Rhombic crystalline

Nickel Carbonate. Nickel carbonate [3333-67-3], NiCO, is a light-green, rhombic crystalline salt, density 2.6 g/cm, that is very slightly soluble in water. The addition of sodium carbonate to a solution of a nickel salt precipitates an impure basic nickel carbonate. The commercial material is the basic salt 2NiCo2 3Ni(OH)2 4H20 [29863-10-3]. Nickel carbonate is prepared best by the oxidation of nickel powder in ammonia and CO2. Boiling away the ammonia causes precipitation of pure nickel carbonate (32). 

Cobalt(II) hydroxide [1307-86-4], Co(OH)2, is a pink, rhombic crystalline material containing about 61% cobalt. It is insoluble in water, but dissolves in acids and ammonium salt solutions. The material is prepared by mixing a cobalt salt solution and a sodium hydroxide solution. Because of the tendency of the cobalt(II) to oxidize, antioxidants (qv) are generally added. Dehydration occurs above 150°C. The hydroxide is a common starting material for the preparation of cobalt compounds. It is also used in paints and Hthographic printing inks and as a catalyst (see Paint). 

We present here the preliminary results of our attempt to develop a new method for the analysis of pyrite in coal and lignite. It is well known that sulfur in coal is present in different forms. In particular, although the iron sulfide in coal is generally pyrite ( 1), other iron sulfides are frequently present. For example, iron disulfide occurs as marcasite, a rhombic crystalline form, as well as pyrite, a cubic crystalline form. Perhaps the term disulfide sulfur should be used to replace the pyritic sulfur more commonly quoted, as recently suggested by Youh (2). Since the chemical reactivity of these two disulfides of iron is similar, our method will record them equally well. Nonetheless, we will continue to refer to the pyrite determinations here, although we are really talking about the chemical species FeS2 rather than a particular crystalline structure. 

When a pure substance is liquefied from the solid state or vaporized from the liquid at constant pressure, there is no change in temperature but there is a definite transfer of heat from the surroundings to the substance. These heat effects are commonly called the latent heat of fusion and the latent heat of vaporization. Similarly, there are heats of transition accompanying the change of a substance from one solid state to another for example, the heat absorbed when rhombic crystalline sulfur changes to the monoclinic structure at 95°C and 1 bar is 360 J for each gram-atom. 

Sulfur melts at approximately 120 °C so it can be melted by high-pressure steam, which allows sulfur to be obtained by the Frasch process described earlier. Solid sulfur exists in several allotropic forms. At room temperature, the rhombic crystalline form is stable, whereas above 105 °C, a monoclinic form is stable. These two crystalline forms are shown in Figure 15.2. Other forms of sulfur include a plastic or amorphous form that can be obtained by rapidly cooling molten sulfur. For example, pouring molten sulfur at 160 °C into water produces the amorphous form. 

Th (or form I), has a rhombic crystalline form [32,33]. One anhydrous form, referred to as T(3 (or form III), is a monoclinic form [34-36]. Another anhydrous form, referred to as Ta (or form II), was first identified by Sussich et al. [37] and its property has been extensively studied by differential scanning calorimetry (DSC), powder X-ray diffractometry [36,38-40] and FTIR [35,41,42]. However it was not until more recently that we succeeded in the X-ray analysis in which its crystalline structure was revealed [43]. 

Properties White, rhombic, crystalline powder very volatile forms urea upon heating. Soluble in water and alcohol. Sublimes at 60C, decomposes in air to evolve ammonia. 

Properties Rhombic, crystalline prisms. D 7.6, mp 1800C. Insoluble in water and acids except hydrogen fluoride. 

Properties Colorless, lustrous, rhombic, crystalline prisms or white powder slight characteristic odor slightly bitter taste. Efflorescent, mp 115-117C, anhydrous mp 105C, bp 258C. Soluble in alcohol and ether slightly soluble in water. Combustible. 

Under these conditions, for example, iodine is normally crystalline, hydrogen is gaseous, and sulphur exists as the rhombic crystalline form. In general, the compound formed will also be in its standard state. (This is not always the case, however. Thus it is possible, by a form of extrapolation, to calculate the standard enthalpy of formation of water vapour at 1 bar and 25°C, although it is normally liquid at this temperature. It is convenient to use such values for reactions at temperatures higher than 100°C, say, because allowance will already have been made for the heat of vaporization of the water.) The omnibus expression for the enthalpy of formation is shown below. 

A portion of the residue is dissolved in a drop or two of H,0, and an equal quantity of colofie concentrated HNO, added if urea be present in sufficient quantity there appear white, shining, hexagonal or rhombic, crystalline plates or six-sided prisms of urea nitrate. 

