1
Introduction to Silver Chemistry

Paramasivam Sivaguru and Xihe Bi

Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, 130024, China

1.1 History and Features of Silver

Silver is a malleable, ductile, and precious metal that has been known since ancient times (its first debut around 5000 BCE) and is located in group 11 (Ib) and period 5 of the periodic table, between the coinage metal copper (period 4) and gold (period 6). Silver is widely distributed in nature. But its abundance in the earth's crust is very low (0.05?ppm) than other metals [1]. It occurs both naturally in its pure form and in ores, particularly derived from all the sulfur bearing lead, copper, gold, tellurides, and zinc, which is extracted through refining [2]. Silver has the atomic number 47 and atomic weight of 107.880, and its ground state electronic configuration is [Kr] 4d105s1, just like copper and gold. Mostly, silver can exist in a mixture of isotopes, 107Ag and 109Ag, approximately occurring in the equal proportions. The most common oxidation states of silver are 0 and +1, although some other oxidation states (+2 and +3) are also known [3]. Among these Ag(II) salts/complexes are less stable than that of Ag(I) and Ag(III) salts/complexes. Silver is noticeably diamagnetic, and its magnetic susceptibility is almost independent of temperature from room temperature to just below the melting point.

The elemental silver has the highest electrical conductivity (1·59?µO?cm at 20?°C) [4], thermal conductivity (429?W?m-1?K-1) [5], and optical reflectivity than any other metal, but it has the lowest electrical contact resistance, and its specific heat capacity is 0.23?J?kg-1?K-1 at 25?°C. The melting and boiling point of silver is 961.9 and 2212?°C, respectively [6]. The heat of fusion of silver is 11.28?kJ?mol-1, and its hardness is 2.7 on the Mohs scale.

1.1.1 Chemistry of Silver

It has been recognized that the outer orbital 5s1 electronic configuration of silver allowed to form numerous silver(I) salts/complexes with a wide variety of counterions (halide, sulfide, nitrate, oxide, acetylide compounds, cyano-derivatives, and olefin complexes). Silver dissolves readily in nitric acid to form silver nitrate (Eq. 1.1), which is a transparent crystalline solid that is readily soluble in water, and is a photosensitive. In addition, it is a precursor for the preparation of various other silver compounds. Silver could also dissolve rapidly with hot concentrated sulfuric acid (Eq. 1.2). However, in the presence of ethanol, silver reacts with nitric acid to give the silver fulminate (AgCNO), which is a powerful touch-sensitive explosive used in percussion caps [7]. Also, silver nitrate reacts with sodium azide (NaN3) to form silver azide (AgN3), which is also used as an explosive [8]. Silver or silver nitrates simply precipitate as silver chloride in the presence of chlorides, which are used in the photographic emulsion:

Furthermore, silver nitrate could easily react with copper to produce the silver crystals (Eq. 1.3). The alkaline solution of copper also reduces the silver nitrate into silver in the presence of reducing sugars. Tollens' test/silver mirror test is a qualitative test to distinguish between an aldehyde and ketone. The Tollens' reagent [Ag(NH3)2]+ is prepared from silver nitrate by two-step process. In the first step, under basic conditions silver nitrate forms an insoluble silver oxide (Eq. 1.4), and it dissolves readily with the addition of sufficient aqueous ammonia (Eq. 1.5), which oxidizes an aldehyde into corresponding carboxylic acid (Eq. 1.6) [9]:

Silver is stable in oxygen and water, but it is tarnishing in the presence of ozone or hydrogen sulfide or sulfur in air/water owing to the formation of a black silver sulfide layer. Besides, silver readily forms soluble silver complexes such as Ag(NH3)2+, Ag(S2O3)23-, and Ag(CN)2- with excess of respective ions. The silver thiosulfate complex is used to dissolve undeveloped AgBr and fix the photography [10]. The silver cyanide complex is frequently used in electroplating [11]. A systematic sequence of reactions outlined in Figure 1.1 illustrated the chemistry of silver.

