Extraction difficulties

Various classes of additives may undergo chemical reaction in hydrolytic conditions. For example, hydrolysis of aromatic phosphites forms phenolic compounds [645]. Additives may also undergo reaction with the polymer chain, which causes extraction difficulties. 

Table 10.32 is a shortlist of the characteristics of the ideal polymer/additive analysis technique. It is hoped that the ideal method of the future will be a reliable, cost-effective, qualitative and quantitative, in-polymer additive analysis technique. It may be useful to briefly compare the two general approaches to additive analysis, namely conventional and in-polymer methods. The classical methods range from inexpensive to expensive in terms of equipment they are well established and subject to continuous evolution and their strengths and deficiencies are well documented. We stressed the hyphenated methods for qualitative analysis and the dissolution methods for quantitative analysis. Lattimer and Harris [130] concluded in 1989 that there was no clear advantage for direct analysis (of rubbers) over extract analysis. Despite many instrumental advances in the last decade, this conclusion still largely holds true today. Direct analysis is experimentally somewhat faster and easier, but tends to require greater interpretative difficulties. Direct analysis avoids such common extraction difficulties as ... 

H2OPPA) U(0PPA)2 + 4H+ (high HE U(OPPA)4 + 4HEt extractant. Difficulties... 

HPLC method development for the fixed combination tablet formulation became more challenging in the presence of the excipients used in the tablet formulations. HPMC and pregelatinized starch in the sustained release portion of the bilayer tablet caused extraction difficulties and interferences in the HPLC chromatogram. 

The active principle in S. divinorum, present in the leaves of the plant, remains unidentified. To date no one has been able to isolate an explicit hallucinogen, though there is some evidence that it may be a terpine rather than an alkaloid. This extraction difficulty is apparently due to the extreme instability of the catalyst after the leaves have been harvested. Although these are said to be viable for a day or two, any infusion made from them loses its potency in a matter of hours. In my own experience, leaves which were frozen immediately after harvest, then later thawed and made into an infusion produced no discernible effect, so apparently freezing doesn t preserve the active principle. 

The determination of vitamin in multivitamin preparations can be hampered by extraction difficulties but the use of sodium diethylene penta-acetic acid facihtates recoveries in excess of 90% (Walker et al., 1981). Subsequently, reversed phase ion-pair chromatography can be carried out using a mobile phase of 0.001 M hexanesulphonic acid in 1% acetic acid-methanol (75 25). Using this method, thiamine, niacinamide, riboflavin and pyridoxine can be quantitated. 

In order to evaluate the catalytic properties of our SILCAs, we chose to test them on a classic and simple reaction. As our laboratory has a great interest in heteroatomic compounds especially sulfurs (Robert et al. 1996 Pees et al. 2001 Thomas et al. 2006), we chose a thioether synthesis, and benzyl phenyl thioether in particular (Harmand, 2009). Conversion rates of reaction between thiols and halides depend on reaction conditions which are in most cases not really environment friendly (extraction difficulties, catalysts loss and so on) To prepare SILCAs, three steps are necessary first the solubilization of IL in chloroform, then the addition of the alumina nanoparticules in the solution and finally the solvent removal evaporation. This method allows the obtention of a supporting material coated with a thin and uniform IL layer. We first compare the reaction between phenyl bromide and thiophenol in a basic aqueous solution in methylene chloride, with (table 4, entry 2-21) or without (table 4, entry 1) prepared SILCAs (Fig. 18). 

Collapsible cores improve extraction difficulties on cooling. 

To prepare gas for evacuation it is necessary to separate the gas and liquid phases and extract or inhibit any components in the gas which are likely to cause pipeline corrosion or blockage. Components which can cause difficulties are water vapour (corrosion, hydrates), heavy hydrocarbons (2-phase flow or wax deposition in pipelines), and contaminants such as carbon dioxide (corrosion) and hydrogen sulphide (corrosion, toxicity). In the case of associated gas, if there is no gas market, gas may have to be flared or re-injected. If significant volumes of associated gas are available it may be worthwhile to extract natural gas liquids (NGLs) before flaring or reinjection. Gas may also have to be treated for gas lifting or for use as a fuel. 

Introduction Difficulties in Extracting Scientific Environmental Information from the Internet... 

Now if K is very small, the amount of X going into the ether layer will also be very small and it would be quite impracticable to carry out more than three or four extractions. This difficulty can, however, be easily overcome by employing continuous ether extraction using the apparatus shown in Fig. 17 (a). 

Attention is directed to the great advantage of continuous extraction over manual shaking in a separatory funnel for liquids or for solutions which tend to froth or which lead to emulsification comparatively little difficulty is experienced in the continuous extraction process. 

Note 2. The separation of the layers may give some difficulties, owing to the presence of aluminium hydroxide. Too vigorous shaking during the extraction procedure should be avoided. The best way to separate the layers is to run off as much of the aqueous layer as possible and subsequently decant the ethereal layer. 

The disadvantage of extraction relative to extractive distillation is the greater difficulty of getting high efficiency countercurrent processing. 

Direct Reduction with Metals. PoUucite can be directly reduced by heating the ore in the presence of calcium to 950°C in a vacuum (20), or in the presence of either sodium or potassium to 750°C in an inert atmosphere (21). Extraction is not complete. Excessive amounts of the reducing metal is required and the resultant cesium metal is impure except when extensive distiUation purification is carried out. Engineering difficulties in this process are significant, hence, this method is not commerciaUy used. 

The leached solids must be separated from the extract by settling and decantation or by external filters, centrifuges, or thickeners, all of which are treated elsewhere in Sec. 18. The difficulty of solids-extract separation and the fact that a batch stirred tank provides only a single equilibrium stage are its major disadvantages. 

Different extraction-spectrophotometric procedures were proposed for the P(V) and As(V) determination as ionic associates (lA) of polyoxometalates with basic dyes. Main disadvantage is difficulty in separation of reagent excess. Flotation, centrifugation or extraction does not allow to create sufficiently sensitive procedures due to worsening of reproducibility. 

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