Precipitation-deposition drying step

Some mention should be made concerning the state of the polymer at the beginning of TLC experiment. Normally the sample is applied to the plate from solution in a relatively nonpolar solvent. This solvent is then evaporated, and the chromatographic plate is eluted with the chosen eluent. The drying step results in a polymer deposit which would be difficult to characterize it is not simply a precipitate, and it probably is not a simple adsorbed (multi) layer. Redissolution and entry into the mobile phase under displacement conditions occur in a minute or less (13). 

It is clear that for those catalyst preparations in which the mobility of the precursor is very low, the drying step is much less of an issue. This is the case, for example, for ion exchange or deposition precipitation, and these procedures are discussed in the next sections. 

A relatively easy method of placing an active phase homogeneously on a monolithic support is by deposition precipitation (76). The advantage of this method is that an insoluble metal salt is deposited on the support, which usually cannot spread readily during the drying step. Satisfactory results are obtained when supersaturation of the liquid occurs everywhere in the monolith at the same rate. This goal can be achieved by a homogeneous deposition precipitation procedure in which both the... 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

After the deposition-precipitation step has been completed, the solid is filtered, washed and dried. Before activation and use, it has to be shaped (e.g. pelleted), and, in most cases, calcined. 

For deposition of the active component, palladium, onto the carbon fibrils a precipitation method is utUized . The method consists of four steps, viz., pretreatment of the carbon surface in boiling nitric acid, suspension of the treated fibrils in an aqueous solution and addition of the solvated palladium precursor, injection of a formaldehyde solution to reduce the palladium ions, and, finally, filtering and drying. 

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