The Influence of Impurities

The materials used, including the solvent, should be as pure as possible. There are several instances recorded (and doubtless a number unrecognized) in which traces of impurities introduced inadvertantly into a system have had catastrophic consequences. 

The pseudo first-order rate constants (k) for reaction of Co(CN)5H20 with N3 concentration in excess were reported as curved (Sec. 1.6.3) and have been interpreted for some 20 years as evidence for a 5-coordinate intermediate and a D mechanism. If however Co(CN)5H20 is generated in solution by aquation of Co(CN)5CP (rather than from Co2(CN) o02 as in the original studies) the plot does not deviate from linearity. 

Reasons are suggested why the latter behavior is correct and importantly, the original material could be shown to contain a slower reacting component which gives false data at higher Nj concentrations. The D mechanism remains a possibility however. Traces of impurities in a solution of Fe(CN)jH20 have led to false conclusions as to the of the coordinated water. The photolytic behavior of aqueous Fe(CN) depends on its history. Copper(II) ion is a potent catalyst even in micromolar concentrations for a number of reactions of Fe(CN) ion.-  

These catalytic effects are usually signaled by irreproducible behavior. If it is suspected that traces of metal ions may be causing peculiar rate effects, a strong ligand may be added to sequester the metal ion. The spontaneous decomposition of H2O2 has been reported as 4.7 X 10 M s at pH 11.6 and 35°C. This is the lowest recorded value and is obtained in the presence of strong chelators. In a similar way the decomposition of permanganate in alkaline solution (3.6) is markedly slowed when the reactants are extensively purified 

4Mn04- -t 40H- - 4MnO - + O2 + 2H2O and metal ion concentrations are reduced below 10 M.  

---

Thermal analysis has been widely and usefully applied in the solution of technical problems concerned with the commercial exploitation of natural dolomite including, for example, the composition of material in different deposits, the influence of impurities on calcination temperatures, etc. This approach is not, however, suitable for the reliable determination of kinetic parameters for a reversible reaction (Chap. 3, Sect. 6). 

The dependence of the C,E curves for a solid metal on the method of electrode surface preparation was reported long ago.10 20 67 70 219-225 in addition to the influence of impurities and faradaic processes, variation in the surface roughness was pointed out as a possible reason for the effect.10 67,70 74 219 For the determination of R it was first proposed to compare the values of C of the solid metal (M) with that of Hg, i.e., R = C-M/c-Hg 10,74.219-221 data at ff=0 for the most dilute solution (usually... 

Raw materials and auxiliary products used in a process as well as materials of construction for equipment items can be the eause of scale-up effects . Pure raw and auxiliary materials must be used in laboratory studies to eliminate the influence of impurities on the ehoice of the process route, catalyst selection, and search for satisfactory process conditions. However, pure chemicals are usually too expensive to use for manufacture on a commercial scale. It is common practice to use raw materials of technical grade in a full-scale plant. These materials contain impurities, which can act as catalysts or inhibitors. They can react with reactants or intermediates, thereby decreasing yields and selectivities of desired produets. Therefore, raw materials of technical grade, even from different suppliers must first be tested on laboratory scale. 

It is important to examine the influence of impurities or additives, such as sizes and lubricants, since these are often prime sources of higher AOX values rather than the substrate itself. It is equally important to assess the contribution of auxiliaries in the bleaching bath to the total AOX value. Kleber [224] has reported several studies of systems requiring compliance with an AOX consent of 3 ppm. Table 10.20 shows that (a) synthetic... 

The influence of impurities on the sign of the effect was studied by Kwan (13), who worked with the classical system ZnO-02 investigated by many authors. The sample of ZnO containing A1 (donor) showed a negative effect. A positive effect was observed with a sample of ZnO to which Li (acceptor) had been added. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

This article would not be complete without reference to the recent studies of Addadi, Berkovitch-Yellin, Lahav, and Leiserowitz on the influence of impurities on crystal growth. However, this topic is the subject of a review elsewhere in this volume, and we therefore limit ourselves to a statement of the nature of the studies. 

Obviously, species with weak bonds like will dissociate more easily than Sg. At lower temperatures the activation energy of radical formation is considerably larger, which has been explained by the influence of impurities, X, which react with the radicals according to... 

The influence of impurities on electrochemiluminescence emission behavior has been difficult to ascertain. The best quantum yield of emission under annihilative conditions thus far achieved is about 1 %.63 The preannihilative emission is one to two orders of magnitude less intense.11 As the concentration of fluorescer in emitting systems is ca. 10 3M, as low a concentration of impurity as 10 7M may be responsible for the entire emission. Investigations thus far conducted have tacitly accepted such impurity levels in these solutions and have concentrated on inferring their action by observing the effects of additives... 