Shiny, rhombic crystalline needles. The pure salt may be stored in a desiccator for many weeks. Slightly soluble in water and liquid NHg. Saponified rapidly to Imldosulfonate in an acid medium. Saponified to amldosulfonate at boiling temperat-jres. 

Rhombic crystalline lamellae. Loses NH3 and H3O in air and converts to the monohydrogen salt. On heating, the aqueous solution loses so much NH3 that the solution corresponds at the end to the dlhydrogen salt. 

Sulfur used by the rubber industry is of a special grind (or mesh size) derived from sulfur in the rhombic crystalline form. Sulfur that is used today comes from either recovered sulfur from natural gas or from the Frasch mining process as shown in Figure 10.4. 

It is not difHcult to make insoluble sulfur. Just simply heating sulfur beyond a certain melt transition temperature and then quench cooling it in water will produce this insoluble sulfur. This has been done many times in chemistry classes. However, this chemistry class product reverts back to crystalline sulfur in a matter of hours. The insoluble sulfur from this simple lab process is not sufficiently stable to be used weeks later in production. So the real commercial challenge is to produce stabilized insoluble sulfur. The Flexsys (now Eastman) process uses proprietary techniques to ensure that their insoluble sulfur does not revert back to the rhombic crystalline form before it is shipped and used by the customer. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

Ammonium sulfate [7783-20-2], (NH 2 U4, is a white, soluble, crystalline salt having a formula wt of 132.14. The crystals have a rhombic stmcture d is 1.769. An important factor in the crystallization of ammonium sulfate is the sensitivity of its crystal habit and size to the presence of other components in the crystallizing solution. If heated in a closed system ammonium sulfate melts at 513 2° C (14) if heated in an open system, the salt begins to decompose at 100°C, giving ammonia and ammonium bisulfate [7803-63-6], NH HSO, which melts at 146.9°C. Above 300°C, decomposition becomes more extensive giving sulfur dioxide, sulfur trioxide, water, and nitrogen, in addition to ammonia. 

Silver nitrate forms colorless, rhombic crystals. It is dimorphic and changes to the hexagonal rhombohedral form at 159.8°C. It melts at 212°C to a yellowish Hquid which solidifies to a white, crystalline mass on cooling. An alchemical name, lunar caustic, is stiU appHed to this fused salt. In the presence of a trace of nitric acid, silver nitrate is stable to 350°C. It decomposes at 440°C to metallic silver, nitrogen, and nitrogen oxides. Solutions of silver nitrate are usually acidic, having a pH of 3.6—4.6. Silver nitrate is soluble in ethanol and acetone. 

Strontium carbonate is a colorless or white crystalline soHd having a rhombic stmcture below 926°C and a hexagonal stmcture above this temperature. It has a specific gravity of 3.70, a melting point of 1497°C at 6 MPa (60 atm), and it decomposes to the oxide on heating at 1340°C. It is insoluble in water but reacts with acids, and is soluble in solutions of ammonium salts. 

Sulfur crystallizes in at least two distinct systems the rhombic and the monoclinic forms. Rhombic sulfur, Sa, is stable at atmospheric pressures up to 95.5°C, at which transition to monoclinic sulfur, SP, takes place. Monoclinic sulfur is then stable up to its natural melting point of 114.5°C. The basic molecular unit of both of these crystalline forms of sulfur is the octatomic sulfur ring Other forms of sohd sulfur include hexatomic sulfur as well as... 

Bornyl Acetate.—The acetic acid ester is the most important of the series. It is a constituent of pine-needle and rosemary oils, and has a most fragrant and refreshing odour. It is prepared artificially by the action of acetic anhydride on borneol, in the presence of sodium acetate, or by the condensation of borneol with glacial acetic acid in the presence of a small amount of a mineral acid. It is absolutely necessary in the reproduction of any pine odour. It is a crystalline body, crystallising from peDroleum ether in rhombic hemihedric crystals melting at 29°. The optical activity depends on that of the borneol from which it has been prepared. It has the following characters —... 

There are two basic crystallisation forms for sulphur - monoclinic and rhombic. Rhombic is the most stable form, at least up to 96 °C the other types revert to this stable form at a rate dependant upon temperature. When sulphur solidifies from the molten state (melting point 114 °C) the crystalline form which occurs is monoclinic (needle-like crystal structure). Below 96 °C, the monoclinic form becomes metastable and changes into the rhombic form. 

Hitchman and his co-workers (121,122,151) have shown how the ground state wave function (in the form ax2 + by2 + cz2) can be obtained for rhombic copper(II) systems the coefficients a, b and c thus obtained are in reasonable agreement with those found by analysis of the e.s.r. spectrum. Marshall and James (123) have attempted an ambitious analysis of the optical and magnetic properties of [Cu(H20)6]2+ in several crystalline environments the AOM was used to parameterise the theoretical expressions for the various experimental properties, in order to see whether a great... 

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