Impurities, effect

The devitrification rate is extremely sensitive to both surface and bulk impurities, especially alkah. Increased alkah levels tend to increase the devitrification rate and lower the temperature at which the maximum rate occurs. For example, a bulk level of 0.32 wt % soda increases the maximum devitrification rate 20—30 times and lowers the temperature of maximum devitrification to approximately 1400°C (101). The impurity effect is present even at trace levels (<50 ppm) and can be enhanced with the addition of alumina. The devitrification rate varies inversely with the ratio of alumina-to-alkah metal oxide. The effect is a consequence of the fact that these impurities lower glass viscosity (102). 

Cooling by means of evaporative cooling towers is required to maintain a constant temperature of 30—40°C. At higher temperatures, the deposit is rougher, impurity effects are more pronounced, lead codeposition is favored, and the manganese dioxide formed at the anode iacreases and tends to adhere rather than fall to the bottom of the cell. 

Testing and Control. Analysis and testing are required whenever a new plating solution is made up, and thereafter at periodic intervals. The analyses are relatively simple and require Httie equipment (78—80). Trace metal contaminants can be analy2ed using spot tests, colorimetricaHy, and with atomic absorption spectrophotometry (see Trace and residue analysis). Additives, chemical balance, impurity effects, and many other variables are tested with small plating cells, such as the Hull cell developed in 1937 (81,82). 

Strained set of lattice parameters and calculating the stress from the peak shifts, taking into account the angle of the detected sets of planes relative to the surface (see discussion above). If the assumed unstrained lattice parameters are incorrect not all peaks will give the same values. It should be borne in mind that, because of stoichiometry or impurity effects, modified surface films often have unstrained lattice parameters that are different from the same materials in the bulk form. In addition, thin film mechanical properties (Young s modulus and Poisson ratio) can differ from those of bulk materials. Where pronounced texture and stress are present simultaneously analysis can be particularly difficult. 

Obviously, the check for protic impurities becomes crucial if the ionic liquid is to be used for applications in which protons are known to be active compounds. Eor some organic reactions, one has to be sure that an ionic liquid effect does not turn out to be a protic impurity effect at some later stage of the research ... 

In order to avoid any source of inaccuracy that might arise from the fact that the absolute intensity line cannot be reproduced, on account of the nature of the instruments themselves, the intensity is always measured with respect to that of a standard sample. Let us suppose that I0/Is represents the ratio of the line height of the compound which is to be irradiated to that of the standard sample. After irradiation, the new ratio has become ///g. On eliminating Is then we get I/I0 which represents the intensity change on going from the irradiated to the nonirradiated compound. Suppose now that the concentration of the new chemical species or, in general terms, imperfections induced by irradiation be proportional to the amount of radiation absorbed in the sample. Then the relation which represents the impurity effect may immediately be written as follows ... 

All of these point defects are intrinsic to the heterogeneous solid, and cirise due to the presence of both cation and anion sub-lattices. The factors responsible for their formation are entropy effects (stacking faults) and impurity effects. At the present time, the highest-purity materials available stiU contain about 0.1 part per billion of various impurities, yet are 99.9999999 % pure. Such a solid will still contain about IQi impurity atoms per mole. So it is safe to say that all solids contain impurity atoms, and that it is unlikely that we shall ever be able to obtain a solid which is completdy pure and does not contain defects. 

Pireaux, J.J., Chtaib, M., Delrue, J.P., Thiry, P.A., Liehr, M. and Caudino, R. (1984) Electron spectroscopic characterization of oxygen adsorption on gold surfaces I. Substrate impurity effects on molecular oxygen adsorption in ultra high vacuum. Surface Science, 141, 211-220. 

The reason for stressing the importance of working with relatively pure reagents and solvents is that the rates of many reactions are extremely sensitive to the presence of trace impurities in the reaction system. If there is reason to suspect the presence of these effects, a series of systematic experinlents may be carried out to explore the question by seeing how the reaction rate is affected by the intentional addition of impurities. In many cases, lack of reproducibility between experiments may be an indication that trace impurity effects are present. 