With regard to the influence of impurities and accidental catalysts in general on the rate of reaction it may be said to be of little account, and there can be no doubt that what is measured is the rate of the uncatalysed decomposition. The reaction has now been investigated under... 

Experimental observation of the Franz Keldysh effect in real semiconductors is complicated by the possibility of absorption at u) < Eg in the absence of an electric field. This absorption can result from various phenomena including exciton excitations, the influence of impurities on the... 

They believe that the results for the film obtained with an extremely high vacuum are representative for the behavior of a clean nickel surface. The opposite effects could be due to the influence of impurities adsorbed on the nickel surface in a moderate vacuum. This opinion is supported by the fact that the adsorption giving rise to a negative surface potential is very rapid, in accordance with experimental evidence about rates of adsorptions... 

The formation of trimethylchlorosilane due to the reduction of the quantity of dimethyldichlorosilane and the absence of trimethylchlorosilane in reactions based on pure silicon seem to testify that trimethylchlorosilane is a product of dimethyldichlorosilane disproportioning, which occurs in the conditions of synthesis under the influence of impurities (first of all, aluminum and its compounds) ... 

Although almost all of the modern ionic liquids are per se air- and water-stable one has to bear in mind that upon addition of SiCU, TaF5, SeCU, AICI3 and other moisture sensitive compounds the resulting solutions are no longer water-stable. Thus, inert gas conditions are required to get reproducible results. From our experience only ultrapure ionic liquids should be employed for fundamental electrochemical studies unless the influence of impurities has been understood. Our own experience and that of many other groups have shown that even a few ppm of impurities can strongly alter fundamental studies. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

The dependence of extraction distribution coefficients on the concentration of HDEHP and HN03 and also the influence of impurities such as sodium, aluminum, iron and fluorine have been studied in our laboratory. These experiments show that if the concentration of HDEHP in kerosene is kept between 0.1 and 0.5M and HN03 kept between 2 and 3M the distribution coefficient of Pu(IV) will exceed 103 and the separation factor of Pu and Am will exceed 103. Under these conditions impurities described above do not affect the extraction of Pu(IV). 

Figure 9-4. Different shapes of sodium chloride crystals as a consequence of the influence of impurities.

In view of wideness of information connected with the influence of impurities on geometric and physical characteristics of nanotubes [3], here are presented only results of investigations of the local defects in zigzag (12,0)-nanotubes to appear after removing one or two neighboring carbon atoms is presented. Especial attention is paid to research of vacancies as more widespread kind of breach of regularity in arrangement atoms in nano tubes. We use for vacancies the... 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

In order to lessen the influence of impurities even further and perhaps get better conductivity at a still-lower leveling-off point, he hit upon mercury. Because it was a liquid at room temperature, it was easier to clean up, and the slippery liquid could be molded readily into wire, in which resistance could then be measured accurately. 

Practical problems, encountered with gas adsorption, like porosity, surface heterogeneity, non-inertness or partial solubility of the solid and the influence of impurities, etc. recur, mutatis mutandis. The extent to which porosity manifests Itself will in first instance be determined by the sizes of the molecules In the mixtures. 

The influence of impurities can be ruled out in tow ways by preparation of high-purity crystals or by elimination of the influence of the charge carriers introduced by the impurities. 

The use of catalytic reactions is very alluring, but optimization can be very difficult. The success of a catalytic reaction is influenced by the choice of catalyst, ligands, solvents, concentration, temperature, aging, and stirring rate. Catalyst decomposition and the influence of impurities, both co-catalytic and inhibitory, can play big roles. Furthermore, conditions that are optimal for one substrate may not be optimal for another. The interplay of all these parameters can make... 

Archer et al. [112] and Paulechka et al. [113] who reported incremental increases in Cp of 40, 30 and 31 J mol-1 K-1, respectively. Ge et al. [110] also reported measurements on the influence of impurities on the heat capacity where it was found that the heat capacity was lowered with increasing water or chloride and that this followed a linear relationship. Overall it was reported that for small chloride or water mass fraction contents, i.e., up to lxlO-3, a decrease in the heat capacity of 0.15% or 1.3% on average when compared with the halide free or dried IL respectively was observed. 

It is usually not difficult to determine the solubility of solids which are moderately soluble (greater than 1 mg/mL), but the direct determination of solubilities much less than 1 mg/mL is not straightforward. Problems such as slow equilibrium resulting from a low rate of dissolution, the influence of impurities, and the apparent heterogeneity in the energy content of the crystalline solid (Higuchi et al. 1979), can lead to large discrepancies in reported values. 

---