Figure 1.1 Reactions in the silver series.

1.1.2 Silver Nanoparticles

Nanoparticulates (colloidal) of silver are fine particles of metallic silver that has been known for about 120 years [12]. Usually, these are synthesized by the reduction of soluble silver with reducing agents such as citrate, glucose, ethyl alcohol, and sodium borohydride as well as an appropriate stabilizing agent. The added stabilizing agent plays a crucial role to prevent the growth and aggregation of the formed silver nanoparticles. The reduction process can be carried out in both aqueous and organic solvents. However, a practical and reproducible synthesis of silver nanoparticle is very difficult than that of expected [13]. This might be due to its different morphologies and crystal sizes when changing reaction conditions such as concentrations, reducing agents, temperature, and additives [13, 14]. Depending on the reaction conditions, there are numerous kinds of silver nanoparticles that have been documented in the literature, including spherical [15], bipyramids [16], discs [17], rods [14, 18], cubes [19], prisms [20], rings [21], platelets [22], triangular prisms [23], and octahedral particles [19c]. Because of the different sizes and morphologies, nanosilver possesses the unique chemical, physical, and optical properties compared with the parent metallic silver. The unique properties of nanosilver are mainly attributed to the high surface area to volume ratio, leading many industrial sectors to incorporate silver nanoparticles into their products. Two main factors such as surface effects and quantum effects to cause nanomaterial behave significantly different than bulk materials [24]. These factors affect the chemical reactivity of materials as well as their mechanical, optical, electrical, and magnetic properties. Due to the unique chemical and biological properties of nanosilver, which are appealing to the consumer products, food technology, textiles/fabrics, catalysis, and medical industries.

1.1.3 Silver Applications

In the earlier years, silver has been used as a precious commodity in currencies, ornaments, jewelry, food decoration, solar cells, and photography [25]. Silver and its compounds have extensive applications in the twentieth century including electrical conductors, electrical contacts, catalysis, electronics, mirrors, assembly of chemical equipment and brazing alloys, drinking water filtration system, swimming pool filtration systems, healthcare products, and medical tools [26]. Silver paints are used for making printed circuits. Silver threads are woven into the fingertips of gloves so that it can be used with touch screen phones. Most importantly, silver/silver nanoparticles have long been used as an effective antibacterial agent against a broad spectrum of gram-negative (Acinetobacter, Escherichia, Pseudomonas, Salmonella, and Vibrio) and gram-positive (Bacillus, Clostridium, Enterococcus, Listeria, Staphylococcus, and Streptococcus) bacteria, which means silver is toxic to bacteria [27]. In addition to this, silver/silver nanoparticles are also found to have antifungal, antiviral, anti-inflammatory, antibiofilm, antiglycoprotein film, surface plasmon resonance, plasmonic heating, and metal-enhanced fluorescence properties [28]. Silver and silver nanoparticles are broadly used in urinary catheters and endotracheal breathing tubes [29]. The silver diammine fluoride complex is a topical drug used to treat and prevent dental caries and relieve dentinal hypersensitivity [30]. One of the most beneficial uses of silver has been as a disinfectant, perhaps, which is routinely used in treating wounds and burns owing to its broad spectrum of toxicity to bacteria as well as its reputation of limited toxicity to humans [31]. Moreover, silver can easily bind with human body proteins (albumins and metallothioneins) and also interact with trace metals in metabolic process [32].

1.2 Silver Catalysis

Recently, catalysis by silver is an important methodology in organic synthesis owing to its more economical than other expensive transition metals (TM), excellent selectivity and stability, and environmentally benign nature. But, in comparison with other TM, silver catalysts have long been believed to have low catalytic efficiency, and the rapid development of silver chemistry was achieved only in the past few decades [33]. Generally, silver salts are mostly utilized as either s-Lewis acid or p-Lewis acid, with preference to s-coordination over p-coordination due to the ready availability of empty f orbitals and relativistic contraction of the electron cloud...