Digestion-Dissolution

The usual equipment for pressure digestion consists of horizontal roll autoclaves or vertical autoclaves equipped with a stirrer. For NaOH leaching. Inconel-lined vessels are necessary, while mild steel can be used in case of sodium carbonate. Heating is performed by direct steam injection. 

Digestion of Wolframite [5.1,5.10-5.15]. Conventionally, wolframite is attacked by sodium hydroxide either under atmospheric pressure in higher concentrated solution or under elevated pressure in a more diluted solution. Today, the less concentrated solution is preferred, because it dissolves less gangue minerals. Table 5.2 describes digest conditions. 

Concentrates low in manganese may be dissolved also by a pressure leach as used for scheelite (Table 5.2). 

Digestion of Scheelite [5.1,5.16-5.22,5.24-5.26], The method mostly applied today in western world countries is dissolution with sodium carbonate under increased pressure. Costs are higher compared to the above-described sodium hydroxide digestion of wolfhimite due to the fact that a high stoichiometric excess of the reagent must be applied. 

The equilibrium constant K = [W04 ]/[CC)3 ] is increased by temperature. Therefore, the digest is performed at elevated pressure and temperature. An increase in carbonate concentration decreases the equilibrium constant, and when it exceeds a certain limit the formation of an insoluble scale at the surface of the seheelite crystals slows down the further dissolution. The scale consists of NaaCOs CaC03 and/or Na2C03 2CaC03. Accordingly, maintaining the sodium carbonate concentration within a certain limit is crucial for a good yield, which should be around 99% or better. There are proposals in the literature to improve the process  

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Minimal pretreatment such as pelletizing, briquetting, grinding Acid digestion/dissolution in open vessels (atmospheric pressure)... 

Sample preparation embraces all processes that convert the analyte into a form suitable for analysis. This includes a possible comminution, homogenization, digestion, dissolution, and filtration of the sample. Ideally, the sample should be dissolved in the eluent being used since the negative peaks that are observed during the dead time in conductivity detection mode do not occur. Before injecting the sample solution, it should be micro-filtrated (0.22 or 0.45 pm) to prevent particulate matter from entering the column. 

Sample preparation embraces all operations which help to bring the samples to be analyzed into the appropriate form. These processes include a possible crushing of the sample, its homogenization, digestion, dissolution, and filtration, all important steps that are usually interconnected. However, difficulties are caused in most cases by the sample pre-purification [99]. At this point, various techniques will be discussed in some detail, with emphasis on procedures that are typical for ion chromatographic analyses. 

Because of the risk of lead poisoning, the exposure of children to lead-based paint is a significant public health concern. The first step in the quantitative analysis of lead in dried paint chips is to dissolve the sample. Corl evaluated several dissolution techniques. " In this study, samples of paint were collected and pulverized with a Pyrex mortar and pestle. Replicate portions of the powdered paint were then taken for analysis. Results for an unknown paint sample and for a standard reference material, in which dissolution was accomplished by a 4-6-h digestion with HNO3 on a hot plate, are shown in the following table. 

Occlusions are minimized by maintaining the precipitate in equilibrium with its supernatant solution for an extended time. This process is called digestion and may be carried out at room temperature or at an elevated temperature. During digestion, the dynamic nature of the solubility-precipitation equilibrium, in which the precipitate dissolves and re-forms, ensures that occluded material is eventually exposed to the supernatant solution. Since the rate of dissolution and reprecipitation are slow, the chance of forming new occlusions is minimal. 

The cake produced by the digestion is extracted with cold water and possibly with some diluted acids from the subsequent processes. During the cake dissolution it is necessary to maintain the temperature close to 65°C, the temperature of iron sulfate maximum solubiUty. To prevent the reoxidation of the Fe " ions during processing, a small amount of Ti " is prepared in the system by the Ti reduction. The titanium extract, a solution of titanium oxo-sulfate, iron sulfate, and sulfuric acid, is filtered off. Coagulation agents are usually added to the extract to faciUtate the separation of insoluble sludge. 

Elemental analysis of organie and inorganie samples requires partial or total dissolution of the sample prior to speetrometrie analysis. Mierowave-assisted extraetion (MAE) provides an alternative to traditional digestion methods and it is reeognized as one of the most effieient methods for the dissolution of soil samples. 

Mais dans la masse il se prodult des effets de digestion, et, au bout de cinq ou six jours, une grande partie des elements constituants des tentacules sent eiitr s en dissolution dans ce liquide glycerin, formant une masse paisse, inucilagineuse, de consistance visqueuse. 

Figure 48-12. Schematic illustration of some aspects of the role of the osteoclast in bone resorption. Lysosomal enzymes and hydrogen ions are released into the confined microenvironment created by the attachment between bone matrix and the peripheral clear zone of the osteoclast. The acidification of this confined space facilitates the dissolution of calcium phosphate from bone and is the optimal pH for the activity of lysosomal hydrolases. Bone matrix is thus removed, and the products of bone resorption are taken up into the cytoplasm of the osteoclast, probably digested further, and transferred into capillaries. The chemical equation shown in the figure refers to the action of carbonic anhydrase II, described in the text. (Reproduced, with permission, from Jun-queira LC, Carneiro J BasicHistology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

While most polymer/additive analysis procedures are based on solvent or heat extraction, dissolution/precipita-tion, digestions or nondestructive techniques generally suitable for various additive classes and polymer matrices, a few class-selective procedures have been described which are based on specific chemical reactions. These wet chemical techniques are to be considered as isolated cases with great specificity. 

For the purpose of the identification and quantification of additives (broadly defined) in polymeric materials extraction and dissolution methods are favoured (Sections 3.3-3.7). However, additives are also made accessible analytically by digestion of the sample matrix (cf. Section 8.2). Such wet chemical techniques, that remove the sample matrix first, are often limited to mg amounts because of pressure build-up in destruction vessels. Another reactive extraction approach to facilitate additive analysis is depolymerisation by acid hydrolysis or saponification, sometimes under pressure. This is then frequently followed by chemical methods such as titrimetry or photometry for final identification and quantification. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

Destructive solid sample preparation methods, such as digestion and mineralisation, are well known as they have been around for some time they are relatively cheap and well documented [13-15]. Decomposition of a substance or a mixture of substances does not refer so much to the dissolution, but rather to the conversion of slightly soluble substances into acid- or water-soluble (ionogenic) compounds (chemical dissolution). 

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