Platinum Tetrachloride, PtCl₄

Platinum Tetrachloride, Platinic Chloride, PtCl₄, was first prepared in a hydrated form by Norton, who, however, failed to isolate the anhydrous salt. This latter may be obtained by heating partially dehydrated chlor-platinic acid in a current of chlorine at 360° C. The possibility of preparing it in this way has been disputed,7 but Gutbier, who carried out a number of experiments with chlor- platinic acid under a variety of conditions, was able to prepare the salt quite easily. In the neighbourhood of 60° C. the crystals melt in their combined water; a little later the water boils off and the mixture thickens. The final stages of the reaction are carried out with a slow rise of temperature.

One method consisted in suspending ammonium chlor-platinate in a small quantity of water, and decomposing it by the passage of chlorine gas. In order to avoid the formation of explosive nitrogen chloride the reaction should be carried out at 100° C. The product, namely, chlor-platinic acid in solution, is evaporated to dryness in vacuo, and heated in dry chlorine at 350° to 360° C., care being taken not to exceed this latter temperature. The mass foams somewhat, on account of the liberation of water and hydrogen chloride, and is gradually converted into the tetrachloride.

Gutbier, however, recommends heating chlor-platinic acid in a current of dry chlorine to a temperature not exceeding 360° C. in an apparatus containing diphenylamine.

Platinum tetrachloride is also obtained by heating chlor-platinic acid to 165° C. in a current of hydrogen chloride for fifteen hours.

Platinum tetrachloride is not formed by merely heating chlor- platinic acid in air, because it decomposes before the whole of the hydrogen chloride is eliminated, yielding the dichloride. When heated to 370° C. in an atmosphere of chlorine it begins to dissociate:

PtCl₄ ⇔ PtCl₂ + Cl₂.

Platinum tetrachloride is reddish brown in colour, somewhat hygroscopic, and moderately soluble in warm water with evolution of heat. It is but slightly soluble in alcohol, but easily soluble in acetone.

When exposed to the air, water is absorbed and the salt assumes a bright yellow colour, yielding a compound of composition represented by the formula H₂PtCl₄.O.4H₂O. Its aqueous solution is stable, admitting even of distillation without decomposition, and has an acid reaction, decomposing carbonates with evolution of carbon dioxide. It is usually regarded as containing tetrachlor-dihydroxy-platinic acid, H₂PtCl₄(OH)₂ or PtCl₄.2H₂O, and this receives support from the fact that upon electrolysis of the solution it is found that platinum is contained in the complex anion.

Platinum tetrachloride is reduced in solution by iodine at the ordinary temperature to the dichloride:

PtCl₄ + I₂ = PtCl₂ + 2ICl.

The heat of formation of platinum tetrachloride is:

[Pt] + 2(Cl₂) = [PtCl₄] + 59.4 Cals.,

and its heat of solution in water is:

[PtCl₄] + Aq. = PtCl₄.Aq. + 19.58 Cals.

When dry, platinum tetrachloride is reduced to the metal by a current of hydrogen even below 80° C. Its solution is reduced by powdered cobalt, with precipitation of metallic platinum. Thus:

PtCl₄. Aq. + 2[Co] = [Pt] + 2CoCl₂.Aq. + 109.98 Cals.

Platinic chloride combines with water to yield a series of hydrates. The octahydrate, PtCl₄.8H₂O, crystallises from aqueous solutions of the salt in the form of red needles.

The pentahydrate, PtCl₄.5H₂O, was first obtained by Norton and studied by Jorgensen. It results on decomposing silver chlor-platinate with boiling water, filtering off the silver chloride and crystallising over sulphuric acid. It crystallises in apparently monoclinic prisms, red in colour. When dissolved in water and the solution evaporated, the octahydrate separates out. Heated to 100° C. the pentahydrate loses 4 molecules of water, being converted into the monohydrate.

