Chemical uniformity

Other papers in this series dealt with silicates which, unfortunately, were not chemically uniform. The mineral tobermorite (Ca3Si207, 2 H20) was made from Ca2Si04 under various experimental conditions180). Strangely, the yalues of nAp derived from the adsorption of nitrogen were in some instances only one-third or one-quarter those calculated from the adsorption of water vapor. When only the latter data were counted, then Qp appeared to be a roughly linear function of nAp, and Us was 388 + 30 erg/cm2. The Us for the trihydrate Ca3Si207, 3 H20 was said to be 320 70 erg/cm2. 

Proper fractionation according to molecular weight can only be expected if the macromolecules are chemically uniform. For polymers that are nonuni-... 

In chemical reactions between low-molecular-weight compounds, new substances are formed that can, in principle, be separated from the unconverted reactants and by-products, e.g., by means of chromatography. With chemical reactions of macromolecular substances the situation is more complicated in that the main reaction and side reactions take place on the same molecular framework. If, for example, only 80 out of 100 CRUs in a polymer chain react in the desired sense while the rest either does not react at all or reacts in some other way, the remaining 20 units cannot be separated from the others since they all belong to the same macromolecule consequently, one cannot obtain a chemically uniform reaction product. 

Oxoalkoxocomplexes are oligomers of varying molecular complexity. The extent and conditions of distillation allow to control the nature of the species in solution, thus influencing the film-formation process [1368], This process of in situ modification ofmetal alkoxide solution by carboxylate ligand may have certain advantages with respect to the chemical uniformity as compared to the techniques based on simple mixing of a titanium alkoxide with alkaline-earth carboxylates. 

Another important result was the finding that inhomogeneous binary mixtures of chemically uniform copolymers did not homogenize upon adding a third copolymer capable of forming clear films with each of the two mixture components. As Molau (4) already found in the styrene-acrylonitrile system there is no homogenization. ... 

In practice, there are effective means for nearly eliminating the spread of chemical compositions (14). The essential condition for the formation of a chemically uniform copolymer is that the monomers must be maintained in the same ratio throughout polymerization. There are two procedures for maintaining this condition. First, it can be achieved by making a continuous addition of either the more reactive monomer alone or certain mixtures of the comonomers as the reaction proceeds. The second method of producing chemically homogeneous copolymers is to proportion a mixture of monomers whose composition is the same as that of the copolymer being formed. 

The local position of the particles in the cell can be recorded in all cases by scanning extinction measurements. Only in the special case of chemically uniform particles can the schlieren optical method also be used. 

The upper part of Figure 4 shows the results of the rapid density gradient centrifugation of a latex mixture of poly(styrene) and two chemically uniform co-... 

There are various factors that can influence the distribution of analytes in a dried blood spot. Water-soluble chemicals uniformly coated on DBS cards would redistribute when the blood was spotted. The redistribution of chemicals may depend on their properties, viscosity of blood, the volume spotted, and the technique used for spotting. Another factor is the viscosity of the blood. Viscosity is normally dependent on the blood composition (hematocrit, protein, lipid levels), and it can affect the physical spread of the blood spot in that the same volume of a less viscous blood will form a larger diameter spot than that of a more viscous blood sample. Viscosity, combined with the chemical redistribution on the sample cards, will increase the complexity of the analyte distribution. 

Greater chemical uniformity (often preferred in electronic applications because of purity)... 

The interstellar carbon grains contain both aliphatic and aromatic C—H and C—C bonds both are observed in absorption and emission in the ISM, but beyond this their exact composition is not known. Given the seemingly uniform elemental abundances in our local ISM we might expect that the dust composition would also be chemically uniform. However, the inferred elemental composition of silicates in the ISM... 

Unit IV if XY is absent), the basal zone, is not physically or chemically uniform across the site, suggesting varying degrees of archaeological disturbance. Unit III appears to be composed of two physically and chemically distinct sections. Units I and II have reasonable internal consistency. 

Mixing in a stream or river can be quantitatively illustrated by instantaneously releasing a mass of chemical uniformly throughout the cross section of a channel this pulse injection is best expressed as mass per cross section of river, [M/L2]. Ideally, one chooses a conservative tracer (i.e., a chemical that does not undergo degradation in the river and is not absorbed to the river channel or suspended particles). The lower panel of Fig. 2-4 shows a chemical concentration in a stream at several different times after a pulse injection. At any instant, the plot of concentration versus distance is bell-shaped ideally, if the mixing is truly Fickian, the curve has the shape of a Gaussian, or normal, curve,... 

It is advisable to carry out practical measurements in order to determine the amount of solvent required, yield and plate number for the process. See the literature [4-6] for further details. The selectivity of the solvent is of special importance with solid-liquid extraction. If the solute to be extracted is chemically uniform, the required solvent can be selected based on the similar polarity with the solutes. Table 2.1 provides further details. 

Such a material prompts reconsideration of the ideas embodied in Figure 19.1, especially the idea of treating a material as substrate plus additive, or a gas dissolved in a continuum, with material parameters K and K . The approach is powerful in Chapters 15 and 16, it facilitates progress it allows us to suppose that nonhydrostatic stress affects only the substrate, which is chemically uniform, and chemical change is affected only by the mean stress at a point, which is direction-invariant. This separates chemical effects from nonhydrostatic effects boxes (i) and (ii) in the lower part of Figure 19.1 become empty. But one cannot believe the mica substitution just described is affected only by the mean stress it must also be affected by the orientation of, for example, the maximum compression, and by the magnitude of the difference... 

MgO 1.50% BaO 0.20% H O 14.80%, which is very close to those reported for the parent rock (12), showing the substantial chemical uniformity of the material. Chabazite content of the grains evaluated through water vapour desorption technique (12,13) on both Na-and K-exchanged samples, has turned out to be close to 55% (mean value). 

---