H. L. Wells — Double Salts of Lead Tetrachloride. 183 



(1) PbCl 4 + 2H 2 = Pb0 2 + 4HCl 



(2) Pb0 9 + 4liCl = PbCl a + Cl a + 2H,0 



The extent to which the second reaction takes place depends 

 upon the dilution and the temperature. If the amount of 

 water present is not too great, a state of equilibrium is 

 reached when a sufficient amount of alkaline chloride, hydro- 

 chloric acid and chlorine have gone into solution, and the de- 

 composition stops. The caesium salt is more slowly decom- 

 posed by water than others. All the salts are decomposed by 

 boiling with an excess of hydrochloric acid, but the decompo- 

 sition of the caesium compound is remarkably slow, especially 

 in solutions containing much caesium chloride. 



When free chlorine is present the caesium salt is almost com- 

 pletely insoluble in strong solutions of caesium chloride and 

 in hydrochloric acid. Although the rubidium salt is consider- 

 ably more soluble, the difference is not great enough so that a 

 quantitative separation can be made. It will be shown in the 

 following article that caesium can be approximately separated 

 from potassium, sodium and lithium by this means, and that 

 when rubidium is also present the caesium can be approxi- 

 mately determined indirectly. 



The salts to be described can be washed with hydrochloric 

 acid containing chlorine. They are perfectly stable on expos- 

 ure to the air. When heated in capillary tubes the ammonium 

 salt begins to whiten at about 225°, the potassium salt at about 

 190° and the caesium and rubidium salts at about 280°. This 

 temperature for the decomposition of the ammonium salt is 

 about 100° higher than that given by Nikolukine. It is prob- 

 able that difference is due to a typographical error. 



Attempts were made to prepare corresponding sodium and 

 calcium salts, without success. 



In analyzing the salts, lead was separated and weighed as 

 sulphate, and, in the filtrate from this, the alkali-metal was 

 determined as sulphate. To determine chlorine, a separate 

 portion was decomposed by a solution of sodium arsenite and 

 chlorine was determined in this as usual. 



Ammonium- Plumbic Chloride, (iV ' R^PbCl^: — In prepar- 

 ing this salt, JNTkolukine's method of using sealed tubes was 

 found to be unnecessary. A solution of lead tetrachloride was 

 made by adding slightly diluted hydrochloric acid to an excess 

 of lead dioxide at 0°. This solution was quickly filtered 

 through asbestus, and a saturated, cold solution of ammonium 

 chloride in dilute hydrochloric acid was added until an abund- 

 ant, yellow, crystalline precipitate was produced. The salt was 

 pressed on paper, and then air-dried. 



