Silver, resistivity

Experiments and calculations both indicate that electron transfer from potassium to water is spontaneous and rapid, whereas electron transfer from silver to water does not occur. In redox terms, potassium oxidizes easily, but silver resists oxidation. Because oxidation involves the loss of electrons, these differences in reactivity of silver and potassium can be traced to how easily each metal loses electrons to become an aqueous cation. One obvious factor is their first ionization energies, which show that it takes much more energy to remove an electron from silver than from potassium 731 kJ/mol for Ag and 419 kJ/mol for K. The other alkali metals with low first ionization energies, Na, Rb, Cs, and Fr, all react violently with water. 

The mechanism of Ag(I) cytotoxicity is unknown. Cell wall damage may be important and it has been shown that Cys-150 in the enzyme phosphomannose isomerase, an essential enzyme for the biosynthesis of Candida albicans cell walls, is the Ag(I) target in this organism (330). Silver resistant bacteria are known, but only recently has significant progress been made in understanding the resistance mechanisms (637). 

On the basis of Eq. (2), it is evident that the lower silver resistivity proportionally reduces the electrical power required to produce a given field. At the same time, it reduces the time constant RjL of the magnet which is an important factor in minimizing the final field-switching times. Section IV C discusses how the magnet time constant RjL and the power supply output voltage affect the maximum achievable slewing rate dBjdt. 

Lowest concentration tested that inhibited growth in 50% of strains during 10-day exposure Silver-resistant strains (55% of all strains tested) survived at least 24 h... 

Metals tarnish when their surface atoms react with gaseous substances in the air. Oxygen is a highly reactive element, as we saw in the previous chapter, and it combines with iron to form the ruddy oxide compound we recognize as rust. Copper reacts with oxygen and carbon dioxide to form a greenish patina of copper carbonate. Silver resists the advances of oxygen but will slowly combine with sulphur compounds in the air to form black silver sulphide. 

These mechanisms are of considerable microbiological and biochemical interest, although not all of the above agents find current use as biocides. The plasmid-mediated efflux pumps are particularly important, since efflux is one means whereby acquired resistance to antibiotics occurs (see earlier) and can be a mechanism of resistance to some clinically useful biocides (see later). No efflux pump comparable to those described for arsenate and cadmium [212] has yet been detected in silver-resistant bacteria [213] however, an up-to-date assessment of this subject is available [212]. 

Nair and Pradeep (2002) have reported that common Lactobacillus strains found in buttermilk, when challenged with silver and gold ions, assisted the growth of microscopic gold, silver, and gold silver alloy crystals of well-defined morphology within the bacterial cells. However, the exact reaction mechanism leading to the formation of silver nanoparticles by this species of silver-resistant bacteria was not... 

Silver, S. 2003. Bacterial silver resistance Molecular biology and uses and misuses of silver compounds. FEMS Microbiology Reviews, 27 341-53. 

Figure 10 The silver resistance genes, transcripts and the organization of the protein products in the membrane of Salmonella (a). Top line shows the mRNAs. The open boxes indicate different genes or open reading frames (ORFs) and their orientations. The nucleotides between the genes (nt) and the amino acids of the proteins (AA) are indicated, (b). The proposed functions and organization of the protein products relative to the bacterial structure are indicated. (Ref 71. Reproduced by permission of Nature Publishing Group (www.namre.com))...

Silver nitrate inhibits cell division and damages the cell envelope and contents of Ps. aeruginosa [110] sensitive cells increased in size and the cytoplasmic contents, cell membrane and outer cell layers all presented abnormalities. A strain of Ps. aeruginosa which was resistant to AgSD was much less affected by Ag in the form of silver nitrate [111]. Spheroplasts of the resistant strain were less susceptible than the sensitive strain to lysis by silver nitrate, which implies that the cytoplasmic membrane is a possible site of silver resistance. 

Studies of the antimicrobial and therapeutic properties of silver and its derivatives have yielded an extensive scientific literature, ranging from clinically- to biochemically-based papers. The latter area includes studies in enzymatic and other macromolecular interactions to silver resistance encompassing plasmid-mediated patterns. The reduction to one silver compound (AgSD) in the current British National Formulary and to two compounds (AgSD and silver nitrate ophthalmic solution) in USP XXII may not be a true reflection of the use of silver in health care today. Silver compounds also find use in non-clinical situations, for example, as water disinfectants and, not considered in this review, as silver staining techniques for microscopy. 

SiAwsoN RM, I Van Dyke M, Lee H and Trevors JT (1992) Germanium and silver resistance, accumulation and toxicity in organisms. Plasmid 27 72-79. 

C Cervantes, G Ji, JL Ramirez, S Silver. Resistance to arsenic compounds in microorganisms. EEMS Microbiol Rev 15 355-367, 1994. 

Amit, G., Maria, M., Simon, S. (1998). Effects of halides on plasmid-mediated silver resistance in Escherichia coli. Appl. Environ. Microbiol, 64, 5042-5045. 

Li X, Nikaido H, Williams K. Silver resistant mutants of E. coli display active efflux of Ag-ions and are deficient in porins. J Bacterial 1997 179 6127-32. 

Li XZ, Nikaido H, Williams KE (1997) Silver-resistant mutants of Escherichia coli display active efilux of Ag and are deficient in porins. J Bacteriol 179 6127-6132 Li Q, Mahendra S, Lyon DY, Brunet L, Liga MV, Li D, Alvarez PJJ (2008) Antimicrobial nanomaterials for water disinfection and microbial control potential applications and implications. Water Res 42 4591-4602... 

Gupta, A., Phung, L.T., Taylor, D.E., and Silver, S. (2001) Diversity of silver resistance genes in IncH incompatibility group plasmids. Microbiology, 147, 3393-3402. 

Silver, % Resistivity, ohms/mif TCRat25-105°C, ppm per °C Noise, db/(ieca(ie... 

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