Effects of impurities

Nucleation rates are sensitive to the presence of foreign solid particles, because these objects may act as catalysts. If a nucleus is created on a solid particle, it will remain attached during part of the subsequent growth process. The growth equations for bubbles attached to solids have not been worked out mathematically, but it is rather obvious that interfacial tensions will be important as long as the bubbles are small. 

Corrosion of a surface may alter nucleate-boiling heat transfer in either direction. If the corroded surface is a good catalyst, the heat transfer rates can increase. Copper oxide is a better boiling surface for water than is copper (S2). On the other hand, oxidized stainless steel is sometimes better, but often poorer than unattacked stainless steel with water (B16). Oxidized chromium is a poor boiling surface for water compared with fresh chromium (J3). 

If fouling (deposition of a solid from the liquid) occurs on a boiling surface, the effect on heat transfer will depend on whether the deposit is a catalyst and whether it is very bulky. If the deposit is thin, the heat transfer may increase. This is an unnatural occurrence to observers not familiar with boiling—a little fouling is sometimes a good thing  

Impurities can be present in the boiling liquid itself. The evidence is strong that traces of dissolved material (excluding surface-active agents of course) have but a small effect on boiling, whereas suspended material is much more important. Wismer s important experiments on superheated liquids show a distinct difference between the maximum superheat attainable with water at atmospheric pressure in the presence of dissolved material and the values resulting for suspended matter. Some of the results are given in Table VI. 

Additional evidence of the small importance of dissolved impurities lies in the observations for water solutions. For glycerine in water, a smooth shift occurs in the values for ATe and h— as the concentration is increased from zero (M10). Similar results occur with sugar in water. Boiling curves for several concentrated solutions of salts in water have been reported (C6). Although these show that an appreciable shift results in the relationship between h and AT (compared with that for pure water) the effect does not seem to be unexpected or unusual. 

The protective oxide film on the surface controls the corrosion rate and pitting tendency of aluminium alloys. The corrosiveness of basin water is due to the ability of impurity ions to penetrate the oxide film to attack the aluminium 

Hie chloride content of storage basin water should be kept low to prevent pitting corrosion. It is difficult to specify a chloride ion limit below which pitting corrosion does not occur, because of the synergistic reactions that take place with other anions in the water. Sverepa found that increasing the chloride content from 0 to 50 ppm in water containing 116 ppm bicarbonate ions increased the number of pits but not their depths [2.21]. The effect was much higher in the presence of copper. Very little attack occurred in the presence of 10 ppm chloride and 116 ppm bicarbonate at pHS.O, while in the presence of 50 ppm chloride and 232 ppm bicarbonate, 0.1 mm deep pits occurred at the same pH. At a lower pH of 6.4, corrosion occurred with only 20 ppm chloride and 116 ppm bicarbonate. 

Heavy metal ions such as copper and mercury are very aggressive with respect to pitting corrosion of aluminium alloys [2.8]. The aluminium reduces the ions of copper, mercury, lead, etc. The heavy metal ions can also plate out on the aluminium surface and form galvanic cells where the aluminium becomes the anode and the heavy metal a very effective cathode. The threshold 

(The impure H2 contained 1 p.p.m. each of H20, 02, and hydrocarbons.) Possibly the effect of H2 O is greater at higher temperatures. 

The presence of soluble impurities can also affect the induction period, (section 5.5), but it is virtually impossible to predict the effect. Ionic impurities, especially Fe + and Cr +, may increase the induction period in aqueous solutions of inorganic salts. Some substances, such as sodium carboxymethyl-cellulose or polyacrylamide, can also increase whereas others may have no effect at all. The effects of soluble impurities may be caused by changing the equilibrium solubility or the solution structure, by adsorption or chemisorption on nuclei or heteronuclei, by chemical reaction or complex formation in the solution, and so on. The effects of insoluble impurities are also unpredictable. 

The effects of soluble impurities on crystal growth and crystallization processes in general are discussed in more detail in sections 6.2.8 and 6.4, respectively. 