The tetrahydrate, PtCl₄.4H₂O, is the stable form at the ordinary temperature. It is obtained by allowing higher hydrates to effloresce in dry air,1 or by dissolving platinum hydroxide in a solution of chlor-platinic acid, and concentrating to crystallisation. The crystals are deliquescent, and, upon warming to 50° C. in a current of dry hydrogen chloride, yield chlor-platinic acid.

The dihydrate, PtCl₄.2H₂O or H₂PtCl₄(OH)₂, has not been isolated in the pure state. It exists in solution when platinum tetrachloride is dissolved in water, and is described under the name of tetrachlor-dihydroxy-platinic acid.

The monohydrate, PtCl₄.H₂O, results when the pentahydrate or the tetrahydrate is heated to 100° C. When heated to 360° C. it leaves a residue of dichloride.

Hexachlor-platinic Acid, H₂PtCl₆, is obtained in solution when platinum tetrachloride is dissolved in aqueous hydrochloric acid.

The usual method of preparing it consists in dissolving platinum in aqua regia.

The metal may, however, contain iridium which, although insoluble in aqua regia when by itself, yet dissolves when alloyed with platinum. The metal is therefore attacked by aqua regia on the water-bath, and the concentrated solution diluted, rendered alkaline with sodium carbonate, and reduced with formate. After warming, the precipitated metals are dried and ignited in a crucible whereby the iridium is rendered insoluble in aqua regia. Treatment with this latter reagent effects the solution of the platinum. To remove all nitrous compounds the solution is repeatedly evaporated and taken up again with concentrated hydrochloric acid. Any chlor-platinous acid, H₂PtCl₄, is converted into chlor-platinic acid by saturating with chlorine in the warm.

Platinum scrap is conveniently worked up into chlor-platinic acid by alloying with zinc under a layer of borax. The melt is treated with hydrochloric acid, which removes most of the zinc, and the finely divided platinum is dissolved in aqua regia.

The solution thus obtained is contaminated with a little zinc. It is therefore treated either with a rod of pure zinc which precipitates the platinum, or with hydrogen sulphide whereby the sulphide is precipitated and ignited. In both cases the resulting metal is dissolved in aqua regia, repeatedly evaporated with hydrochloric acid, excess of acid being finally removed by evaporation and the residue taken up with water.

Platinum sponge is soluble in concentrated hydrochloric acid saturated with chlorine; platinum black, if it has not been ignited, dissolves in concentrated hydrochloric acid in the presence of hydrogen peroxide; whilst compact platinum passes into solution when treated with a mixture of chloric acid and concentrated hydrochloric acid.

In each of these cases a solution of chlor-platinic acid is obtained entirely free from nitrogen compounds, and in a high state of purity if the original metal consisted of pure platinum.

The acid may also be obtained electrolytically by passing an electric current through concentrated hydrochloric acid containing precipitated platinum in a fine state of subdivision, using platinum foil as electrodes. The anodic chlorine attacks the finely divided metal, yielding the tetrachloride, which dissolves in the liquid to form the acid. The finely divided metal is conveniently prepared for this purpose by precipitation with metallic zinc from a solution of a platinum salt. On concentrating the solution to obtain the pure chlor-platinic acid it is advisable to introduce a small quantity of chlorine to ensure the absence of platinous compounds.

On concentrating the solution, chlor-platinic acid crystallises out in the form of reddish brown needles containing 6 molecules of water, thus: H₂PtCl₆.6H₂O. The crystals are deliquescent, soluble in water, alcohol, and ether; density 2.431.

Equivalent solutions of chlor-platinic acid and its sodium salt exhibit the same absorption spectrum. It is reasonable, therefore, to assume that similar negative ions exist in both solutions. If the formula Na₂PtCl₆ be accepted for the sodium salt, that of the free acid is H₂PtCl₆, and not 2HCl. PtCl₄.

