Precipitation by electrolytes

It will be observed that even small quantities of gelatin exert a most important influence, retarding the decomposition of the peroxide very considerably, as is usual with a protective colloid. The gelatin, however, increases the stability of the colloidal metal solution, and tends to prolong its period of activity by preventing its precipitation by electrolytes, thereby enabling many reactions to be studied other than the decomposition of pure hydrogen peroxide solution. 

Colloidal Molybdic Acid.—When solutions of sodium molybdate (1 molecule) and hydrochloric acid (4 molecules) are warmed together, or when a solution of molybdic acid dihydrate is evaporated over sulphuric acid under diminished pressure at 20° C., colloidal molybdic acid is formed. From the solution of the hydrosol so obtained, molybdic acid is precipitated by electrolytes. Graham considered that, by dialysis of a solution of sodium molybdate in hydrochloric acid, he obtained colloidal molybdic acid. ... 

See also Sabaaieeff, Chem. Zentr., 1891, i., 10 UUik, Annalen, 1867, 144, 329 1870,153, 373. Rosenheim and Davidsohn (loc. cit.) consider that the solutions previously-prepared by Graham s method, since not precipitated by electrolytes, in all probability contained no colloidal acids. 

Graham distinguished between sols and gels (V, 1864 see p. 731). It was generally recognised that sols are of two types (i) those like colloidal arsenious sulphide, non-viscous, not easily gelatinising, and easily precipitated by electrolytes and (2) those like albumin solution, viscous, easily gelatinising, and not easily precipitated by electrolytes. For these two classes the names colloidal suspensions and colloidal solutions, respectively, were proposed by... 

Emulsoids differ in several ways from suspensoids they are usually more viscous, not easily precipitated by electrolytes, and usually their surface tension is markedly lower than that of the solvent. They also protect suspensoids from precipitation. Graham (V) said the flow of liquid colloids through a capillary tube is always slow compared with the flow of crystalloid solutions, so that a liquid-transpiration tube may be employed as a colloidoscope. The precipitation of emulsoids by electrolytes follows a different course from that... 

The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

It is then precipitated by adding Zn dust. Electrolytic refining may then be used to provide gold of 99.99% purity. ... 

The isolation of zinc, over 90% of which is from sulfide ores, depends on conventional physical concentration of the ore by sedimentation or flotation techniques. This is followed by roasting to produce the oxides the SO2 which is generated is used to produce sulfuric acid. The ZnO is then either treated electrolytically or smelted with coke. In the former case the zinc is leached from the crude ZnO with dil H2SO4, at which point cadmium is precipitated by the addition of zinc dust. The ZnS04 solution is then electrolysed and the metal deposited — in a state of 99.95% purity — on to aluminium cathodes. 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

When foreign electrolytes which do not react with the soap are present, the ions responsible for bringing about coagulation are the ones with charges opposite in sign to those on the dispersed particle. The effect of the valence of the ion follows the Schulze-Hardy rule for the precipitation of sols by electrolytes wherein the coagulating power of an ion increases considerably with the increasing valence of the ion. 

By definition, in a solution all ions belong to the same phase, even though counterions may cluster more or less diffusely around the macroions. When significant amounts of a simple 1 1 electrolyte (such as KCl) are added to a polyelectrolyte solution, dissociation of the polyelectrolyte macromolecule is repressed in an extreme case the polyelectrolyte may be salted out. An undissociated polyacid may be precipitated by generous addition of a simple acid such as HCl. 

Excer A process for making uranium tetrafluoride by electrolytic reduction of a uranyl fluoride solution, precipitation of a uranium tetrafluoride hydrate, and ignition of this. 

Even the mildest pulping process, hydrotropic extraction, i.e. repeated extraction with concentrated aqueous solutions of organo-philic electrolytes such as sodium xylenesulfonate or cymenesulfonate, has failed because of an outlet for the extracted lignin, wliich is precipitated by mere dilution of the solution. Other methods based on hydrolysis of the wood with acids, e.g. HCl, HNO3, AICI3, acetic acid, acetyl chloride, SOg, or phenol, are merely of laboratory interest. 

The redispersion of a sol which has been precipitated by the addition of an electrolyte may also occasionally be effected by the removal of the precipitating agent through washing thus certain precipitates such as silver chloride, zinc sulphide and copper ferro-cyanide are readily redispersed in water where the precipitating salts are removed by thorough agitation and filtration. 

The coagulation and eventual precipitation of colloidal suspensions by electrolytes is the result of a series of reactions which in general cannot be isolated one from the other. 

The azo dye is either formed as an insoluble precipitate and is filtered off from the mixture or, if it contains solubilising groups such as sulfonic acid groups, further precipitated by adding an electrolyte such as salt. Isolation via membrane technologies, e.g. reverse osmosis, is practised on commercial dyesmff plants. 

Hypovanadous oxide resembles the metal in many of its properties. It is insoluble in water, but dissolves in acids without evolution of hydrogen to yield the lavender-coloured solutions which are characteristic of solutions of hypovanadous salts. These salts are, however, most conveniently prepared in solution by electrolytic reduction in an inert atmosphere of solutions of vanadium pentoxide in the various acids.7 Hypovanadous salts are isomorphous with salts of divalent iron, chromium, and manganese. On being treated with caustic alkalis, a brown precipitate of hypovanadous hydroxide, V(OH)a, is obtained, which rapidly oxidises to the greyish-green vanadous hydroxide, V(OH)s. 

Nearly pure cadmium sponge is precipitated by the addition of high-purity. lead-free zinc dust. The cadmium sponge then is redigested in spent cadmium electrolyte, alter which the cadmium is deposited by electrolysis onto aluminum cathodes. The metal is then stopped from the electrodes, melted, and cast into various shapes. Reactions which occur during the electrolytic process are (Roasting) ZnS +1,0 — — ZnO +... 

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