Carbon can block permeation and create porosity at higher temperatures, which is detrimental to both membrane stability and permselectivity [76-79], espedaUy in combination with oxygen [71]. Exposure to unsaturated hydrocarbons at elevated temperatures is particularly detrimental [77, 80]. The formation of carbon on both the membrane and catalyst is promoted in palladium membrane reactors because of the selective removal of hydrogen [81], which necessitates the study of membrane, reactant/product gas mixture, and spedahzed catalyst in concert [51, 82]. For example, a Pd75-Cu25 (aU compositions in this chapter are given in 

Certain metals, such as mercury and germanium (a catalyst component), have been found to irreversibly poison the surface of palladium and should be avoided [74, 99, 106]. Palladium membranes may not be able to withstand direct contact 

From a practical viewpoint, it is probable that an azide in storage will absorb ions and organic species from the environment or by handling it is desirable to understand the influence of these impurities on the sensitivity of the stored material. Several investigations of the effects have been made with lead azide. 

To test the theory that metal deposited on the surface of lead azide would act as an electron trap, Reitzner et al. [6] deposited silver on the surface of lead azide. A sensitization was found for both slow and explosive decomposition. Lead nuclei are produced on the surface of lead azide by the action of ultraviolet light with the concomitant production of nitrogen. It has been postulated that the lead nuclei behave at elevated temperatures as electron sinks during the induction period and account for the observed shortened ignition delays [4,7]. 

Lead azide exposed to strongly adsorbed normal propylamine and di- 

Figui-e 2. Effect of bismuth on the ignition delay of lead azide [17], 

The postulated, ground-state, molecular-charge-transfer complex, 

Variation of the content of impurities in the different CNT preparations [21] offers additional challenges in the accurate and consistent assessment of CNT toxicity. As-produced CNTs generally contain high amounts of catalytic metal particles, such as iron and nickel, used as precursors in their synthesis. The cytotoxicity of high concentrations of these metals is well known [35, 36], mainly due to oxidative stress and induction of inflammatory processes generated by catalytic reactions at the metal particle surface [37]. Another very important contaminant is amorphous carbon, which exhibits comparable biological effects to carbon black or relevant ambient air particles. 

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Effect of impurities upon the melting point. Let us take a specific example and examine the effect of the addition of a small quantity of naphthalene to an equilibrium mixture of pure solid and liquid a-naphthol at the temperature of the true melting point (95 5°) at atmospheric pressure. 

Osmotic pressure experiments provide absolute values for Neither a model nor independent calibration is required to use this method. Experimental errors can arise, of course, and we note particularly the effect of impurities. Polymers which dissociate into ions can also be confusing. We shall return to this topic in Sec. 8.13 for now we assume that the polymers under consideration are nonelectrolytes. 

Rehable deterrnination of the solubihty of sihca in water has been comphcated by the effects of impurities and of surface layers that may affect attainment of equihbrium. The solubihty behavior of sihca has been discussed (9,27). Reported values for the solubihty of quartz, as Si02, at room temperature are in the range 6—11 ppm. Typical values for massive amorphous sihca at room temperature are around 70 ppm for other amorphous sihcas, 100—130 ppm. Solubihty increases with temperature, approaching a maximum at about 200°C. Solubihty appears to be at a minimum at about pH 7 and increases markedly above pH 9 (9). 

Because of the effects of impurity content and processing history, the mechanical properties of vanadium and vanadium alloys vary widely. The typical RT properties for pure vanadium and some of its alloys are hsted in Table 4. The effects of ahoy additions on the mechanical properties of vanadium have been studied and some ahoys that exhibit room-temperature tensile strengths of 1.2 GPa (175,000 psi) have strengths of up to ca 1000 MPa (145,000 psi) at 600°C. Beyond this temperature, most ahoys lose tensile strength rapidly. 

Figure 12 contrasts the decrease in conductivity of ETP copper with that of oxygen-free copper as impurity contents are increased. The importance of oxygen in modifying the effect of impurities on conductivity is clearly illustrated. Phosphoms, which is often used as a deoxidizer, has a pronounced effect in lowering electrical conductivity in oxygen-free copper, but Httie effect in the presence of excess oxygen. 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effec ts on materials. The chloride ion is a good example small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommena a particular material of construction. 