When heated, the crystals melt at 60° C. in their own combined water; if maintained at 100° C. in vacuo in the presence of potash, pentachlor-hydroxy-platinic acid, PtCl₄.HCl.2H₂O or H₂PtCl₅(OH).H₂O, is left as a residue in the form of reddish brown crystals. At higher temperatures platinum dichloride is formed. It does not, upon mere ignition in air, yield the anhydrous tetrachloride, PtCl₄, since this decomposes before the whole of the hydrogen chloride has been eliminated. Consequently, to prepare the tetrachloride it is necessary to heat chlor-platinic acid either in dry hydrochloric acid or in chlorine.

When a concentrated solution of chlor-platinic acid is poured into concentrated sulphuric acid, the tetrahydrate, H₂PtCl₆.4H₂O, is obtained as a yellow precipitate. The anhydrous acid has not been isolated.

Chlor-platinic acid decomposes metallic carbonates, and unites with bases to form salts known as chlor-platinates, M₂PtCl₆. Hydrogen sulphide reduces it to chlor-platinous acid, H₂PtCl₄, and after a time precipitates some of the platinum in the form of its black disulphide. PtS₂.

Sulphur dioxide likewise reduces the acid to chlor-platinous acid, and if added in excess gives rise to complex sulphite derivatives. Yellow phosphorus reduces it to the metal, whilst hydrogen phosphide causes hypophosphites or phosphides to form, according to circumstances.

The heats of formation of chlor-platinic acid are as follow: [PtCl₄] + 2HCl.Aq. = H₂PtCl₆.Aq. + 24.8 Cals.

and

[Pt] + 2(Cl₂) + 2HCl.Aq. = H₂PtCl₆.Aq. + 84.6 Cals.

The heat of solution of the hexahydrated crystals is:

[H₂PtCl₆.6H₂O] + Aq. = H₂PtCl₆.Aq. + 4.34 Cals.

Hexachlor-platinates

Salts of hexachlor-platinic acid may be conveniently divided into three groups, namely:

1. Anhydrous salts of general formula M₂PtCl₆.
2. Hexahydrated salts, chiefly of the divalent metals, of general formula RPtCl₆.6H₂O.
3. Salts of divalent, trivalent, and tetravalent metals, containing varying quantities of water.

They may be obtained in several ways, namely:

1. By direct action of chlor-platinic acid upon the hydroxides or carbonates of metals. For example:
   
   2KOH + H₂PtCl₆ = K₂PtCl₆ + 2H₂O.
2. By mixing solutions of soluble metallic chlorides and chlor-platinic acid. For example:
   
   2NH₄Cl + H₂PtCl₆ = (NH₄)₂PtCl₆ + 2НСl.
   
   This is a particularly useful method to adopt when a quantitative determination of platinum is required since ammonium chlor-platinate is relatively insoluble in water, and still less soluble in alcohol.
3. A similar reaction to the preceding takes place when other salts than chlorides are added to chlor-platinic acid. For example, silver chloride is insoluble in water, but on adding the nitrate to a cold solution of the acid, silver chlor-platinate is immediately precipitated:
   
   2AgNO₃ + H₂PtCl₆ = Ag₂PtCl₆ + 2HCl.
4. By double decomposition of a soluble chlor-platinate, such as the sodium salt Na₂PtCl₆, with a salt of a metal yielding an insoluble chlor-platinate, such as, for example, silver.

The chlor-platinates are mostly of a yellow colour, the insoluble salts, namely, those of potassium, rubidium, caesium, ammonium, and silver, crystallising in the anhydrous condition in the form of cubic octahedra, isomorphous with the chlor-iridates and chlor-osmates. The more soluble chlor-platinates, namely, those of sodium, magnesium, zinc, etc., crystallise with 6 molecules of water, and are isomorphous with the chlor-stannates.

Both ammonium and potassium chlor-platinates, on account of their insolubility in alcohol, are used in the quantitative estimation of platinum. The former salt upon ignition yields a very pure platinum sponge. The solubilities of the more important chlor-platinates in water at 18° C. are given in the following table:

Chlor-platinate	Grams of M2PtCl6 in 100 grams of Water.
Silver	Almost nil
Caesium	0.08
Rubidium	0.14
Ammonium	0.67
Potassium	1.03
Sodium	c.40