Colloidal solution dialysis

When superheated selenium vapour is passed into air-free water, colloidal solutions are formed which are usually rose-coloured, but at first of a blue tint and cloudy. Under the most favourable conditions clear yellowish-red or deep red sols may be obtained,4 the former being the more highly dispersed. The blue sols after dialysis are extremely stable, but non-dialysed sols decompose after a few days, selenious acid being detected except in the yellowish-red sols. The dialysed sols may be frozen to an almost colourless ice which at the ordinary temperature thaws and decomposes. The sols are negative and are readily coagulated by the addition of chlorides. 

Very stable colloidal solutions of selenium may be prepared by the regulated action of concentrated hydrazine hydrate solution on selenium dioxide or grey crystalline selenium and subsequent dilution of the solutions with water and purification by dialysis. According to the degree of dispersion the colour of the solutions varies from intense yellow to blood red. These sols are completely irreversible. The dilute solutions are stable at the boiling-point, but are readily coagulated by barium sulphate. Sodium and potassium carbonates appear to increase the stability of the system. 

Colloidal Tellurium.—On reduction of dilute solutions of tellurium dioxide or telluric acid by means of hydrazine, hydroxylamine, hypo-phosphorous acid, sulphurous acid or salts of these compounds, brown or blue colloidal solutions can be obtained, the stability of which is greatly increased by the presence of an organic colloid such as gum arabic, sodium lysalbate or sodium protalbate.8 Similarly the addition of an extract of the seeds of Plantago psyllium, in amount not exceeding 0-3 per cent., renders extremely stable the sols obtained by the reduction of telluric acid with hydrazine hydrate.9 Stable sols may also be obtained by the reduction of telluric acid with dextrose in the presence of ammonia. Dialysis does not completely remove the adsorbed dextrose and this undoubtedly contributes to the remarkable stability.10 The particles of colloidal tellurium in these sols are negatively charged. 

Color Plate 31 Colloids and Dialysis (Demonstration 27-1) (a) Colloidal Fe(III) (left) and ordinary aqueous Fe(III) (right). (J ) Dialysis bags containing colloidal Fe(III) (left) and a solution of Cu(ll) (right) immediately after placement in flasks of water, (c) After 24 h of dialysis, the Cutll) has diffused out and is dispersed uniformly between the bag and the flask, but the colloidal Fe(III) remains inside the bag. 

Silver Arsenite Sol.—A colloidal solution of silver arsenite has been prepared by the following method.11 A 10 per cent, solution of sodium lysalbinate was added to aqueous silver nitrate and the precipitate separated, washed, and dissolved in an alkaline solution of sodium dihydrogen arsenite. After dialysis, the solution melded on evaporation to dryness a pale yellowish-brown mass which could be dispersed in water to form a sol. 

Dialysis is particularly useful for removing small dissolved molecules from colloidal solutions or dispersions - e.g. extraneous electrolyte such as KNO3 from Agl sol. The process is hastened by stirring so as to maintain a high concentration gradient of diffusible molecules across the membrane and by renewing the outer liquid from time to time (Figure 1.5). 

Dialysis The purification of colloidal solution by this method is based on the inability of the sol particles to pass through an animal membrane or a parchment paper which allows only the molecules or the ions to pass through. The vessel in which dialysis is carried out is known as dialyser [Fig. (2)]. A dialyser consists of a special type of vessel open at both the ends. To the lower end a membrane is stretched. This membrane allows only the solvent and other molecules to pass through it, but it is impermeable to the colloidal particles. The dialyser is then suspended in a larger vessel... 

Attempts to determine the molecular weight of colloidal ferric hydroxide lead to very high values. Thus, a colloidal solution prepared by addition of ammonium carbonate to ferric chloride solution was purified by dialysis, and the freezing-point determined of that portion which would not pass through a collodion membrane. The point was only slightly lower than that of the filtrate, indicative of a molecular weight of 3120 for the colloid.2... 

Purification of colloidal solutions is based on the ability of contaminating ions and molecules to penetrate freely through special membranes which hold back colloidal particles (dialysis). Inasmuch as low-molecular impurities in sols usually are electrolytes, dialysis can be accelerated by imposing an electrical field on the liquid to be dialyzed (electrodialysis). Prolonged dialysis leads not only to the removal of impurities from the sol, but also to the removal of an electrolyte-stabilizer which could lead to coagulation. 

In laboratory conditions hydrolysis usually is carried out as follows a solution of FeClj is added drop by drop to distilled water heated to boiling, stirring constantly. The whole liquid quickly takes on the red-brown color characteristic of hydrous ferric oxide, but remains transparent. If the solution is allowed to cool, the color fades somewhat, as some of the Fe(OH)3 changes back into FeCl3. To prevent the reverse reaction, the solution is boiled for a few minutes to remove HCl with water vapor, or it is removed by dialysis. In this way one obtains stable colloidal solutions of iron hydroxide containing from 5 to 5000 mg/1 of iron. 

