18 THE ANALYSIS OF POTASSIC CHLORIDE. 



ably because of loss of previously adsorbed water. Moreover, when a 

 dilute solution of argentic nitrate, potassic nitrate, and nitric acid, such as 

 remains after an analysis, was passed through the filter, and this was five 

 times washed with water, and dried at 150, yet 0.15 more was gained, 

 probably due to adsorbed salts. Very- thorough washing after this 

 brought the crucible back to its original weight after drying at 150. 



These experiments showed that the washing of the platinum sponge 

 must be very thorough, and that a definite temperature must always be 

 used for drying. Other experiments showed that constant weight could 

 always be obtained, if these conditions were fulfilled. 



It is well known that argentic chloride sometimes clings to a platinum 

 surface, and always shrinks much on drying. On this account the 

 somewhat delicate upper surface of the sponge was protected by a closely 

 fitting disk or diaphragm of platinum punched with many fine holes. 

 This diaphragm, merely laid upon the top of the sponge at the base of the 

 crucible, was easily loosened with the precipitate, and formed an effectual 

 protection for the sponge. After this was done, no platinum was ever 

 lost from the upper surface ; and none was ever carried away mechanically 

 from below in the wash-water. 



In spite of the presence of the disk, small particles of argentic chloride 

 always clung here and there to the sponge. As a possible means of 

 removing these when preparing for a new analysis, potassic cyanide was 

 tested. It was found, however, that not only was this salt adsorbed by 

 the sponge, but that in the presence of air platinum was dissolved even 

 to the extent of a milligram --a fact not new, but none the less pertinent. 

 Thiosulphate was not tried, because sulphur was an impurity little wished. 

 Finally, while ammonia for a short time was inefficient, this liquid, when 

 concentrated and applied for twelve hours or more, dissolved every trace 

 of the silver salt without harming the crucible. 



The technique of the Gooch-Munroe crucible .having thus been mas- 

 tered, this utensil was used with satisfaction as a means of collecting 

 and weighing the precipitate in hand. The latter was first washed often 

 by decantation, as is recounted below, and finally on the filtering crucible, 

 which was dried to constant weight by a temperature gradually rising to 

 150. After careful weighing, the main mass was separated from the 

 clinging platinum disk, and was carefully fused in porcelain, as has often 

 been described. The accurately determined loss on fusion, amounting 

 sometimes to a milligram, was calculated from the part to the whole, 

 and applied as a correction to the total weight of the precipitate. 



This weight, even as thus corrected, did not, however, exactly represent 

 the total in weight of the chlorine, as some of the argentic chloride was 

 dissolved bv the wash-water. The ablution had been conducted in three 



