Purity, of precipitates

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

The improved purity of precipitated solute implies that R(Partition) for the impurities is different from that for the major solute. 

If there is introduced into the solution from some other source an ion that is in common with an ion of the insoluble solid, the chemical equilibrium is shifted to the left, and the solubility of that solid will be greatly decreased from what it is in pure water. This is called the 11 common-ion effect." This effect is important in gravimetric analysis, where one wishes to precipitate essentially all of the ion being analyzed for, by adding an excess of the "common-ion" precipitating reagent. There is a practical limit to the excess, however, which involves such factors as purity of precipitate and possibility of complex formation. You can calculate the solubility under a variety of conditions, as illustrated in the following problem. 

MORPHOLOGICAL STRUCTURE AND PURITY OF PRECIPITATES Precipitation is probably the most frequently used method in the practice of qualitative analysis. The occurrence of a precipitate as the result of the addition of a certain reagent may be used as a test for a given ion. In such... 

Reprecipitation A method of improving the purity of precipitates involving formation and filtration of the solid, followed by redissolution and re-formation of the precipitate. 

Zinc cyanide. Solutions of the reactants are prepared by dis solving 100 g. of technical sodium cyanide (97-98 per cent. NaCN) in 125 ml. of water and 150 g. of anhydrous zinc chloride in the minimum volume of 50 per cent, alcohol (1). The sodium cyanide solution is added rapidly, with agitation, to the zinc chloride solution. The precipitated zinc cyanide is filtered off at the pump, drained well, washed with alcohol and then with ether. It is dried in a desiccator or in an air bath at 50°, and preserved in a tightly stoppered bottle. The yield is almost quantitative and the zinc cyanide has a purity of 95-98 per cent. (2). It has been stated that highly purified zinc cyanide does not react in the Adams modification of the Gattermann reaction (compare Section IV,12l). The product, prepared by the above method is, however, highly satisfactory. Commercial zinc cyanide may also be used. 

A precipitation gravimetric analysis must have several important attributes. Eirst, the precipitate must be of low solubility, high purity, and of known composition if its mass is to accurately reflect the analyte s mass. Second, the precipitate must be in a form that is easy to separate from the reaction mixture. The theoretical and experimental details of precipitation gravimetry are reviewed in this section. 

The plutonium extracted by the Purex process usually has been in the form of a concentrated nitrate solution or symp, which must be converted to anhydrous PuF [13842-83-6] or PuF, which are charge materials for metal production. The nitrate solution is sufficientiy pure for the processing to be conducted in gloveboxes without P- or y-shielding (130). The Pu is first precipitated as plutonium(IV) peroxide [12412-68-9], plutonium(Ill) oxalate [56609-10-0], plutonium(IV) oxalate [13278-81-4], or plutonium(Ill) fluoride. These precipitates are converted to anhydrous PuF or PuF. The precipitation process used depends on numerous factors, eg, derived purity of product, safety considerations, ease of recovering wastes, and required process equipment. The peroxide precipitation yields the purest product and generally is the preferred route (131). The peroxide precipitate is converted to PuF by HF—O2 gas or to PuF by HF—H2 gas (31,132). 

Physica.1 Sta.bihty, Physical stabiHty depends primarily on the purity of the enzyme. Impurities remaining from the fermentation broth may precipitate or form a hazy solution. Unwanted sedimentation is often related to Ca " or acidic polysaccharides. The solubiHty of some enzymes can be increased by optimizing the ionic strength or changing the dielectric constant of the solution by a dding low molecular-weight polyols. 

The acid is analytically pure. There is no satisfactory solvent for the recrystallization of large amounts of terephthalic acid. Small quantities may be recrystaUized from acetic acid, but the purity of a properly precipitated and washed sample is not thereby improved. 

