Colloidal precipitates peptization

Washing of the precipitate (AgCl) with 0.01 N HN03 is always recommended to prevent loss of AgCl by virtue of its return to colloidal condition (peptization) and to get rid of the soluble salts, namely AgNO and NaN03, and... 

Peptization is the process by which a coagulated colloid returns to its original dispersed state as a consequence of a decrease in the electrolyte concentration of the solution in contact with the precipitate. Peptization can be avoided by washing the coagulated colloid with an electrolyte solution rather than pure water. 

X sorbed-precipitated in attached colloids Detachment-peptization —> <— Attachment-filtration X sorbed-precipitated in mobile colloids... 

Ono S., Yamada N., Hirano S. Pottasium lithium niobate films derived from aqueous precursor solution. J. Am. Ceram. Soc. 2001 84 1415-1420 Panda A.B., Pathak A., NandagoswamiM., PramanikP. Preparation and characterization ofnanocrys-talline Sri j Bi2+jTa209 powders. Mater. Sci. Eng. 2003 B97 275-282 Perez-Maqueda L.A., Wang L., Matijevic E. Nanosize indium hydroxide by peptization of colloidal precipitates. Langumuir 1998 14 4397-4401... 

Hydrated Stannic Oxide. Hydrated stannic oxide of variable water content is obtained by the hydrolysis of stannates. Acidification of a sodium stannate solution precipitates the hydrate as a flocculent white mass. The colloidal solution, which is obtained by washing the mass free of water-soluble ions and peptization with potassium hydroxide, is stable below 50°C and forms the basis for the patented Tin Sol process for replenishing tin in staimate tin-plating baths. A similar type of solution (Staimasol A and B) is prepared by the direct electrolysis of concentrated potassium staimate solutions (26). 

Hydrolysis of solutions of Ti(IV) salts leads to precipitation of a hydrated titanium dioxide. The composition and properties of this product depend critically on the precipitation conditions, including the reactant concentration, temperature, pH, and choice of the salt (46—49). At room temperature, a voluminous and gelatinous precipitate forms. This has been referred to as orthotitanic acid [20338-08-3] and has been represented by the nominal formula Ti02 2H20 (Ti(OH). The gelatinous precipitate either redissolves or peptizes to a colloidal suspension ia dilute hydrochloric or nitric acids. If the suspension is boiled, or if precipitation is from hot solutions, a less-hydrated oxide forms. This has been referred to as metatitanic acid [12026-28-7] nominal formula Ti02 H2O (TiO(OH)2). The latter precipitate is more difficult to dissolve ia acid and is only soluble ia concentrated sulfuric acid or hydrofluoric acid. 

Positively-charged hydroxide sols can be prepared from many metals. To produce a colloidal solution from the precipitate, a process termed peptization, a solution of the salt, itself, is not... 

Walcr-peptizable colloidal substances such as gelatin, dextrin, gum arable, and soap peptize many precipitates, and are often culled protective colloids. Gelatin in the solution prevents the precipitation of silver dichromatc upon mixing silver nitrate and potassium dichroinalc solutions. (See Condensation Processes, below.)... 

In the particulate-sol method a metal alkoxide dissolved in alcohol is hydrolyzed by addition of excess water or acid. The precipitate that results is maintained as a hot solution for an extended period during which the precipitate forms a stable colloidal solution. This process is called peptization from the Greek pep—to cook (not a misnomer many descriptions of the sol-gel process have a strong culinary flavor). The colloidal solution is then cooled and coated onto the microporous support membrane. The layer formed must be dried carefully to avoid cracking the coating. In the final step the film is sintered at 500-800 °C. The overall process can be represented as ... 

Washing the precipitates It is essential to wash all precipitates in order to remove the small amount of solution present in the precipitate, otherwise it will be contaminated with the ions present in the centrifugate. It is best to wash the precipitate at least twice, and to combine the first washing with the centrifugate. The wash liquid is a solvent which does not dissolve the precipitate but dilutes the quantity of mother liquor adhering to it. The wash liquid is usually water, but may be water containing a small amount of the precipitant (common ion effect) or a dilute solution of an electrolyte (such as an ammonium salt) since water sometimes tends to produce colloidal solutions, i.e. to peptize the precipitate. 

At room temperature colloidal solutions of iron hydroxide can be obtained only by way of prolonged dialysis (Glazman et al., 1958). And finally, experiments are known in which sols were obtained by peptization, by treating freshly precipitated, washed Fe(OH)3 sediment with ferric chloride while heating. A dilute solution with a certain amount of HCl acts on freshly precipitated Fe(OH)3 as ferric chloride does. The sols of Fe(OH)3 obtained by peptization are no different in structure from the sols obtained by hydrolysis. 

The reverse of the coagulation process is peptization. It is well known from analytical chemistry that fresh precipitates are easier to disperse than old ones, which indicates qualitatively that an aggregate of colloidal particles is not in equillibrium and that irreversible, time-dependent processes occur in coagulation. As pointed out Frens and Overbeek (1971) the interpretation of peptization phenomena with aggregated systems is not possible unless the data are obtained in experiments with a shorter time scale than the aging time of the aggregate. They demonstrated that it was possible to follow the kinetics of peptization by suddenly diluting the sol... 

Preparation of TiO Colloid Solution. Titanium tetraisopropoxide (Aldrich Chemical Company) at a concentration of 5 ml in 25 ml of isopropanol, was added dropwise to 0.1 M aqueous HC1 solution while stirring. After the addition was complete, the solution was stirred for another 10 minutes and then heated slowly to remove the solvent the residue was dried under vacuum at 118°C. The Ti02 powier readily peptized in water. Aqueous colloidal solutions tended to precipitate in basic solutions addition of large amounts of inert salts also precipitated the colloids. Photoplatinization was done as reported earlier (9 ). 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

The normal way to obtain colloidal sols from oxide precursors is therefore a two-step process. In the first step, a precipitate of hydroxylated condensed species is formed from hydrolysed precursors. As described below, it can be seen that hydroxylated species capable of further condensation and precipitation in aqueous media can also be obtained from hydrolysis of metal alkoxides with excess water. In the second step this precipitate is transformed into a stable sol through a peptization reaction using basic or acid electrolytes. After adding appropriate organic binders, if requested, this sol can be directly used to form supported membranes. 

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