Platinum Hydrosol or Colloidal Platinum

A solution of platinum hydrosol or colloidal platinum in water is easily prepared by sparking between platinum electrodes immersed in ice-cooled water, a current of about 10 amperes and 40 volts being employed. The electrodes consist of thick platinum wire, and, when placed from 1 to 2 mm. apart, sparking takes place, particles of the metal being torn off and suspended in the water. The liquid thus obtained is allowed to stand overnight, and decanted from any sediment. It has a dark colour, but the individual metallic particles cannot be distinguished even with the aid of a microscope.

Colloidal platinum may also be prepared by reduction of the chloride with hydrazine hydrate, in the presence of a protective colloid (vide infra).

Castoro recommends acraldehyde as a suitable reducing agent.

When platinum wire is heated to incandescence and plunged into distilled water the presence of colloidal metallic particles can be detected by ultra-microscopic examination.

As mentioned above, the stability of colloidal metal solutions is considerably enhanced by the addition of certain organic substances or protective colloids. For example, gelatin has been frequently employed, 0.5 gram per litre of colloidal solution proving very effective in preventing the coagulation or precipitation of the metal. Extracts of Iceland moss and of quince seed have also been recommended.

Paal employs the sodium salts of protalbinic and lysalbinic acids, which are prepared from egg albumen by heating at 100° C. with a 3 per cent, solution of sodium hydroxide. Addition of acetic acid, effects the precipitation of protalbinic acid, which is washed free from salts by dialysis. The mother-liquors contain lysalbinic acid, which is separated by concentrating to small bulk and acidifying with sulphuric acid. It may be purified by dissolving in water and pouring the solution into alcohol. An alkaline solution of either of these acids is prepared, and to it a solution of platinic chloride is added and a slight excess of hydrazine hydrate, the whole being allowed to stand for five hours. The salts produced are washed away by dialysis and the colloidal solution concentrated, the product being dried at 100° C., when it yields black scales, readily soluble in water to a brown opalescent solution.

Amberger has used lanolin as protective colloid. The lanolin is impregnated with an aqueous solution of a platinous salt, and intimately mixed with the requisite quantity of alkali hydroxide to precipitate out the platinum as hydrated oxide. This is reduced to colloidal metallic platinum by hydrazine hydrate. Both the lanolin and the colloidal metal are dissolved by light petroleum or chloroform, and the whole of the metal with a portion of the lanolin may be re-precipitated on addition of alcohol - a reaction affording a means of increasing the concentration of the metal in the preparation.

When stannous chloride is added to solutions of platinum salts a red coloration is produced, which is due to the formation of so-called red colloidal platinum, which is kept in a fine state of division by the colloidal products of hydrolysis of the stannous chloride. In the absence of a protective colloid the red platinum changes to brown colloidal platinum. An interesting analogy may be traced between this red colloidal metal and the better-known colloidal gold, termed "purple of Cassius."

Colloidal platinum possesses considerable catalytic activity. For example, it stimulates the decomposition of hydrogen peroxide solution, a dilution of one gram atom of platinum in 70 million litres of water having a pronounced accelerating effect.

The decomposition of hydrogen peroxide in this manner is a convenient reaction to study, since its rate can be followed by titration with potassium permanganate. Both in acid and in neutral solution it proceeds strictly in accordance with the monomolecular reaction:

H₂O₂ = H₂O + O.

In alkaline solution the catalytic action of the colloidal platinum increases to a maximum with increase of alkali concentration and then decreases, in which respect it behaves in a precisely similar manner to certain organic ferments.

Colloidal platinum still further resembles organic ferments in its action upon hydrogen peroxide in that its activity is reduced or partially paralysed by the addition of poisons such as hydrogen cyanide, hydrogen sulphide, or mercuric chloride. After a time, however, the metal may recover from these.

The reaction is not affected by light, but increases with the concentration of the platinum, although not directly in proportion to the same; it is also enhanced by rise of temperature.

The rate of decomposition of hydrogen peroxide by colloidal platinum has been made use of in determining the absolute and relative influences of different protective colloids upon the colloidal metal, the times required for the decomposition of a definite percentage of the peroxide under varying conditions being noted.

With gelatin the results are typical, and are given in the following table:

Percentage of Gelatin	Relative Times required to decompose 50 per cent, of the Peroxide Solution.
0.000	100
0.001	437
0.01	460
0.05	620
0.10	983

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.

When colloidal platinum is added to mixtures of Caro's permonosulphuric acid and aqueous hydrogen peroxide, oxygen is rapidly evolved; thus:

H₂SO₅ + H₂O₂ = H₂SO₄ + H₂O + O₂.

Solutions of hydrogen peroxide and potassium persulphate interact slowly in the cold, the rate of reaction being greatly enhanced by the addition of colloidal platinum:

K₂S₂O₈ + H₂O₂ = K₂SO₄ + H₂SO₄ + O₂.

Carbon monoxide combines with oxygen to yield carbon dioxide when shaken at ordinary temperatures with colloidal platinum solution, whilst hydrogen and oxygen gases unite under similar conditions to yield water.

Colloidal platinum solution absorbs hydrogen gas, a rough proportionality existing between the concentration of the metal and the volume of hydrogen absorbed. Certain protective colloids, such as gum acacia, dextrin, and albumen, tend to reduce the amount of hydrogen absorbed, but sucrose has a negligible effect. It was to be expected, therefore, that colloidal platinum would assist in the reduction of many substances in the presence of hydrogen gas. This has, indeed, proved to be the case. Acetylene is reduced to ethylene and ethane, and ethylene is reduced to ethane. The activity of the metal diminishes after repeated use, and, just as in the decomposition of peroxide already referred to, the reactions are increased by increasing the concentration of the colloidal metal, such increase is by no means directly proportional to the concentrations employed.

Ferric salts are reduced to ferrous, and ammonium molybdate likewise suffers reduction.

Platinum hydrosol also catalytically assists in the reduction or hydrogenation of many organic substances such as unsaturated oils. To this end an aqueous solution of colloidal platinum is added to an alcoholic solution of the organic substance and hydrogen gas bubbled through. The temperature, pressure, and extent of agitation of the liquid are important factors. In this manner linseed and other oils are readily reduced, saturated or " hardened," to yield white fatty substances.

The activity of colloidal platinum is reduced by such poisons as hydrogen cyanide or sulphide, but increased by short exposure to ultraviolet light. On the other hand, prolonged exposure to ultra-violet light causes the deposition of the metal as a black, flocculent precipitate, its catalytic activity being thereby totally destroyed.

Colloidal platinum may also be coagulated by introducing plates of various metals into its aqueous solutions. It is found that plates with roughened surfaces are more active than those that are smooth, other things being equal. The following is the order of activity of the five metals that have been experimented with, the first named causing the most rapid coagulation of the platinum, and the last named the slowest: zinc, steel, nickel, tin, and copper. The explanation appears to lie in the solution of traces of the metals upon introduction into the colloidal solution, whereby positively charged ions are formed which neutralise the negative charges of the colloid particles. In support of this, it is interesting to note that the presence of copper has been demonstrated in the coagulated mass precipitated from colloidal solution by the introduction of a copper plate.