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Estimation of Palladium

Palladium may be estimated gravimetrically by addition of potassium chloride and alcohol to a concentrated solution of palladium chloride in hydrochloric acid. The brown precipitate of potassium chlor-palladite is collected, ignited, the potassium chloride washed out, and the residual palladium weighed as metal.

Palladium may be separated out from copper in this manner, but clearly cannot be separated from such metals as ruthenium, etc., which yield insoluble double chlorides with potassium chloride.

Palladium is frequently estimated by precipitation of palladous cyanide. To this end the chloride solution is almost completely neutralised with sodium carbonate and a solution of mercuric cyanide added. On gently warming until the odour of hydrogen cyanide has ceased, a whitish precipitate of palladous cyanide separates out. This is well washed, ignited, and the palladium weighed as metal.

This cyanide method is particularly useful because palladium is unique amongst the platinum metals in yielding a precipitate of cyanide on addition of mercuric cyanide; hence palladium admits of separation from its allies in this manner, and from most of the commoner metals too, notable exceptions being lead and copper.

Palladium may also be estimated by adding sodium hydroxide to a solution of the metal until the precipitate first formed has redissolved. Excess of alcohol is now added, and the whole kept on the water-bath until the alcohol has disappeared. This is repeated, the liquid thoroughly boiled, and the precipitated palladium collected in a filter and ignited, first in air, then in hydrogen, and finally cooled in carbon dioxide.

Another method consists in precipitating the metal from its hot solution by addition of a hydrazine salt in acid solution. In this way palladium may be separated from aluminium and chromium, and indeed from nickel and cobalt if dilute solutions are employed. But the metal cannot be separated from mercury, lead, silver, platinum, copper or gold by this method, for although none of these metals are precipitated from solution by hydrogen salts alone, the presence of the palladium induces precipitation owing to the production of hydrogen.

When acetylene gas is passed through an acidified solution containing palladium, a brown precipitate is obtained which, upon ignition, yields palladium. In this way palladium may be quantitively separated from copper.

Separation of Palladium from Other Metals

Palladium may be separated from its allies also by addition of freshly precipitated silver iodide to a solution of their chlorides. Palladium chloride is the only one acted upon; it is converted into a black insoluble residue of iodide, the other metals remaining in solution. The washed precipitate is treated with potassium iodide solution or with aqua regia, whereby the palladium is dissolved out, and may be determined by ignition to the metallic condition.

Palladium may be conveniently estimated by precipitation with α-nitroso β-naphthol in a similar manner to cobalt. The solution is acidified with hydrochloric and acetic acids, heated to boiling and precipitated with a hot, saturated solution of α-nitroso β-naphthol in 50 per cent, acetic acid. A red coloration is produced, giving rise to a voluminous precipitate of the same colour, and having the composition (Cl10H6O2N)2Pd. This is filtered off, washed, ignited in a current of hydrogen, and cooled in carbon dioxide, the palladium being weighed in the metallic condition. Since the other platinum metals are not precipitated in this manner, the foregoing affords a useful method of separation from them. The reaction is very sensitive, yielding a distinct precipitate, on standing, from a solution containing only 0.001 mg. of palladium ammonium chloride per c.c.

Palladium may also be precipitated with dimethyl glyoxime, and estimated in a similar manner.

Palladium and Platinum may be separated from each other by repeatedly evaporating their solution with hydrochloric acid, whereby the palladium is reduced to palladous chloride. On addition of ammonium chloride the platinum is precipitated as ammonium chlor-platinate, (NH4)2PtCl6, leaving the palladium in solution, from which it may be obtained by the glyoxime method.

Electrical Methods of Palladium Estimation

Palladium may be estimated electrolytically by acidifying solutions of its salts with sulphuric acid, heating to 65° C. and electrolysing at this temperature with a current of 0.25 ampere at 1.25 volts. The cathode may conveniently consist of a semicircular piece of platinum-gauze and the anode of a platinum-spiral. If tin is present, it remains in solution, a complete separation of the two metals being thereby effected.

When a dilute solution of palladium is acidified with hydrochloric acid and reduced with carbon monoxide the electric conductivity increases considerably, the increase being proportional to the palladium and independent of the acidity . The increase in conductivity multiplied by 1.21×10-4 gives the number of milligrams of palladium in 100 c.c. with a mean error of about 1 per cent.

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