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. 

After the synthesis, impurities of sodium, chlorine, pyrophosphate ions and unreacted tripolyphosphate ions remained in the colloidal solutions that can affect optical properties and stability. Thereby, after the synthesis the solution was purified with dialysis membranes (cut-off 12 kD). The purification efficiency was controlled by the measurement of the dialysate electroconductivity. During the first hour of dialysis, a slight increase in dialyzates electroconductivity occurs. After 24 h, the electroconductivity of dialysate and distilled water became equal. During the whole time of dialysis the terbium luminescence intensity was constant. 

An industrial application of dialysis is the recovery of caustic from hemi-cellulose solutions produced in making rayon by the viscose process. Flat-sheet membranes are placed parallel to each other in a filter-press arrangement (see Chap. 30, p. 1004) and water is passed countercurrent to the feed solution to produce a dialyzate with up to 6 percent NaOH. Recovery of salts or sugars from other natural products or other colloidal solutions could be achieved by dialysis, but ultrafiltration is more likely to be used because of the higher permeation rates that can be obtained. 

The common method for preparing a colloidal solution of hydrous ferric oxide, for example, has been to hydrolyze ferric chloride in solution by heat, and remove the hydrochloric acid by dialysis. Colloidal silica can be made in a similar way, that is, by dialyzing a solution of sodium silicate. One hydrolysis product—hydrochloric acid or sodium hydroxide—can pass through the membrane used for dialysis, whereas the colloidal particles of hydrous oxide cannot. The process is rather tedious and takes hours or even days. A much simpler method is to take out the acid or alkali with one of the solid ion exchangers or acid absorbers used in water purification. ... 

In order to free Congo Red from foreign electrolytes Bayliss precipitated it with HCl and dialyzed in a parchment osmometer until free from electrolytes In this manner he obtained a blue colloidal solution of the free acid that showed a very small rise in the osmometer. To get the sodium salt he dialyzed against a solution of sodium hydroxide, which caused an extraordinary rise in the osmometer. The excess of alkaU was removed by distilled water. After a week of dialysis the column retained a constant height of 50 mm. When the ordinary distilled water was replaced by conductivity water free from CO2 there was a further rise in the osmometer, and the column remained at 97 per cent of what it should from calculation. Only a faint cloud was to be seen in the ultramicroscope. The extraordinary sensitiveness to carbonic acid, as revealed by the last experiment, has also been observed by Biltz and v. Vegesack.f In these experiments membrane hydrolysis must be considered, which, according to Donnan, plays a part even in the case of strong electrolytes. 

Electrodialysis is an accelerated form of dialysis. The colloidal solution to be purified is contained within a membrane permeable to electrolyte ions, and two electrodes in the outside solution apply an electric field across the column of colloid. The ions to be removed move in each direction into the outer solution, and may be washed away with fresh solvent. 

After preparation, colloidal suspensions usually need to undergo purification procedures before detailed studies can be carried out. A common technique for charged particles (typically in aqueous suspension) is dialysis, to deal witli ionic impurities and small solutes. More extensive deionization can be achieved using ion exchange resins. 

Dialysis. If a solution containing colloidal particle is placed on one side of a dialysis membrane, the water on the other side will allow the solution to be reduced in concentration as it passes through the membrane. 

The colloidal palladium solution is prepared as follows A solution of a palladium salt is added to a solution of an alkali salt of an acid of high molecular weight, the sodium salt of protalbinic acid being suitable. An excess of alkali dissolves the precipitate formed, and the solution contains tine palladium in the form of a hydrosol of its hydroxide. The solution is purified by dialysis, and the hydroxide reduced with hydrazine hydrate. On further dialysis and evaporation to dryness a water-soluble product is obtained, consisting of colloidal palladium and sodium protalbinate, the latter acting as a protective colloid. 

Interstitial Water. The differentiation between solutes and particles is of great importance in the sampling of interstitial water. Most conveniently so-called peepers are used. These consist usually of plexiglass plates in which small compartments (0.5 cm deep and 0.5 - 1 cm high) are separated from the sediments by a dialysis membrane. The compartments are initially filled with degassed distilled water. After 1 - 2 weeks for equilibration subsequent to the retrieval of the peeper, the "dissolved" components are measured in each component. For this type of application the pore size does not seem to be very critical colloids do not seem to accumulate in the compartments (low diffusion coefficients). 

Destructive Interference when waves combine to cancel each other Deuterium isotope of hydrogen containing 1 proton and 1 neutron Dialysis the separation of particles from a colloid suspension by the passage of suspension solution through a semipermeable membrane... 

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