Sulfur reducing acids, mixed

Putidaredoxin. Cushman et al. (36) isolated a low molecular iron-sulfur protein from camphor-grown Pseudomonas putida. This protein, putidaredoxin, is similar to the plant type ferredoxins with two irons attached to two acid-labile sulfur atoms (37). It has a molecular weight of 12,000 and shows absorption maxima at 327, 425 and 455 nm. Putidaredoxin functions as an electron transfer component of a methylene hydroxylase system involved in camphor hydroxylation by P. putida. This enzyme system consists of putidaredoxin, flavoprotein and cytochrome P.cQ (38). The electron transport from flavoprotein to cytochrome P.cq is Smilar to that of the mammalian mixed-function oxidase, but requires NADH as a primary electron donor as shown in Fig. 4. In this bacterial mixed-function oxidase system, reduced putidaredoxin donates an electron to substrate-bound cytochrome P. g, and the reduced cytochrome P. g binds to molecular oxygen. One oxygen atom is then used for substrate oxidation, and the other one is reduced to water (39, 40). 

NITRATING ACID [legal label name for various concentrations of sulfuric and nitric acid mixtures (mixed acid often 61% sulfuric/36% nitric)]. A strong oxidizer. Violent reaction with reducing agents, bases, 1,2-dimethyl-2-trimethylsilylhydrazine, phthalic anhydride, lithium-2,2-dimethyltrimethylsilyl hydrazide, nitronaphthalene, organic solvents, and other organic materials. Violent reaction when water is added to acid to dilute, always add acid to water. See Nitric Acid and Sulfuric Acid. 

The addition of nitric acid to hydrochloric or sulfuric acid significantly reduces corrosion rates. Titanium is essentially immune to corrosion by aqua regia (3 HCl 1 HNO,) at room temperature. ASTM grades 7 and 12 show respectable corrosion rates in boiling aqua regia. Corrosion rates in mixed acids will generally rise with increases in the reducing acid component concentration or temperature. 

Silicon carbide (SiC) has a tetrahedral structure and is the only known alloy of silicon and carbon. Both elements have four electrons in the outer shell, with an atom of one bonded to four atoms of the other. The result is a very stable structure not affected by hydrogen or nitrogen up to 1600°C. In air, SiC begins decomposing above 1000°C. As with other compounds, a protective oxide layer forms over the silicon, reducing the rate of decomposition. Silicon carbide is highly resistant to both acids and bases. Even the so-called white etch (hydrofluoric acid mixed with nitric and sulfuric acids) has no effect. 

Sodium bicarbonate is generally added to increase alkalinity and muriatic acid (HCl) or sodium bisulfate (NaHSO ) to reduce it. In general, with acidic sanitizers such as chlorine gas or trichloroisocyanuric acid, ideal total alkalinity should be in the 100—120 ppm range, whereas, with alkaline products such as calcium, lithium, or sodium hypochlorite, a lower ideal total alkalinity of 80—100 ppm is recommended (14). Alkalinity is deterrnined by titration with standard sulfuric acid using a mixed bromcresol green—methyl red indicator after dechlorination of the sample with thiosulfate. Dechlorination with thiosulfate causes higher readings due to formation of hydroxyl ion (32) ... 

Laboratory Preparation of Absolute Nitric Acid, Strong nitric acid, 99%+, free from oxides (white) is used for many nitrations. Since nitric acid of this quality is not generally commercially available, it is usually prepd in small quantities in the laboratory as needed. The customary procedure is to mix one p of coned nitric acid ( d 1.42g/cc) with 2p of sulfuric acid (d 1.84g/cc) and to distill the nitric acid from this mixt under reduced press, using an all glass system... 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

---