Trace elements, solvent precipitation

The average value for the THF insolubles of a filtered extract solution produced in this laboratory is 2-4 % and it was found (1 that about 60 % of the trace elements were precipitated out with the insolubles. Thus, it was considered that THF might give low ashes in the filtrate with less material precipitated. Table 8 shows the results obtained when THF was used as the solvent The results obtained were disappointing, in that there was little reduction in ash level. It may be that lower temperatures or more solvent are required. However, both would reduce the practicality of using THF. 

In this paper, a description will be given of the previous work carried out to find ways to reduce metallic trace elements in the filtered extract to low levels. This will be compared with new work where solvent precipitation was carried out on filtered extract solution as an alternative method of producing low levels of trace elements. 

The final solvent to be tried was pentane. In the normal insolubles test, the pentane insolubles is normally over 25 % for the filtered extract solution. In the runs that were carried out with pentane, there were many operational problems, mainly associated with the large quantities of material precipitated. The filtrates produced contained very low levels of trace elements, but it was considered that the large amounts of material precipitated, most of which was difficult to remove from the autoclave, ruled this out as a potential solvent. 

An alternative to adjusting process conditions is to have a secondary solvent precipitation stage. Toluene was found to be the best solvent tried and, again, low levels of trace elements were obtained. Using this method, however, there is a loss of product, but this is mainly high molecular weight species, which are difficult to hydrocrack and are probably responsible for deposition of carbonaceous material on the catalyst. 

Procedures for the determination of 11 elements in coal—Sb, As, Br, Cd, Cs, Ga, Hg, Rb, Se, U, and Zn—by neutron activation analysis with radiochemical separation are summarized. Separation techniques include direct combustion, distillation, precipitation, ion exchange, and solvent extraction. The evaluation of the radiochemical neutron activation analysis for the determination of mercury in coal used by the Bureau of Mines in its mercury round-robin program is discussed. Neutron activation analysis has played an important role in recent programs to evaluate and test analysis methods and to develop standards for trace elements in coal carried out by the National Bureau of Standards and the Environmental Protection Agency. 

Instead of passage of gas bubbles through the solution, the aqueous pseudo-solution can be shaken with non-polar solvent. In such cases, hydrophobic sparingly soluble compounds accumulate at the phase boundary or adhere to the wall of the separating funnel used. After careful removal of both liquid phases, and washing, the precipitate adhering to the wall can be dissolved in a polar solvent and the isolated trace elements can be determined. 

Trace elemental analysis is under constant development and the challenging analyses of today become the routine of tomorrow. In order to analyze the trace components, liquid samples must pre-concentrated by column retention (Akatsuka et al., 1998, Santelli et al., 1994), ion exchange (Anezaki et al., 1998, Ferreira et al., 2000), precipitation (Atanassova et al., 1998, Chen et al., 1997) or chelation followed by solvent extraction. 

Preparation 4r-l Candida strains may be cultivated in a nutritive medium containing assimilable carbon and nitrogen sources, essential mineral matter, trace elements and the like under aerobic conditions, and the medium may be constructed in a conventional manner. After the cultivation, insoluble substances are removed by filtration or centrifuging to prepare a concentrated solution of liquid enzyme, and a culture solution may be subsequently evaporated or concentrated by reverse osmosis. The concentrated solution may be precipitated in a solvent capable of being mixed with salts or water, for example ethanol, or may be dried in a conventional spray manner to prepare a solid enzyme preparation. 

MDP, as all the N-acetyl-muramyl-peptides, can be precipitated by addition of ether from its ethanol or methanol solution, but it occludes a considerable amount of solvent judging from elemental analysis. Repeated lyophilization allows the elimination of all traces of solvent 33). 

A major problem in the analysis of lead-based alloys is the selection of a suitable solvent mixture. Price [7] has described a method based on hydro-bromic acid/bromine. The disadvantage of this method is that the solvent mixture is unpleasant to handle and precipitation often occurs on dilution. Hwang [8] has described a method for the analysis of trace and minor elements in lead/tin solders using a solvent mixture of fluoboric acid and nitric acid. 

Radiochemical purification of the induced activity in the presence of carrier involves chemical operations such as precipitation, distillation, solvent extraction, chromatography, and ion exchange. While the chemistry performed on the carrier and sample should be designed to isolate the material in a pure state, a useful operation frequently carried out is scavenging. Strongly adsorptive precipitates such as ferric hydroxide, lanthanum fluoride, and antimony sulfide may be formed in the solution. These precipitates, by coprecipitation, occlusion, and surface adsorption can be used to remove unwanted traces of activity. A scavenging agent should be chosen that wdll not carry down appreciable amounts of the carrier from solution. An alternative method is to add hold-back carriers for the unwanted traces of activity and precipitate the required element in their presence. 

In order to prepare thin-film samples, the desired elements must usually be chemically or physically separated from the host compound. The elements being determined are collected in a physical form suitable for x-ray analysis using such methods as ion exchange, solvent extraction, or precipitation. Metallic ions, for example, may be collected on resin-loaded paper, which also serves as the mechanical support in the x-ray spectrometer. The absolute sensitivity for elements isolated from the host compound is 0.01 to 1 /ug analysis of elements present in trace concentrations is possible with this preconcentration approach, if the total sample size is sufficiently large. 

Probably the most widely used procedure for trace metal analysis of seawater over the last 20 years has been preconcentration followed by ETAAS because of the wide availability, good sensitivity, and large range of elements that can be measured by this method. Preconcentration procedures have in general been one of three types. First, co-precipitation, with iron(III) hydroxide or cobalt pyrrolidinedithio-carbamate, being the most widely used co-precipit-ants. Second, cornplexation followed by solvent extraction with a number of different complexants, of which dithiocarbamates, 8-quinolinol, and dithi-zone are particularly popular, and with extraction into a range of solvents. The third approach is extraction on to a chelating column (usually Chelex-100 but recently also other complexants such... 

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