PREPARATION OF THE MATERIAL. 11 



in the spectroscopic tests. This method, therefore, appeared very well 

 suited for the preparation of pure potassium material, which might easily 

 be converted into chloride after the other metals had been eliminated. 



Pure potassic nitrate of commerce, obtained from Germany, was dis- 

 solved in a little water, freed from solid substances by nitration, and 

 crystallized twice in Jena glass flasks. The salt was each time freed 

 from the mother liquor in a porcelain centrifuge. 1 The resulting material 

 was four times subsequently recrystallized in platinum vessels, and whirled 

 in a platinum centrifuge. The pure crystals were now divided into two 

 parts; one part was dried in a vacuum desiccator, while the other part 

 was subjected to yet six more crystallizations and whirlings in platinum 

 vessels. 



It now became a question how this nitrate could best be converted into 

 chloride. The method recommended by Stas of beating the salt with 

 pure ammonium chloride involves not only the preparation of this latter 

 salt in a pure state, but also great danger of contamination from the 

 vessel, whether of platinum or porcelain. Platinum is especially attacked 

 because of the action of the oxychlorides of nitrogen and of chlorine. The 

 more favorable method appeared to be to convert the nitrate into chloride 

 by repeated evaporation with hydrochloric acid in quartz dishes. This 

 method was found to have two objections. In the first place, dishes 

 large enough for the raw material were not to be had. Moreover, the 

 tendency of the branching crystals to grow over the edge of the dish 

 caused serious loss of material and danger of impurity. Neither could 

 potassic nitrate be heated to a high temperature with ammonic chloride, 

 or in the steam of hydrochloric acid in quartz dishes ; for there is 

 always danger that the reaction 4KNO 3 -f 2SiO 2 = 2K 2 SiO 3 + 4NO 2 -f O 2 

 would occur. 2 



Because Stas had never been able to obtain a salt free from silica when 

 working in glass vessels, we desired to avoid completely the use of glass. 

 Therefore, only one way remained, namely, to convert the potassic nitrate 

 into chloride by precipitation with hydrochloric acid in a platinum dish. 

 The resulting aqua regia must naturally attack the platinum, but we pre- 

 ferred this impurity to that of silica, and found, moreover, that the amount 

 of platinum dissolved is very small. In the presence of large masses 

 of potassic salts, aqua regia, especially in the cold, dissolves platinum 

 but little. For example, in one experiment in which about half a kilo- 

 gram of potassic nitrate was treated with hydrochloric acid gas, only 

 three-tenths of a gram of platinum was converted into the form of 

 potassic chlorplatinate. In subsequent crystallizations the amount was 



JRichards, Journ. Am. Chem. Soc., 27, 105 (1905). 

 2 Richards and Archibald, loc. cit. 