The effect of impurities in either structural material or corrosive material is so marked (while at the same time it may be either accelerating or decelerating) that for rehable results the actual materials which it is proposed to use should be tested and not types of these materials. In other words, it is much more desirable to test the actual plant solution and the actual metal or nonmetal than to rely upon a duphcation of either. Since as little as 0.01 percent of certain organic compounds will reduce the rate of solution of steel in sulfuric acid 99.5 percent and 0.05 percent bismuth in lead will increase the rate of corrosion over 1000 percent under certain conditions, it can be seen how difficult it would be to attempt to duplicate here all the significant constituents. 

J.R. Asay and Y.M. Gupta, Effect of Impurity Clustering on Elastic Precursor Decay in LiF, J. Appl. Phys. 43, 2220-2223 (1972). 

What is the effect of impurities on chemical reactions and upon process mixture characteristics ... 

In the next paper [160], Villain discussed the model in which the local impurities are to some extent treated in the same fashion as in the random field Ising model, and concluded, in agreement with earlier predictions for RFIM [165], that the commensurate, ordered phase is always unstable, so that the C-IC transition is destroyed by impurities as well. The argument of Villain, though presented only for the special case of 7 = 0, suggests that at finite temperatures the effects of impurities should be even stronger, due to the presence of strong statistical fluctuations in two-dimensional systems which further destabilize the commensurate phase. 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

It would appear that the effects of impurities at the grain boundary must be either (a) to increase the diffusion rates or (b) to influence the microstructure and increase the number of short-circuit paths. However, theoretical modelling of the grain boundary structure by Duffy and Tasker and... 

We have seen that the adverse effect of impurities can, within limits, be controlled by alloying additions. Thus silicon and aluminium are added to zinc, and manganese to aluminium and magnesium, to counter the effect of iron. 

The uncertain effects of impurities are avoided by periodic or continuous electrolysis of the solution at low current densities to remove metallic contaminants and by filtration through active carbon to remove organic substances. A concise review of the effects of impurities and their removal is given by Greenall and Whittington". 

By pre-titration of the generating solution before the addition of the sample, more accurate results can be obtained. Since the effect of impurities in the generating solution is minimised. 

Our study has led us to the point where we can realize that the primary effect of impurities in a solid is the formation of defects, particularly the Frenkel and Schottky types of associated defects. Thus, the primary effect... 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

W e know of many examples of the effect of impurities of crystallization. In many cases impurities will completely inhibit (2-4) nucleus formation. Reading the literature on this subject impresses one with the frequent occurrence of hydrocolloids as crystal modifiers, particularly where sugar or water is the material being crystallized. The use of gelatin, locust bean gum, or sodium alginate in ice cream is just one example of many practical applications of hydrocolloids in crystal modification. 

AU these features—low values of a, a strong temperature dependence, and the effect of impurities—are reminiscent of the behavior of p- and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. As a rule (although not always), ionic semiconductors have unipolar conduction, due to ions of one sign. Thus, in compounds AgBr, PbCl2, and others, the cation transport number is close to unity. In the mixed oxide ZrOj-nYjOj, pure 0 anion conduction t = 1) is observed. 

The effects of impurities are less important for oxygen-depolarized than for hydrogen-depolarized corrosion, since the values of polarization for oxygen reduction found at different metafs differ fess strongfy than those for hydrogen evofution. 

T. T. M. (2004) Effect of impurities on the mobility of single crystal pentacene. Appl. Phys. Lett., 84, 3061-3063. 

This long assessment of the analysis of the level of error of measurement that goes with flashpoint will be completed later (see para 1.3.7) by considering the effect of impurities that can be found in substances at their flashpoint. Nevertheless, it is sufficient to prove that it is not possible to have any confidence in the data of flash-points that can be found in the technical literature, especially when the safety expert has unique data only. To the author s knowledge, there were not until now... 

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