A suspension of 6-aminopenicillanic acid (36.4 grams) in water was adjusted to pH 7.2 by the addition of N aqueous sodium hydroxide and the resulting solution was treated with a solution of 3-(2-chloro-6-fluorophenyl)-5-methylisoxazole-4-carbonyl chloride (46.1 grams) in isobutyl methyl ketone. The mixture was stirred vigorously for hours and then filtered through Dicalite. The layers were separated and the isobutyl methyl ketone layer was shaken with saturated brine. Then, precipitation of the sodium salt only took place after dilution of the mixture with ether. In this way there was obtained 60.7 grams of the penicillin sodium salt having a purity of 88% as determined by alkalimetric assay. 

The formation and stability of peroxoniobates and peroxotantalates can be used successfully in the technology of tantalum and niobium oxide production. Belov, Avdonina and Mikhlin [512] investigated processes of precipitation and thermal decomposition of high-purity ammonium tetraperoxoniobate and tetraperoxotantalate as precursors for the production of tantalum and niobium... 

The method based on the precipitation of peroxometalate precursors enables to achieve additional purification during the process. Thus, the addition of complexonates, such as OEDP or EDTA, which form stable complexes with some polyvalent metals, prevents co-precipitation of the main impurities such as Fe, Co, Ni, Mn, Mg, etc., which in turn significantly increases the purity of the final product. Enhanced purification can also be achieved by recrystallization of the precursor. Particularly, the precipitation of ammonium peroxofluorometalates, such as ammonium peroxofluoroniobate ((NH4)3Nb04F4), as a primary precursor, leads to significant reduction of the titanium contamination. Ammonium peroxofluoroniobate, (NH4)3Nb04F4, is... 

It was assumed throughout that the compound which separated out from the solution was chemically pure, but this is not always the case. The purity of the precipitate depends inter alia upon the substances present in solution both before and after the addition of the reagent, and also upon the exact experimental conditions of precipitation. In order to understand the influence of these and other factors, it will be necessary to give a short account of the properties of colloids. 

Note on the gravimetric standardisation of hydrochloric acid. The gravimetric standardisation of hydrochloric acid by precipitation as silver chloride is a convenient and accurate method, which has the additional advantage of being independent of the purity of any primary standard (compare Section 10.38). Measure out from a burette 30-40mL of the, say, 0.1M hydrochloric acid which is to be standardised. Dilute to 150 mL, precipitate (but omit the addition of nitric acid), and weigh the silver chloride. From the weight of the precipitate, calculate the chloride concentration of the solution, and thence the concentration of the hydrochloric acid. 

If there is any doubt concerning the purity of the potassium titanyl oxalate, standardise the solution by precipitating the titanium with ammonia solution or with cupferron solution, and igniting the precipitate to TiOz. 

Wash solutions for precipitates, 426 Washing of precipitates 118, 426 by decantation. 119 solubility losses in, 119, 427 Washing soda D. of sodium carbonate in, 295 Water absorbents for, 477 ammonia-free, 679 deionised, 90 D. of hardness, 332 D. of total cation concentration, 210 D. with Karl Fischer reagent 637 distilled, 90 high purity, 91 ionic product of, 36 types and standards for, (T) 90 volume of 1 g at various temperatures, (T)87... 

The main disadvantage of precipitation with a polar (water-soluble) solvent is the need for a costly distillation stage to recover the relatively laige volumes of solvent used. Another disadvantage is the precipitation of proteins, salts and, in some cases, pigments which reduces the purity and leads to discoloration of the product. To overcome these problems, precipitation using less polar solvents, such as methylethylketone, has been proposed. Only 23% (w/v) methylethylketone is sufficient to saturate the aqueous phase and precipitate exopolysaccharides quantitatively. 

Fractionation. The process by which components are extracted firm bacterial eells or from the medium in whieh the baeteria are grown and obtained in a purified form. The polysaccharide antigens of Neisseria meningitidis are separated from the bacterial cells by treatment with hexadecyltrimethylammonium bromide and those of Streptococcus pneumoniae with ethanol. The purity of an extracted material may be improved by resolubilization in a suitable solvent and precipitation. After purification, a component may be dried to a powder, stored indefinitely and, as required, incorporated into a vaccine in precisely weighed amounts at the blending stage. 

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