Stable orthorhombic crystals

Five chemical compositions constitute these pigment classes. Medium chrome yellow is essentially PbCr04 and is the reddest hue of yellow. Solid solutions of PbCr04 with PbS04 in varying proportions yield primrose and lemon chrome yellows. Their hue difference is also attributable to the their polymorphic nature— primrose yellow is orthorhombic in crystal structure and lemon yellow is monoclinic. The less stable orthorhombic crystal is treated during primrose yellow manufacture to minimize hue shifts and conversion. 

For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

Aminophenol. This is the most stable of the isomers under atmospheric conditions. It forms white prisms when crystallized from water or toluene. The orthorhombic crystals have a tetramolecular unit and a density of 1.195 g/cm (1.206 and 1.269 also quoted) (15,16) (see Tables 3—5). 

In the thermodynamic sense, a phase is defined as part of a chemical system in which all the material has the same composition and state. Appropriately, the word comes from the Greek phasis, meaning appearance . Ice, water and steam are the three simple phases of H20. Indeed, for almost all matter, the three simple phases are solid, liquid and gas, although we must note that there may be many different solid phases possible since H20(S) can adopt several different crystallographic forms. As a related example, the two stable phases of solid sulphur are its monoclinic and orthorhombic crystal forms. 

Thermodynamically stable sulfur forms deep yellow, nonodorous orthorhombic crystals with a space group Fddd-D, containing 16 molecules, i.e., 128 atoms in the unit cell. It has a density of 2.069 gm/cm and is well soluble in CS2. Its molecular unit is Sg, cyclooctasulfur, a crown-shaped molecule with a symmetry of i>4d. The pale flowers of sulfur, prepared by alchemist by distillation, are insoluble in CS2, and the structure is not yet fully understood. Another form, plastic sulfur, is obtained by melting sulfur to about 180°C, where it forms as highly... 

Grayish orthorhombic crystal or powder stable at ambient temperature density 1.70 g/cm melts at 816°C reacts with water and alcohol. 

The oxide exhibits two crystalline modifications, the reddish or orange-red alpha form, known as litharge, and the yellow beta form, massicot. The alpha form constitutes tetragonal crystals while the beta modification is a yellow amorphous powder of orthorhombic crystal structure. The alpha form is stable at ordinary temperatures, converting to the beta form when heated at 489°C density 9.35 g/cm (beta form) Moh s hardness 2 (alpha form) the oxide melts at 888°C vaporizes at 1,472°C with decomposition vapor pressure 1 torr at 943° C and 5 torr at 1,039°C practically insoluble in water (the solubdity of alpha form is 17 mg/L at 20°C and that of beta form 23 mg/L at 22°C) insoluble in ethanol soluble in dilute nitric acid and aUtahes. 

White orthorhombic crystals stable in air density 1.636 g/cm at 25°C soluble in water, about 8.3g /lOOmL at 20°C and 33 g/lOOmL in boiling water pH of 0.05M aqueous solution 4.005 at 25°C slightly soluble in ethanol. 

Elemental sulfur exhibits complicated allotropy, that is, it exists in many modifications.4 The stable, prismatic crystal form at room temperature, a-S or orthorhombic sulfur, is built up of stacks of Ss rings (Section 3.4). If heated quickly, it melts at 112.8 °C. If it is heated slowly, however, it changes to needlelike crystals of /3-S or monoclinic sulfur, which is the stable form above 95.5 °C and which melts at 119 °C. Both 0-S and the yellow mobile melt (below 160 °C) are composed exclusively of Ss rings. Solids containing S7, Sg, S10, S12, and other rings are known, but all slowly revert to Sg below 160 °C. 

Physical, Sulfamic acid is a dry acid having orthorhombic crystals. The pure crystals are nonvolatile, nonhygroscopic, colodess, and odorless. The acid is highly stable up to its melting point and may be kept for years without change in properties. Selected physical properties of sulfamic acid are listed in Table 1. Other properties are available in the literature (5—8). 

This compd is reported by Blatt(Ref 33) to exist in at least three polymorphic forms. The stable form crystallizes at RT as orthorhombic plates and flat rods... 

Potassium Nitrosochlor-ruthenate or Potassium Ruthenium Ni-trosochloride, 2KC1. RuCl3. XO or K2RuClsNO, is obtained by direct precipitation of concentrated solutions of potassium chloride and ruthenium nitrosochloride it also results on evaporation of mixed dilute solutions of the two substances.3 It yields black orthorhombic crystals which dissolve in water to a reddish violet solution. The aqueous solution is stable, its electric conductivity showing no alteration whatever after standing for two weeks. Its solution apparently contains three ions,4 namely, K , K and RuCls.XO". 

Palladous Nitrate, Pd(N03)2, may be obtained by dissolving the metal in nitric acid of density 1-35 to 1-4, and cooling the concentrated solution, when the salt crystallises out in brownish yellow, orthorhombic crystals. These are deliquescent, and readily dissolve in water, yielding a solution that is not very stable. When heated, the crystals decompose, yielding palladous oxide.2... 

The salt yields orthorhombic crystals isomorphous with the corresponding nitrosochlor-ruthenate (see p. 150). It is very stable for a compound of osmium. Its aqueous solution is quite stable, and even nitric acid does not attack the salt as vigorously as usual. 

Jolles and Berthou (1972) observed that tetragonal crystals of lysozyme were unstable above 25 C, especially at physiological temperatures, and transformed into orthorhombic crystals which are stable up to 55°C (see also Berthou and Jollds, 1974). Berthou et al. (1983) found that, although the conformations obtained from orthorhombic and tetragonal forms are similar, there are differences caused by crystal contact. Thus, Trp-63 and Pro-71 are much better ordered than in the tetragonal form, where they are exposed to solvent. These differences may account for the observed difficulty of inhibitor binding in the high-temperature crystalline form, but do not seem to reflect the behavior of lysozyme in solution at the same temperature. 

PROP Exists in 2 forms (1) red to reddish-yellow, tetragonal crystals stable at ordinary temps. (2) Yellow, orthorhombic crystals stable >489°. D 9.53, mp 897°. Insol in water, ale sol in acetic acid, chi HNO3, warm solns of fixed alkah hydroxides. 

PROP White or colorless platelets or orthorhombic crystals formed from quartz temperamres >870° (NTIS PB246-697). Stable from 870-914° at atmospheric pressure, but persists as a metastable phase below 8° forming low tridymite below 1° and middle tridymite from 117-1°. IDLH 25 mg/m ... 

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