Electrolyte, peptizing

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Nonionic cellulose ethers, hydroxyethyl(HE) and hydroxypropy1 (HP) cellulose, of variable molar substitution (M.S.) levels, were adsorbed on peptized sodium montmorillonite surfaces from fresh and saline (NaCl) aqueous solutions. The amounts adsorbed for 2 M.S. HEC and HPC and 4 M.S. HEC were insensitive to electrolyte concentration the 4 M.S. 

Another example of subdivision/dispersion is given by the classic disintegration of metals that occurs when two electrodes of the same metal are submersed and subjected to an electric potential that causes arcing (Bredig s arc). Here, metal melts at the electrode tips and becomes dispersed into suspension. Such suspensions can be stabilized by the presence of peptizing electrolyte. 

For clay minerals the natural processes of weathering and erosion tend to produce small particle sizes so that usually only mild dispersion in simple mixers, blenders, or ultrasonic baths are required. Also for days, having inherent lattice charge means that when in contact with water an electric double layer is immediately created and no stabilizing (peptizing) electrolyte may be needed in this case. The converse may also apply. That is, a sample may contain too much electrolyte to be easily dispersed. Clay and other suspensions that contain a large, aggregating amount of electrolyte can be purified by a number of means to remove this electrolyte and create a reasonably stable dispersion. 

Peptization, usually by dilution, of a once-stable dispersion that was aggregated (coagulated or flocculated) by the addition of electrolyte. 

Sols are obtained via either colloidal or polymeric routes. In the first method, colloids are formed and stabilized by adding peptizing agents (acidic or basic electrolytes) to metal hydroxides, and the gel is obtained by evaporating the solvent. In the second (polymeric) method, alkoxides are used as starting materials and hydrolysis and condensation reactions control the size of the resulting clusters (temperature and pH are the critical parameters). Additives such as surfactants may also play an important role in the sol characteristics by controlling the hydrolysis step of the alkoxides [25]. 

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. 

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. 

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. 

Peptization is the reverse of coagulation (the precipitate reverts to a colloidal state and is lost). It is avoided by washing with an electrolyte that can be volatized by heating. 

Besides hydrolysis, one can also utilize other exchange reactions in the preparation of disperse systems. It is, however, important to remember that a substantial amount of electrolyte, which is often present in the solution, may result in a loss of colloidal stability. One can sometimes remove excess electrolyte by washing and subsequent peptization of the precipitate. It is advantageous to prepare disperse systems at high supersaturations, which can be reached upon mixing concentrated solutions of reactants. The sols of Prussian Blue, various sulfides, stannic acid and its compound with colloidal gold (Cassian Purple) are all made by this method. 

In the case when the depth of potential minimum is smaller than several kT, the coagulation (i.e., the combination of two particles) becomes thermodynamically unfavorable even at low height of the potential barrier (Chapter VII, 1), and the stability of colloidal system towards coagulation is of thermodynamic nature. This is confirmed by observed peptization of coagulated precipitates upon washing out the excess of coagulating electrolyte and by stabilization of sols by specifically adsorbed ions. 

Potential Determining Ions Ions whose equilibrium between two phases, frequently between an aqueous solution and a surface or interface, determines the difference in electrical potential between the phases, or at the surface. Example For the Agl surface in water, both Ag+ and I- would be potential-determining ions. If such ions are responsible for the stabilization of a colloidal dispersion, then they are sometimes referred to as peptizing ions. See also Indifferent Electrolyte. 

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