THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE. 17 



THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE. 



For purpose of analysis, the solution of potassic chloride was carefully 

 and completely transferred to a large Jena glass Erlenmeyer flask with a 

 finely ground glass stopper, and diluted to the volume of a liter or more. 

 To this in the dark-room under the red light was added exactly the calcu- 

 lated weight of argentic nitrate, never more concentrated than one-fiftieth 

 normal ; and the mixture was shaken for a short time in order to aggre- 

 gate the great mass of the precipitate. Because of the absence of an 

 excess of argentic nitrate, the danger of the occlusion of this salt was 

 slight. On the following day the supernatant liquid was usually quite 

 clear, and to it now was added the excess of perhaps 0.05 gram of argentic 

 nitrate needed for complete precipitation. This method of treatment 

 was found to be more satisfactory than the immediate addition of all the 

 argentic nitrate at once, as in this case long and tiresome shaking and 

 washing of the precipitate was needed to eliminate wholly the occluded 

 argentic nitrate. Thus was one of the chief difficulties met by previous 

 experimenters almost entirely overcome. It was shown in the paper on 

 sodium that the simplest test for the presence of argentic nitrate is the 

 appearance of the argentic chloride after fusion, even a small trace of 

 nitrate causing a perceptible gray-violet cloudiness. In all cases when the 

 above precaution was used the fused mass was perfectly clear and trans- 

 parent. This is, however, an anticipation of the later part of the process ; 

 the collection, ablution, and desiccation of the precipitate should first be 

 described. 



In preliminary experiments, the Gooch perforated platinum crucible 

 with asbestos mat was used with all the precautions previously adopted 

 in this laboratory. In this way it is certain that good results may be 

 obtained ; but the necessary second filtration of the filtrate in order to 

 collect the traces of disintegrated asbestos is a tiresome process. On this 

 account preliminary experiments were made with Gooch crucibles pro- 

 vided with a smooth and burnished mat of platinum sponge, as prepared 

 by Heraeus. 1 Such crucibles are named by him after Neubauer, but 

 might more properly be called Gooch-Munroe crucibles. In these tests 

 it was found that such crucibles as a matter of fact answer very well 

 for the purpose in hand if certain special precautions are taken. It 

 is not permissible to ignite them at a high temperature, unless the pre- 

 cipitate also is to be ignited at a high temperature. A crucible brought 

 to constant weight at 150 was found to lose 0.15 mg. on ignition, prob- 



1 Zeit. Angew. Chem., 14, 923 (1901). A number of experimenters have recom- 

 mended and used a mat of platinum sponge in a Gooch crucible, but Munroe was 

 probably the first. J. Analyt. Chem., 2, 241 (1888). Also Chem. News., 58, 101. 

 1885. 



