Precipitation by salting out

The dihydrate Na2S204.2H20 can be precipitated by salting out with NaCl. Air and oxygen must be excluded at all stages in the process to avoid reoxidation. The dithionite ion can also be produced in situ on an industrial scale by reaction... 

All these phenomena have a common denominator. Ions dissolved in water bind the water tightly. The addition of salt to an aqueous solution reduces the amount of free water and promotes any molecular interactions that result in a release of water molecules. At the same time, water—or, more precisely, structured water—surrounds the hydrophobic part of the surface of all molecules dissolved in water. This water is released as these molecules bind to each other or to another hydrophobic surface. Therefore, any interaction of hydrophobic surfaces is enhanced by an increase in the salt concentration. If the molecules interact with each other, precipitation by salting out results. If the hydrophobic surface is provided by a packing, we are dealing with HIC. 

In recent years, other fractionation methods have been proposed, such as ultraflltra-tion (Yu et al. 2008) or fractional precipitation by salting out (Zavarzina, Vanifatova, and Stepanov 2008). Size-exclusion chromatography (SEC, which is discussed in Section 10.3.2.3) in preparative form (low-pressure gel-permeation chromatography) has also been employed (De Nobili and Chen 1999 Perminova 1999 Clapp et al. 2005). 

S ts can be used to precipitate proteins by salting out effects. The effectiveness of various salts is determined by the Hofmeister series, with anions being effective in the order citrate > PO4" > SO4" > CH3COO > Cl > NO3 , and cations according to NH4 > > Na ... 

Many cytosolic proteins are water soluble and their solubility is a function of the ionic strength and pH of the solution. The commonly used salt for this purpose is Ammonium Sulphate, due to its high solubility even at lower temperatures. Proteins in aqueous solutions are heavily hydrated, and with the addition of salt, the water molecules become more attracted to the salt than to the protein due to the higher charge. This competition for hydration is usually more favorable towards the salt, which leads to interaction between the proteins, resulting in aggregation and finally precipitation. The precipitate can then be collected by centrifugation and the protein pellet is re-dissolved in a low salt buffer. Since different proteins have distinct characteristics, it is often the case that they precipitate (or salt out ) at a particular concentration of salt. 

Anhydrous hydrosulphites are obtained as products of methods described on p. 225. Commercially the anhydrous sodium salt is manufactured in large quantities on account of its greater stability than the hydrated Na2S204.2Ha0. The latter can be dehydrated by extraction with warm alcohol or acetone at 60° to 70° C. It is also possible to produce the anhydrous salt directly by precipitation of the aqueous solution with alcohol at 60° to 70° C., or even by salting-out the solution at this temperature by the addition of sodium hydroxide, chloride, sulphate, carbonate, nitrate or acetate with sodium hydroxide solution of 50 per cent, concentration the anhydrous salt can be separated even at 20° C.1... 

Summarizing the experiences on the formation of fines by salting-out precipitation the supersaturation has the... 

With di- and polysulfonic acids, the formation of acid salts is possible, of course, by neutralizing only part of the sulfo groups, and it is conceivable that in some cases these acid salts would be suflBciently insoluble to separate out. However, this is the exception as a rule, the precipitate formed by salting out consists of the neutral salt, even with polysulfonic acids. 

Figure 6 The main broad avenues of nanoparticle production. Comminution or size reduction (say by high-pressure homogenization or critical solution technology), precipitation methods by salting out or other solvency changes in solutions of drag-polymers mixtures and molecular assembly of amphipathic components. Methods are listed in Table 2.

The reaction need not be limited to liquid and gases, as shown in Figure 5.14. In Figure 5.15, for example, an ore is dissolved (decomposed) in mother liquor with the resultant supersaturation and crystallization of another crystalline material. Materials may also be precipitated by common-ion effect or by salting out through the addition of solutions or other solids. These techniques are important because they frequently reduce energy consumption. 

Crystallization by Salting Out. Thompson and Molstad suggested that crystallization using organic precipitants has definite commercial possi-... 

Mullin, J.W., Teodossiev, N. and Sohnel, O. (1989) Potassium sulphate precipitation from aqueous solution by salting-out with acetone. Chemical Engineering and Processing, 26, 93-99. 

The phenomena described by means of the Hofmeister anion series cover a very broad spectrum. They range from the original one of protein precipitation to salting out of other solutes, enzyme activity, macromolecular conformational transitions. 

Crystallization temperature is that temperature at which a component in a solution fertilizer will precipitate or salt-out. This is a useful measurement in determining the optimum storage temperature range of a solution fertilizer. Crystallization temperature is determined by slowly cooling a solution fertilizer until crystallization occurs. Typical crystallization temperatures of some common solution fertilizers are Ested in Table 18.14. 

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