Impurities crystallization

Properties. Anhydrous aluminum chloride is a hygroscopic, white soHd that reacts with moisture ia air. Properties are shown ia Table 1. Commercial grades vary ia color from light yellow to light gray as a result of impurities. Crystal size is dependent upon method of manufacture. At atmospheric pressure, anhydrous aluminum chloride sublimes at 180°C as the dimer [13845-12-0] Al2Qg, which dissociates to the monomer beginning at... 

Boiling in the bulk of the fluid generally takes place at submicron nucleation sites as impurities, crystals, or ions. When there is a shortage of nucleation sites in the bulk of the liquid, its boiling point can be exceeded without boiling then the liquid is superheated. There is, however, a limit at a given pressure above which a liquid cannot be superheated, and when this limit is reached, microscopic vapor bubbles develop spontaneously in the pure liquid (without nucleation sites). 

The second term on the right-hand side of Eq. (146) vanishes in the continuum limit when use is made of electrical neutrality. For the defects in the impure crystal the term is again zero. In the intrinsic case it is not identically zero but is much smaller than the other terms (details can be found in Ref. 4). The final term to be evaluated in Eq. (146) is found, by substituting the asymptotic value of mu, to be... 

Also the effect of impurities in a crystal on the Vickers hardness was analysed. In Figure 4 are shown the force dependency curves of the Vickers hardnesses of a pure sodium chloride crystal and a sodium chloride crystal grown in a solution with an impurity of 10 % urea. The hardness of the pure sodium chloride crystal is up to 25 % higher than the hardness of the impure crystal. 

Silicon glass is made by fusing pure quartz crystals or glass sand (impure crystals), and it is typically about 99.8% silicon dioxide. It is high-melting and difficult to fabricate. 

Once more, the warning concerning using the commercial raw materials must be uttered. All crystals growth and nucleation phenomena are pronouncedly dependent on small amounts of impurities. Crystal habit, extent of twinning, etc, may be altered by unknown impurities in the commercial materials, it is essential, therefore, that any rate measurements which are to be of use for plant design must use the approach reagents. 

The bulk properties of a material are best obtained from large single crystals. However, measurements on single crystals of zinc oxide of desirable purity and size are not yet available. Some work, however, has been done on small pure crystals and larger impure crystals. 

Dissolve the impure crystals in about 400 cc. of hot water to which a little acetic acid has been added and then allow the solution to become thoroughly cold. The yellow or brown crystals are quite pure. 

If the second mother liquor is evaporated to about half of its original volume, a small third crop of very impure crystals may be obtained. 

The entire crop of impure crystals is extracted for a total of seven or eight times with two separate 500ml portions of boiling water, and filtered while hot. After each extraction, the filtrate is cooled in ice and the liquid is decanted from the orange crystals. 

The poles of t(z) give the eigenenergies of the impurity-crystal coupled system when A is substantially larger than the band width of W, there exists a discrete impurity level, which is solution of... 

In the early part of the 1800s chemists in Europe took willow leaves and boiled them with different solvents to try to extract the active ingredients. In 1825 an Italian chemist filtered such a solution and evaporated away the solvent. He obtained impure crystals of a compound containing some of the active ingredient. Repeated recrystallization and refinement of his experimental technique produced a pure sample of the unknown material (Figure 1.1). 

With all crystalline substances, the crystal form is an intrinsic indication of purity. Impurities are often recognized macroscopically, but more generally their presence is revealed by microscopic observation of a different crystal habit or amorphous character. There are cases where the impurity crystallizes with the main product and is not directly detectable. Such mixed crystals, however, frequently have a different crystal form from that of the pure compound, and are distinguishable if a direct comparison is made with a pure sample. The control of purity of sulfonic acids, which is made difficult by the lack of characteristic melting points, is simplified somewhat by the fact that numerous salts can be prepared from them, and one or more of these will usually be found to have a recognizable characteristic crystal form. 

Appendix C contains the chemical formulae for the minerals used in this book. There are very few minerals that have the ideal crystalline structures discussed above. There are sufficient substitutional impurities, crystal defects, and distortions that make the CBPC structure significantly different from the models discussed above. Several well-established minerals exhibit these features, as are many of those listed in Appendix C. For example, Ca(UO2)2(PO4)2T0H2O is formed by the substitution of Ca in autunite by uranyl (UO2) ions, making the autunite a mineral of radioactive uranium. Similarly, (Ce,Th)P04 is formed by the substitution of the Ce in monazite by Th. Numerous minerals can be formed by substitutions and provide a researcher sufficient degree of freedom to synthesize very complex minerals to produce useful CBPCs. 

Limited information is available on the uv-visible spectral properties of BjO, and related materials, both as pure materials and with transition-metal impurities. The lowest-energy fundamental absorption in B2O3 is at about 8.6 eV (Izumitani and Hirota, 1985), close to the value of 8.4 eV calculated by Tossell for B(OH)j (Tossell, 1986). To assess the environment encountered by metal ion impurities, crystal-field (d-d) transitions, uv absorption spectral transitions (e.g., 6s-)-6p in Pb ), or ESR spectra of odd electron species can be studied. For example, Lin and Angell... 

If the compound is relatively impure, crystallization usually entails considerable loss of material, and several recrystallizations are required to effect complete purification. The procedure often may be... 

The conditions for stable growth of a crystal are closely associated with segregation of dopants or impurities. Crystal growth theory includes an expression for the effective segregation coefficient K, for a crystal growing from a medium in which the dopant distribution is characterized by a boundary layer of thickness 5 ... 

A large majority of chemical products as such, whether natural or synthetic, are first produced as solutions of greater or less purity and concentration. In many cases where these solutions are of sufficient purity, i.e. contain besides the solvent either no impurities which will interfere with later use, or only small amounts of such impurities, crystallization offers no advantages. A few cases may be cited ... 

The best Na +, K +, Rb, and Cs ion conductors all belong to the five families described above. Some other high-conductivity materials are represented in Table 7.2 and Figure 7.17, but descriptions of their structures are omitted here. Only a very specific class of oxide structures with a 3 A translation (types I-XIV in Figure 7.19 [48,49,53,168-187]) will be reviewed briefly. This short distance precludes the location of any atom or ion between two large alkali ions, guaranteeing their free motion in that direction [62]. The freedom may, however, be restricted by impurities, crystal defects (e.g., anions on cation sites [188]) and grain-boundaries in ceramics. 

---