Sulfuric acid metastable

The white basic sulfate, 2Po02-S03, results when polonium(IV) hydroxide or chloride is treated with 0.02 N-0.25 N sulfuric acid. Like the selenate, it is yellow above 250°C and decomposes to the dioxide at 550°C. Solubility studies indicate that it is metastable in contact with 0.1 N-0.5 N sulfuric acid (10). 

Vapour pressures for a number of atmospherically relevant condensed systems have been measured with mass spectrometry. These systems include hydrates of HC1, HjS04 and HNO, supercooled liquids and pure water-ice, as well as the interactions of HC1 vapour with die solids, ice and NAT [23,47,50-55]. Vapour pressure measurements over HNOj/HjO hydrates have also been made using infrared optical absorption with light originating from a tunable diode laser [29]. This technique allowed the identification of the metastable NAD in presence of the more stable NAT under temperature and vapour pressure conditions near to those found in the polar stratosphere. Vapour pressures of Up, HN03, HC1, HBr over supercooled aqueous mixtures with sulfuric acid have been calculated using an activity model [56]. It provides a parameterized model for vapour pressures over the stratospheric relevant temperatures (185-235 K). 

In the saturator process (see Figure 12.7), neutralization and crystallization are carried out in the same vessel. The sulfuric acid is delivered to the suction side and the ammonia to the pressure side of the forced circulation pump. Crystallization of the metastable solution gives particle sizes generally between 0.5 and 3 mm. The salt is continuously discharged at the lower end of the saturator. The salt is separated in centrifuges, dried, and cooled. The mother liquor is returned to the saturator. Impurities in the sulfuric acid can adversely affect crystallization. Small quantities of phosphoric acid, urea, or inorganic salts are added to promote crystal growth295. 

Fig. A.2. Sulfuric acid freezing point temperature versus mass % H2S04 in acid. The dashed lines show metastable phases. Source Gable, C.M., Betz, H.F. and Maron, S.H. (1950) Phase equilibria of the system sulfur trioxide-water, Journal of the American Chemical Society, Vol. 72, 1445 1448. www.chemistry.org...

Less common among the metastable sulfur species are the polythionates (S Oi" with n = 4 to 9). TTiey are formed, for example, in volcanic lakes by the bubbling of H2S gas through sulfurous acid (Takano 1987). The polythionates break down eventually into sulfate and elemental sulfur. The polysulfides and polythionates are themselves metastable relative to sulfite, thiosulfate, S , and the sulfide species. In order to examine the redox-stability environments of these species, it is useful to construct an Eh-pH diagram ignoring sulfate, which is relatively inert. Such a plot is given in... 

Although anhydrous thorium sulfate dissolves in water at 0°C to the extent of 20 w/o, the solution is metastable and deposits hydrates on standing. Stable solutions at higher temperature require the presence of free sulfuric acid, as in solutions used to leach thorium minerals. Sec. 

Sometimes, the traditional methods (such as salt formation) used for increasing the dissolution rate and thereby the bioavailability of the drug in the body may not be feasible. This was the case for 2-[4-(4-chloro-2-fluorophenoxy)-phenyl]pyrimidine-4-carboxamide, a sodium channel blocker. It was a weak base with very low solubility of 0.1 pg mL in water, simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). The extremely low pATa of —0.7, made it non-amenable to salt formation, even with strong acids such as hydrochloric acid, sulfuric acid and phosphoric acid. Also, its glass transition temperature (Jg) at 43 °C, was close to that at room temperature, which limited the formation of the amorphous phase reproducibly. It should also be noted that the amorphous phase, which might be an option in some cases, was not desirable either. This is because it could lead to the formation of a metastable state that could limit its shelf life. As a result, co-crystal formation was attempted in order to manipulate its physicochemical and pharmacokinetic properties. 

Figure A.2 Sulfuric acid freezing point temperature versus mass% H2SO4 in acid. The dashed lines show metastable phases.

Table A.3 Metastable sulfuric acid freezing point data ...

A number of other hydrates of potential stratospheric importance have been reported in the literature. All these hydrates are metastable with respect to NAT or SAT, but the formation of metastables preceeding the stable phase is a thermodynamically common feature according to Ost-wald s rule . Most prominent metastable hydrates are nitric acid dihydrate (NAD), sulfuric acid hemihexahydrate (SAH), nitric acid pentahy-... 

Figure 2.17 shows that although the metastable elemental sulfur maybe present at pyrite surface, pyrite does not exhibit self-induced collectorless floatability except in very strong acidic media and a narrow oxidized Eh range. Such behavior may be similar to that of arsenopyrite in alkaline solutions due to the formation of hydroxides, thiosulphate. 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

The stabilities of species in the system S-Oj-HjO have been studied by numerous researchers. Much of this work has been summarized or critiqued by Garrels and Naeser (1958), Boulegue and Michard (1979), Morse et al. (1987), Schoonen and Barnes (1988), and Williamson and Rimstidt (1992). Thermodynamic data for some substances in the system S-O2-H2O are given in Table A 12.3 in the chapter appendix. The many aqueous sulfur species that are either thermodynamically stable or are important metastably are shown in Fig. 12.14. Acid-base reactions among the sulfur species are generally rapid and reversible. The redox reactions, however, may be fast and reversible (e.g., H2S/SJ see Boulegue and Michard 1979) or more often irreversible in the absence of bacterial activity (e.g., SO4 reduction to H2S). 

Dissociation constants and reaction enthalpy data for the stable and most important metastable sulfur species are summarized in Table 12.4. The log K values in Table 12.4 indicate the pH at which the acid and conjugate base have equal concentrations. Bisulfate (HSO4) is a relatively strong acid. 

Metastable sulfur allotropes are light-sensitive and should be protected from direct exposure to sun-light or other intense illumination. These materials are also very sensitive towards nucleophiles including alkaline glass smfaces. Therefore, pure and dry solvents should be used and the glassware should be treated with concentrated hydrochloric acid followed by rinsing with water and drying in an oven prior to use. 

Figure 9.3-5 is a Pourbaix. predominant-area, diagram for the S-O-HjO system for a total activity of all dissolved sulftir species of 10 1 ( S — 10" ). The only stable sulfiir species are HSOJ. SO2-. H.S. HS". and elemental sulfur. The formation of elemental sulftir films occurs in acid solutions as indicated. In basic solution, during the oxidation of sulfur-bearing compounds, intermediate metastabte solfor species such as thiosulfate, dithionate. and polythionates form. This is a problem previously described in the Sherrill Gordon process. Under acid conditions, during the dissolution of sulfide minerals, elemental sulfur layers often form but metastable solfor intermediates such as thiosulfate and sulfite are not observed. 

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