There are several sources of irreproducibility in kinetics experimentation, but two of the most common are individual error and unsuspected contamination of the materials or reaction vessel used in the experiments. An individual may use the wrong reagent, record an instrument reading improperly, make a manipulative error in the use of the apparatus, or plot a point incorrectly on a graph. Any of these mistakes can lead to an erroneous rate constant. The probability of an individual s repeating the same error in two successive independent experiments is small. Consequently, every effort should be made to make sure that the runs are truly independent, by starting with fresh samples, weighing these out individually, etc. Since trace impurity effects also have a tendency to be time-variable, it is wise to check for reproducibility, not only between runs over short time spans, but also between runs performed weeks or months apart. 

If a reaction is reversible and if one has assumed a rate function that does not take the reverse reaction into account, one observes a downward curvature. As equilibrium is approached, the slope of this curve approaches zero. Another cause of curvature is a change in temperature during the course of the experiment. An increase in temperature causes an increase in the reaction rate, leading to an upward curvature. Bunnett (3) has discussed a number of other sources of curvature, including changes in pH and ionic strength, impurity effects, autocatalysis, and side reactions. 

Nonpolymeric amorphous dyes for electron transport, some of them containing an oxadiazole ring, were prepared and theoretically studied. It was concluded that reversible electron injections and ejection properties without impurity effects could be obtained for the symmetric and globular amorphous molecules <1997PCA2350>. Amplified spontaneous emission laser spikes were observed for some simple 2,5-diaryl oxadiazoles <1997PCA3260>. 

A few years later, Davison et al (1988) applied the ANG model of chemisorption to supported-metal catalysts. The key parameters were found to be the metal film thickness and the metal-support bond strength. Related papers followed, studying impurity effects (Zhang and Wei (1991), Sun et al (1994b)) and variation with metal substrate (Xie et al (1992)). 

Of course, even for the cyclic formals there are still unresolved problems, especially the mechanism by which metal halides initiate their polymerisation [16]. Once again it has become evident that attention to impurity effects and side-reactions is of paramount importance if the conclusions from chemical studies of catalytic reactions are to be valid. 

Impurity Effects in the Polymerisation of N-vinylcarbazole by Mercuric Chloride, M.A. Hamid, M. Nowakowska and P.H. Plesch, Die Makromolekulare Chemie, 1970, 132, 1-5. 

Attention should be paid to possible problems in the measurement of fluorescence quantum yields (some of which are discussed Section 6.1.5) inner filter effects, possible wavelength effects on Op, refractive index corrections, polarization effects, temperature effects, impurity effects, photochemical instability and Raman scattering. 

Figure 6.8. An example of the impurity effects. (Adapted from Gschneidner (1993)).

Intentional impurities, doping After the comments previously reported on impurity effects and on the path towards higher and higher purity, a few remarks may be noteworthy on intentional addition of Impurities . To underline this point we quote... 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

KlMURAH, H. J. Cryst. Growth 73 (1985) 53-62. Impurity effect on growth rates of CaCl2-6H20 crystals. 

The extent of this impurity effect is surprising and is worthy of examination. Consideration of Eq. (6.111) readily shows [23] for heterogeneous oxidation that there is no apparent gas-phase reaction. This equation is now written in the form... 

The work discussed in the previous paragraphs provides the framework for the prediction of crystal habit from internal structure. The challenge is to add realistic methods for the calculation of solvent and impurities effects on the attachment energies (hence the crystal habits) to allow this method to provide prediction of crystal habit. Initial attempts of including solvent effects have been recently described (71. 721. The combination of prediction of crystal habit from attachment energies (including solvent and impurity effects) and the development of tailor made additives (based on structural properties) hold promise that practical routine control and prediction of crystal habit in realistic industrial situations could eventually become a reality. 

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