Precipitating agent

As noted earlier, one of the most powerful techniques for producing a supersaturated protein solution is adjustment of the pH to values where the protein is substantially less soluble. This may be done in the presence of a variety of precipitants so that a spectrum of possibilities can be created whereby crystals might form. The gradual alteration of pH is particularly useful because it may be accomplished by a variety of gentle approaches that do not otherwise perturb the system or introduce unwanted effects. Chief among these are dialysis and vapor diffusion. 

As with pH, proteins may vary in solubility as a function of temperature, and some are quite sensitive. One can take advantage of this property with both bulk and microtechniques (Jacoby, 1968 McPherson, 1999). Many of the earliest examples of protein crystallization were based on the formation of concentrated solutions at elevated temperatures followed by slow cooling. Osborne in 1892 successfully crystallized over 20 plant seed globulins by cooling relatively crude extracts from 60°C to room temperature in the presence of varying concentrations of sodium chloride. 

Most protein and nucleic acids are conformationally flexible or exist in several conformational equilibrium states. In addition they may assume a substantially different conformation when they have bound coenzyme, substrate, or other ligand. Frequently a protein with bound effector may exhibit appreciably different solubility properties than the native protein. In addition, if many conformational states are available, the presence of effector may be used to select for only one of these, thereby engendering a degree of conformity of structure and system microhomogeneity that would otherwise be absent. 

Protein precipitants fall into four broad categories (1) salts, (2) organic solvents, (3) long-chain polymers, and (4) low-molecular-mass polymers and nonvolatile organic compounds. 

Ammonium phosphate Ethanol Polyfethylene glycol) 2-Methyl-2, 

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Heavy metals often can be removed effectively by chemical precipitation in the form of carbonates, hydroxides, or sulfides. Sodium carbonate, sodium bisulfite, sodium hydroxide, and calcium oxide are all used as precipitation agents. The solids precipitate as a floe containing a large amount of water in the structure. The precipitated solids need to be separated by thickening or filtration and recycled if possible. If recycling is not possible, then the solids are usually disposed of to a landfill. 

Used in ethanoic acid solution as a precipitating agent for nitrates. CI04 , PF , Re04 , W04 " also form precipitates. 

Precipitation can also occur upon chemical reaction between the impurity and a precipitating agent to form a compound insoluble in the molten metal. The refining of cmde lead is an example of this process. Most copper is removed as a copper dross upon cooling of the molten metal, but the removal of the residual copper is achieved by adding sulfur to precipitate copper sulfide. The precious metals are separated by adding zinc to Hquid lead to form soHd intermetaHic compounds of zinc with gold and silver (Parkes process). The precious metals can then be recovered by further treatment (see Lead). 

FUll-methode, /, precipitation method, -mittel, n. precipitating agent, precipitant. 

The polymers obtained by emulsion polymerization are seldom prepared in a powdered form because the material is highly hydroscopic and does not allow easy preparation of aqueous solutions. If it is necessary to obtain a finely dispersed powder, a precipitating agent is added to the latex on continuous stirring (methanol, isopropanol, etc.), and the precipitate formed is then filtered and dried. 

Like ammonia, hydrogen sulfide can act as a precipitating agent toward metal cations. The reaction with Cd2+ is typical ... 

The increase in solubility of a precipitate upon the addition of excess of the precipitating agent is frequently due to the formation of a complex ion. A... 

A procedure which is commonly employed to prevent supersaturation from occurring is that of precipitation from homogeneous solution. This is achieved by generating the precipitating agent within the solution by means of a homogeneous reaction at a similar rate to that required for precipitation of the species. 

A modified DP method with urea as a precipitating agent, called the DP urea method, has been developed for Ti02 support [27]. Deposition of gold NPs by this method was appHed to AI2O3 [28], Ce02 [28], and Mn203 [29] supports. Aqueous urea solution produces hydroxyl ions above 60 °C as foUows ... 

Usually resolve within 12-24 h with cessation of precipitating agent or supportive therapy but can be prolonged with long half-life drugs... 

Magnetic material (mean size, nm) Precursor/reagents Surfactant/reducing or precipitating agent Solvent Method T(°C)b> Time (h)c Ref. 

Sodium sulfite is also used to absorb sulfur dioxide, producing a bisulfite that is converted back to sodium sulfite by the addition of a calcium precipitating agent. 

There is a consensus amongst published recommendations for the management of C. difficile infection [19-21], The most important first step in the treatment is cessation of the precipitating agent, most commonly antibiotics, if this is deemed to be medically appropriate. In mild disease, this is often sufficient for full recovery. In more... 

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