Silicates samples

It is important to remember that in AAS, as in most analytical techniques where a solid sample is converted into a solution for analysis, it is the concentration of the element which is determined, whereas it is sometimes the concentration of the oxide which is often required. This is always the case for silicate samples (e.g., glass, pottery, and some minerals), but not of course for... 

Diffusion from spherical silicate samples can be studied readily by observing the loss of volatile components of the silicate as a function of time. Where the sphere is initially uniform in composition and subsequent vaporization allows one to assume a zero surface concentration of the vaporizing component thereafter, the solution to the differential equations and boundary conditions governing concentration-independent diffusion is given by... 

Examples of the application of this method are given in Ref. 12. An extension of this method directly to fission products formed in situ in silicate particles is presented here. Also in this study an attempt was made to improve the geometrical match between the mathematical model and the actual silicate sample. 

Summary of D(efl/R2 for different sorbates in silicate sample Si-MCM-41 with different... 

Although acid attack is a classical means of dissolving silicate samples, the use of lithium metaborate fusion is a new departure, originating in the work of Ingamells [182] who showed that a clear aqueous solution could be easily and quickly prepared from silicates. Fluxes decompose most substances at the high temperature required for their use (500-1000 °C) and the high concentration of... 

Lippmaa et al. (86) have successfully studied solid state Si NMR of several trimethylsilyl silicate samples using proton-enhanced nuclear induction spectroscopy and magic angle spinning. They showed that the high resolution Si NMR of solids can be useful for structural studies of certain soluble and insoluble silicates, because many of these compounds with well defined molecular structures in the solid state tend to undergo very complicated rearrangements and condensation in solution. (93)... 

The 2D INADEQUATE experiments on ZSM-S gave substantially better results than 2D COSY experiments on the same highly siliceous samples. In the case of the orthothombic form (12 T sites), almost all of the expected cotmectivities were observed, whereas for the monoclinic form (24 T sites), 38 of the total of 48 connectivities were clearly registrated, allowing the assignment of the spectrum (16). 

Fyfe and coworkers (28) modified a ZSM-11 synthesis method developed by Chu (29) to produce a very highly crystalline siliceous sample from which highly resolved X-ray diffraction patterns and ID and 2D 29si MAS NMR spectra were obtained. 

The effect of the addition of F to the starting mixture on the adsorption and the catalytic properties is quite significant. For example, the hydrophobicity of purely siliceous samples is considerably enhanced (less Si-OH groups). Figure 6 shows that such a material adsorbs more n-propanol from a dilute n-propanol + water solution than a material which was prepared in alkaline medium. 

Thus the combined use of X-ray and adsorption methods can be especially useful to determine the variation of the silicate sample structure resulting from the modification. 

Samples containing silicate minerals are fused with Na2C03 or NaOH before HF is distilled off. Larger quantities of silica precipitate in the still and interfere in the distillation. If the silicate sample is fused with Na2C03 and ZnO, silica remains in the solid phase during subsequent leaching with water. Fluorine is distilled from the silica-free filtrate. 

A glass membrane electrode was used to measure the pH of the silicate solutions. Adsorption of the smaller alkali metal cations, K, Na, and Li, caused an underestimation of the pH. The pH error reached several pH units for the most alkaline Li silicate samples. On the other hand, the errors for Na silicates were around 0.5 pH unit and the errors for larger cations were relatively small. Corrections to the measured pH were made by comparison of the electrode response for the silicate solutions with that for alkali metal hydroxide solutions of known concentration. Since the hydroxide content of the base solutions is known, the interference from cation adsorption was calculated and added to the pH measured for the silicate solutions (16). 

As indicated above, Si MAS NMR can provide a wealth of structural information on zeolites, limited in the case of highly siliceous samples only by the spectral resolution. In solution, the application of two-dimensional NMR techniques has provided considerable information on the two-dimensional connectivities between atoms within molecular structures [63]. For example, the HSC (heteronuclear shift correlation) sequence establishes heteronuclear connections such as Si/ H, etc. the COSY sequence defines homonuclear correla-... 

All the vanadium silicate samples have been characterized to investigate their thermal stability, the location, co-ordination number and oxidation state of vanadium in different atmospheres by a variety of techniques. Some of the physical properties of representative vanadium silicates are given in Table 1. The oxidation reactions reported here are confined to calcined fin air) samples of VS-2 and V-NCL-1 with SiA = 79 and 250, respectively. Absence of ESR signals due to paramagnetic vanadium species indicated that all vanadium is in pentavalent oxidation state in these samples. For comparison, data on their respective silica polymorphs viz., silicalite-2 (S-2) and Si-NCL-1 are also included in Table 1. These samples as well as those with different Si/V ratios, (Figs. 1 and 2) probably contain vanadium in the silicalite framework as may be inferred from the following observations - 1) The unit cell parameters of V-samples increase linearly with vanadium content, as shown in Rg. 1... 

A silicate sample requires separation in order to remove interferences. 

For comparison with the above silicic acids from concentrated silicates, samples of the same silicates were diluted from 5 M to 1.0 M and aged at 23 C for 3 months. 

Although accurate results were also reported for leaching road dust with aqua regia under high-pressure and temperature without HF (Muller and Heumann 2000), addition of HF is mostly required for siliceous samples. Efficient HF digestion procedures have been recommended to dissolve strongly bound Pd from silicates (Limbeck et al. 2003). Excess HF is evaporated in open vessels or masked... 

A sodium silicate sample containing 5 mol% soda is originally equilibrated at 600 °C. It is then heated to 700 °C. Describe the original and final microstructures, including composition and amounts of each phase. 

FIGURE 10-1 Scanning electron micrographs of Li-layer silicate (sample El) (upper) and NH4- layer silicate (sample E4) (lower). 

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