Conditions of precipitation

No universal rules can be given which are applicable to all cases of precipitation, but, by the intelligent application of the principles enumerated in the foregoing paragraphs, a number of fairly general rules may be stated  

Precipitation should be carried out in dilute solution, due regard being paid to the solubility of the precipitate, the time required for filtration, and the subsequent operations to be carried out with the filtrate. This will minimise the errors due to co-precipitation. 

Precipitation is effected in hot solutions, provided the solubility and the stability of the precipitate permit. Either one or both of the solutions should be heated to just below the boiling point or other more favourable temperature. At the higher temperature (a) the solubility is increased with a consequent reduction in the degree of supersaturation, (b) coagulation is assisted and sol formation decreased, and (c) the velocity of crystallisation is increased, thus leading to better-formed crystals. 

The precipitate should be washed with the appropriate dilute solution of an electrolyte. Pure water may tend to cause peptisation. (For theory of washing, see Section 11.8 below.) 

If the precipitate is still appreciably contaminated as a result of coprecipitation or other causes, the error may often be reduced by dissolving it in a suitable solvent and then re-precipitating it. The amount of foreign substance present in the second precipitation will be small, and consequently the amount of the entrainment by the precipitate will also be small. 

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Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Selenium and precious metals can be removed selectively from the chlorination Hquor by reduction with sulfur dioxide. However, conditions of acidity, temperature, and a rate of reduction must be carefliUy controlled to avoid the formation of selenium monochloride, which reacts with elemental selenium already generated to form a tar-like substance. This tar gradually hardens to form an intractable mass which must be chipped from the reactor. Under proper conditions of precipitation, a selenium/precious metals product substantially free of other impurities can be obtained. Selenium can be recovered in a pure state by vacuum distillation, leaving behind a precious metals residue. 

Bismuth subcarbonate [5892-10 ] (basic bismuth carbonate) is a white or pale yellow powder that is prepared by interaction of bismuth nitrate and a water-soluble carbonate. The exact composition of this dmg depends on the conditions of precipitation it corresponds approximately to the formula (Bi0)2C02. It has been widely used as an antacid (183). 

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. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

In agreement with previous suggestions [9], the present results have confirmed that whatever be the nature of the cation or of the heteroatom, the acidic salts appeared in the form of pure acid embedded in a neutral salt. The conditions of precipitation and more particularly the initial stoichiometry determine the texture and the composition of the resulting precipitate. 

According to Equation 48 calcite should precipitate from waters having a Mg/Ca ratio below a certain value, while dolomite should precipitate from waters having a Mg/Ca ratio above that critical value. This rule is obeyed under conditions of precipitation from very slightly supersaturated aqueous solutions like those occurring in certain areas of the ocean. Ocean water is close to equilibrium with both calcite and dolomite (53). 

Examine under a microscope the shape of the crystals remaining in the beaker and in the bowl. Filter off the crystals formed in the porcelain bowl, dry them in the air, and determine the yield in per cent. Using Table 4 containing the solubilities of the salts that can form in the solution, explain the conditions of precipitation of the salts formed from the solution. 

Coprecipitates. Silica-alumina coprecipitates are immensely complicated. As a result of structural disorder and substitution of Al(III) for Si (IV) on tetrahedral sites, they exhibit cation exchange behavior (34). The fraction of total Al which occurs on tetrahedral sites and the CEC vary widely in response to the conditions of precipitation and subsequent sample history, especially thermal history. In general, the fraction... 

Silver bromide, AgBr.—Precipitation of a solution of a silver salt with a bromide solution yields a flocculent to pulverulent, amorphous precipitate of silver bromide, its colour being white, yellowish-white, or lemon-yellow according to the conditions of precipitation. Its melting-point is given as 422° C.5 and 427° C.,e and the density of the precipitated salt as 6-89 to 6-52, and after fusion as 6-82 to 6-49. 

Fig. 24. Conditions of precipitation of carbonates and silicates of iron, magnesium, and calcium in the presence of excess silica. Graphic representation of model with initial =UMg + =0.015...

Fig. 43. Conditions of precipitation of iron as a function of pH, Upe Fistires on curves of...

Zinc sulfide, ZnS, is the only tvhite sulfide among the sulfides of the common metals. Its conditions of precipitation have been discussed in Chapter 22. 

Regardless of the conditions of precipitation or crystallization, a polymorphic solvate is obtained which consists of 2 BNP acid l pyridine H2O, according to elemental analyses. Pyridine peaks are apparent in the NHR spectrum (6 8.71 and 8.78 in d5-DHS0). Desolvation occurs at 210-230°C on the Kofler bench or by heating to reflux in 6 N hydrochloric acid. 

Where two or more components are co-precipitated, special steps must be taken to ensure homogeneity of the final catalyst. This can be achieved by adding a solution of both components to an excess of the precipitating agent, rather than the other way round. The physical properties of precipitated catalysts will often depend on the conditions of precipitation e,g. concentration of solutions, order and rate of mixing, temperature of precipitation, washing, drying and calcination) all of which must be carefully studied. 

Basic salts may be prepared in various ways, which usually involve—directly or indirectly-hydrolysis of a normal salt. The hydrolysis may be carried out under conditions of controlled temperature, acidity, and metal-ion concentration, or indirectly by heating a hydrated salt. Many hydroxy-salts are formed by the latter process instead of the anhydrous normal salts, for example, Cu2(0H)3N03 by heating hydrated cupric nitrate. The precipitates formed when sodium carbonate solution is added to metallic salt solutions are often hydroxy-carbonates. Some metals do not form normal carbonates (e.g. Cu, see p. 887) others such as Pb, Zn, Co, and Mg form normal or hydroxy-salts according to the conditions of precipitation. Lead, for example, forms 2 PbC03. Pb(OH)2 and PbC03. Pb(OH)2, both of which occur as minerals, while Co forms Co4(OH)6C03 in addition to the normal carbonate. 

Figure 5.7 shows particle size distribution of synthetic barium sulfate. The characteristic feature of these curves is their steepness which denotes a very narrow particle size distribution which was obtained by controlling the conditions of precipitation. The development of this kind of particle size distribution in a small particle sized filler allows for substantial improvement in the gloss of coatings. 

Moreover, even if all conditions of precipitation of hydroxides (temperature, concentration, nature of the precipitant, etc.) are observed, the hydroxides are not identical as each of them has individual pH of initial and final precipitation, solubility, shape of particles and calcination temperature. It is well known that different hydroxides calcinated under... 

Moreover, even though the precipitation conditions are observed most strictly, the process is periodical because it is unsteady in its nature and occurs, as a rule, with changing conditions of precipitation. For example, if at the beginning of precipitation the system is supersaturated,at the end it has different parameters. As a result, hydrogel particles or crystals that were formed upon initial precipitation can have different sizes, especially because for many hydroxides the pH of initial and final precipitation occur in the acid region, i.e., pH < 7, while for others, the pH of initial precipitation lie in the acid medium and those of the final precipitation, in the alkaline medium, etc. 

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