178 RESEARCHES UPON THE CHEAHCAL PROPERTIES OF GASES. 



chloric acid. The reaction begins and is completed without application of external 

 heat, the temperature of the mass becoming very high, as the above equation would 

 indicate. The production of hydrogen chloride in this reaction renders palladium 

 chloride a I'eagent of great sensitiveness for the recognition of free hydrogen in gas 

 mixtures. In order to study the reaction more fully it was necessary to prepare 

 pure, dry palladium chloride. When a solution of palladium in aquaregia is evapo- 

 I'ated to dryness, brown amorphous crusts are formed which are very imperfectly 

 soluble in water or hydrochloric acid. Analyses were made of the compound so 

 obtained, but the results showed varying amounts of chlorine and it was evident 

 that the salt had been partially decomposed during the evaporation. In order to 

 obtain pure palladium chloride the following method of preparation was adopted. 



Palladium dissolved in aquaregia was heated for several days in a covered 

 beaker over the water bath. Hydrochloric acid was added from time to time to 

 insure the destruction of any lower oxides of nitrogen. The solution was then 

 evaporated to dryness and the residue heated to 180° in a glass tube through which 

 a current of dry hydrogen chloride was passed. The excess of hydrogen chloride 

 was then expelled by a stream of carbon dioxide, and after the escaping carbon 

 dioxide was found to carry with it no more hydrogen chloride the compound was 

 considered pure. Hydrogen was now passed through the tube and into standard 

 soda solution. The palladium chloride was then reduced and the resulting hydrogen 

 chloride absorbed by the soda. The chlorine was determined volumetrically. Two 

 analyses were made. The results showed the salt to consist of palladium dichloride. 



Experiments undertaken for the purpose of comparison of the properties of the 

 two preparations demonstrated that palladium chloride prepared by the method 

 above detailed is a much more sensitive reagent towards free hydrogen than the 

 compound obtained on merely evaporating to dryness a solution of palladium chlo- 

 ride on the water bath. So delicate is the reaction that a neutral gas, such as nitro- 

 gen, containing 1-20 of 1 per cent, of free hydrogen, will rapidly give an indication 

 when passed over the palladium chloride and into a solution of silver nitrate. It is 

 absolutely necessary, especially where traces of hydrogen are suspected, that the gas 

 should be dry, as moisture is liable to condense with the hydrogen chloride in drops, 

 and thus the hydrogen chloride may be prevented from reaching the silver nitrate 

 solution. 



Ethylene reduces palladium chloride at temperatures above 100°. 



The double chloride of palladium and potassium, PdCL 2 KCl, is reduced at 

 about 300° by ethylene. 



The hydrocarbons of crude gasolene do not reduce palladium chloride at 190°. 



