The formation of Bi(s) can be recognized as a black coating materialized on the surface of the nail. Note: galvanized nails are available at hardware stores, and Pepto Bismol® pills are available as over-the-counter drugs in most drugstores.

Hazards and disposal

The use of HCl for the hydrolysis of bismuth subsalicylate requires that the experiment be done following strict safety protocols and using personal protection equipment (NCBI Hydrochloric Acid, 2007). It is recommended to carry out the experiment in a well- J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement 3

Figure 1: Atomic view of the Bi–Zn galvanic displacement in an acidic medium. ventilated place to avoid inhaling the generated vapors. If the melting of bismuth is desired, this must be done in a well-ventilated space because of the possible generation of oxide gases. The Bi-plated nail represents no hazard to humans nor to the environment and so it can be disposed of with regular solid waste. The liquid residues, after neutralizing the HCl excess, contain no hazards and can be disposed of according to local regulations.

Results and discussion

The acid-base reaction that hydrolyzes the Pepto Bismol® tablet yields salicylic acid and bismuth trichloride as follows:

(2)

According to Figure 1, the trichloride in turn becomes a complex tetrachloride ion in excess chloride as follows: 4 J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement

Figure 2: Pourbaix diagrams for Bi in: (A) Pure water, and (B) 0.6 M Cl− (adapted from Nam & Choi, 2017).

BiCl + Cl− ⇌ BiCl− 3()aq ()aq 4(aq) (3) The HCl medium also helps to avoid the precipitation of undesired bismuth oxychloride, BiClO, which is used in the cosmetic industry to give a pearly effect to their products (Barton, Eastham, Isom, McIaverty, & Ling Soong, 2020). Pepto Bismol® tablets typically contain bismuth subsalicylate as the main active ingredient, as well as calcium carbonate, ﬂavorings, magnesium stearate, mannitol, povidone, saccharin sodium, and talc (P&G, 2021) – none of them interferes with the plating reaction. The carbonate produces a side reaction of the excipient involving the formation of carbonic acid and its decomposition into carbon dioxide:

CaCO3(s) + 2HCl(aq) → CaCl2(s) + H2O(l) + CO2(s) (4) This reaction is responsible for the bubbling observed during hydrolysis. Lastly, the pregelatinized starch that serves as the excipient is left out of the reaction and floats (it can be removed with a spoon). Salicylic acid also separates because of its low solubility in water, making it easy to be ﬁltered (although this step is not absolutely required). If the extraction of pure Bi were desired, it would be necessary to ﬁlter the excipient and make it react with Al foil to precipitate Bi in the form of a black powder. This appearance is due to the different orientations in which it disperses light. J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement 5

The half-reactions that occur are therefore (Bard, Parsons, & Jordan, 1985; Lide, 2005): Reduction reaction: BiCl− + e− ⇌ Bi + Cl− E∘ =+ . V 2 4(aq) 6 2 (s) 8 0 16 (5) Oxidation reaction:

2+ − ∘ 3Zn(s) ⇌ 3Zn(aq) + 6e E =−0.763 V (6)

Global reaction: BiCl− + Zn ⇌ Bi + Zn2+ + Cl− E∘ =+ . V 2 4(aq) 3 (s) 2 (s) 3 (aq) 8 (aq) 0 923 (7) − The BiCl4 ion therefore undergoes a spontaneous/exergonic reaction with Zn. As long as there is still some Zn leftover, this reaction will be favored over the oxidation of the Fe in the nail due to the more negative potential of the former (i.e., −0.763 vs. −0.447 V). The behavior of a galvanized nail in an acidic BiCl3 solution versus that in an HCl solution alone is compared in Figure 3 to establish that the black coating is not simply due to the corrosion caused by the acidic medium but rather by the deposition of Bi.

Figure 3: Comparative behavior of a galvanized nail in an acidic

BiCl3 solution vs. HCl solution alone, after being submerged for 1 min in each solution.

Figure 4: Simple model for the ionic plating of Bi on a galvanized nail. 6 J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement

Figure 5: Bi2O3 produced by heating the Bi deposit.

Iron, while generally not behaving this way, acts as an inert electrode since the electrons lost by Zn during its oxidation are conducted by the Fe in the nail, thus allowing the deposition of Bi on its surface, as shown in Figure 4. Contrary to the action of Zn, the Bi plating cannot protect the iron of the nail from dioxygen-originated ambient oxidation (ΔE°Bi/O2 = +0.912 V, ΔE°Zn/O2 = +1.992, ΔE°Fe/O2 = +1.669 V) (Bard, 1985). If desired, once Bi is deposited on the surface of the nail it can be gently heated for a few seconds over the flame of a kitchen stove above its melting point (i.e., 270 °C) (Bismuth, 2019) and allowed to cool in air to form a thin, yellowish patina (see Figure 5).

Conclusions

The extraction of Bi from an over-the-counter medicinal drug like Pepto Bismol® enables a demonstration of galvanic displacement as one of the forms of electroless deposition in a clear, fast, and safe way with a small amount of materials. Some learning outcomes from the student’s global distance-learning hands-on experi- ence were gathered from a voluntary survey and included the following: 67% of students improved their mastery on the related topics; 50% of the total learned very well to use basic glassware and laboratory equipment following the proper techniques; 93% learned to design sound scientific procedures to generate new experiments or solve basic experimental problems; 87% understood and applied green chemistry principles, such as decreasing the amounts of substances used and minimizing the generation of waste; 87% discussed with their classmates and the professor the scientific or mathematical theories and models devel- oped through data analysis to explain some observed phenomena. Lastly, 87% said that using scientific data published in reliable sources helped them understand or report better the observed results. J.L. Aguilar-Charfen et al.: Homemade bismuth plating by galvanic displacement 7

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission. Research funding: None declared. Conﬂict of interest statement: The authors declare no conﬂicts of interest regarding this article.

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