Colloidal Selenium

Selenium is readily obtained in the form of a colloidal solution, red by reflected light and blue by transmitted light. Such solutions were first obtained by Schulze by the reduction of selenium dioxide in dilute aqueous solution by means of the requisite quantity of sulphur dioxide:

SeO₂ + 2H₂O + 2SO₂ = Se + 2H₂SO₄.

With more concentrated solutions some selenium was also precipitated, which, however, on dilution dissolved in the hydrosol.

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, 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.

Caoutchouc mixed and warmed with finely divided black amorphous selenium assumes the dark red colour of colloidal selenium. This is the first observed case of the direct reduction of an element to the colloidal condition by intimate contact with a colloid. The effect is probably due in large measure to stresses produced and heat generated during the mechanical working of the mixture. The presence of another colloidal substance such as albumen, gum arabic or the sodium salt of protalbic acid, renders the colloidal selenium more stable, so that it may even be separated in a solid state without losing its power of again yielding a colloidal solution on the addition of water.

When warmed with neutral organic substances selenium frequently passes into colloidal solution and remains in this condition even after solidification of the solvent. Even when the solid suspension in anthracene or phthalic acid is melted the selenium does not pass into the grey crystalline condition although the points of fusion of these solvents are higher than the transformation temperature. The higher the temperature reached in the preparation of these solutions and the slower the cooling the smaller are the selenium particles.

Selenium prepared by the reduction of selenious acid with sodium or ammonium hydrogen sulphite forms colloidal solutions, that precipitated by means of the former salt passing more easily into solution than that precipitated by the ammonium salt. Reduction with sodium hydro-sulphite produces reversible and stable sols so long as the relationship 1SeO₂:2 to 2.8Na₂S₂O₄ is not exceeded in the reduction.

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 selenium produced by means of hydrazine hydrate can be frozen to a blue ice which melts with complete coagulation, but the presence in the solution of hydrogen chloride, sodium carbonate or potassium chloride, exerts a protective action which is a maximum at certain definite concentrations.

If an aqueous solution containing equimolecular quantities of selenium dioxide and dextrose is evaporated on a water-bath to a syrupy consistency and concentrated ammonia is then added, drop by drop, care being taken that the solution remains syrupy, then after cooling and mixing with water, reddish-brown sols of colloidal selenium are obtained. The stability of these sols is increased by the presence of a slight excess of dextrose. They are stable on boiling. If there is no excess of dextrose present then on freezing the sols are completely and irreversibly coagulated. The sols are also sensitive to electrolytes. Stable selenium sols may be obtained by the reducing action of quadrivalent titanium. If a solution of titanium trichloride (1.5 per cent.) is boiled for some time, hydrolysis and oxidation occur; on addition of this solution to one of selenium dioxide (0.2 per cent.) reduction to selenium occurs and any unchanged titanic acid, Ti(OH)₄, remains in colloidal solution and exerts a protective action.

By the action of dilute sulphuric acid on sodium selenosulphate according to the equation

Na₂SeSO₃ + H₂SO₄ = Se + SO₂ + Na₂SO₄ + H₂O,

particles of colloidal selenium are obtained which are positively charged. The colloidal selenium produced by the solution of selenium in hydrazine hydrate is negatively charged.

The electrical method, commonly applied to the preparation of colloidal metals, can also be extended to selenium. When an electric discharge is made to pass under pure water between a platinum anode and a cathode prepared by fusing a small piece of selenium on to platinum foil, colloidal selenium is slowly formed. When a dilute solution of selenious anhydride is electrolysed using platinum electrodes, a colloidal solution of selenium and a black cathodic deposit of selenium are formed. According to Gutbier and Weise, when dilute aqueous selenium dioxide is electrolysed between platinum poles (in the presence of a trace of alkali) with a tension of 220 volts, a moderate evolution of a gas which does not contain hydrogen selenide is first observed. As soon as the solution attains its boiling-point the formation of colloidal selenium commences and the solution becomes consecutively yellow, yellowish-red, red bluish-red and finally blue. Selenium is not deposited in an irreversible form until the last stage is reached provided that the original solution is not too concentrated. The red solutions invariably show a tendency to become bluish-red when cooled, a sign of incipient coagulation. They can only be obtained in a moderately stable condition if dialysed while still hot and then immediately diluted with pure water or with a solution of a protective colloid such as gum arabic.

Selenium bromide, Se₂Br₂, decomposes in contact with water according to the equation

2Se₂Br₂ + 2H₂O = 3Se + SeO₂ + 4HBr;

a small proportion of the selenium passes into solution, but the greater part is precipitated. Selenium tetrabromide dissolves in water, but much more readily in hydrobromic acid, and on dilution of the acid solution colloidal selenium is formed, which may be purified by dialysis.

The coagulation of selenium sols is greatly accelerated by freezing. It has been shown that the destruction produced by freezing is greater the more completely the solutions have been purified by dialysis. The nature of the reducing agent employed in the preparation of the sols and the temperature of the preparation also have a great influence on the stability towards freezing. The more concentrated sols are more readily destroyed than the more dilute sols, but even with the latter it is found that on keeping the sol in the frozen state for some time a non-homogeneity is produced and a red ring of precipitated selenium formed at the top and bottom of the frozen mass.

With selenium sols prepared by means of hydrazine hydrate at the ordinary temperature, by pouring into a large volume of water it has been shown that the stability of the colloid depends mainly on the degree of dispersion. An optimum concentration of electrolyte is necessary for the stability of the hydrosols. In the absence of electrolytes the system is quite unstable towards freezing.

The sodium salts of protalbic and lysalbic acids exert a protective action on selenium sols. Saponin also exerts a protective action on sols prepared by the reduction of selenious acid with hydrazine hydrate. The protected sols after dialysis may be preserved for long periods; although they have the tendency to settle into two layers, they may readily be made homogeneous again by simply shaking. Sols prepared in this way are relatively stable towards concentration either by freezing or warming. A chloroform extract of the seeds of plantago psyllium renders selenium sols prepared by reduction with hydrazine hydrate very stable. Evaporation of such stabilised colloidal solutions gives reversible residues containing up to 65 per cent, of selenium.

The rate of flocculation of selenium sols by solutions of potassium or barium chloride of various concentrations at temperatures between 15° and 20° C. has been determined. Results show that a very high concentration of these electrolytes is necessary for rapid flocculation. Smoluchowski's theory holds when the velocity of flocculation is not far removed from that obtaining when the colloidal particles are totally discharged. The results, however, deviate largely from this theory when the concentrations of the electrolytes are lower.