Detection of Palladium

Metallic palladium is the most readily fused of the platinum metals. When heated in air the surface becomes coated with oxides which tint it. The metal is soluble in aqua regia, sparingly soluble in nitric acid, but more readily soluble in the fuming acid, yielding palladous nitrate, from which a brown precipitate of basic salt may be obtained on dilution with water provided the acid is not present in too great an excess. A mixture of concentrated hydrochloric acid with chloric acid is recommended as a useful solvent for palladium, inasmuch as it liberates free chlorine, which readily attacks the metal. Thus:

5HCl + HClO₃ = 3Cl₂ + 3H₂O.

Boiling, concentrated sulphuric or hydrochloric acid slowly dissolves the metal. Fused potassium hydrogen sulphate attacks palladium.

Both hydrogen sulphide and ammtmium sulphide effect the precipitation of black palladium monosulphide, PdS, from solutions of its salts. The precipitate is insoluble in excess of ammonium sulphide, but dissolves on boiling with concentrated hydrochloric acid, and is soluble in aqua regia.

Caustic alkalies precipitate basic salts as yellowish brown deposits, soluble in excess. Water, alone, causes partial precipitation from acidulated solutions if acid is only present in very slight excess.

A characteristic reaction consists in the formation of a black precipitate of palladous iodide, PdI₂, on addition of alkali iodide. This tends to dissolve in excess of the alkali iodide, but is insoluble in hydrochloric acid.

Mercuric cyanide, Hg(CN)₂, precipitates palladous cyanide, Pd(CN)₂, from solution in the form of a yellowish white, gelatinous deposit, which is readily soluble in potassium cyanide, as also in ammonium hydroxide. This reaction is unique amongst the platinum metals, and serves as the basis of a useful method of separating and estimating palladium.

Addition of potassium chloride to a solution of palladous chloride causes the precipitation of the red potassium tetrachlorpalladate, K₂PdCl₄, which dissolves in water to a red solution, but is insoluble in alcohol.

Palladium may be distinguished from copper, since ammonium thiocyanate yields no precipitate even after passage of sulphur dioxide.

Metallic zinc reduces palladous chloride solutions with deposition of finely divided metallic palladium.