BAXTER AND COFFIN. — ANALYSIS OF SILVER ARSENATE. 1S9 



salt showed no perceptible change in weight. The third determination 

 also was made with a sample of salt which had been exposed to bright 

 light for three weeks in a dry state. In the second determination the 

 salt was exposed to light under water for one week. 



Two facts show that the presence of the small proportion of the 

 residue in the arsenate could have had no important effect upon the 

 results. In the first place, the formation of the insoluble matter 

 under the influence of light is not attended by change in weight. In 

 the second place, the silver content of the residue is very near that of 

 silver arsenate. Nevertheless care was taken to protect the arsenate 

 as far as possible from exposure to light. 



The Determination of Moisture in Silver Arsenate. 



T. W. Richards 10 and others have already drawn attention to the 

 fact that it is not possible, without fusion, to dry completely a sub- 

 stance formed in aqueous solution, owing to the mechanical retention 

 of liquid in pockets within the solid. In the case of silver arsenate, 

 although it is possible to fuse the salt, the temperature necessary is so 

 high that decomposition of the salt takes place to some extent. Hence 

 the loss in weight on fusion cannot be used as a true measure of the 

 water content of the salt. Since decomposition of the salt could pro- 

 duce only easily condensible substances and oxygen, the difficulty was 

 overcome in the present instance by fusing weighed quantities of the 

 salt in a current of pure dry air and collecting the water vapor in a 

 weighed phosphorus pentoxide tube. Of course great pains were taken 

 to treat the salt used in the water determinations in exactly the same 

 way as that used in the analyses for silver. 



The procedure was as follows : A sample of salt very nearly as pure 

 as that used in the silver analyses was weighed out in a copper boat 

 which had been previously cleaned and ignited in the blast lamp to 

 remove organic matter. The boat was placed in a hard glass tube and 

 was heated for between seven and eight hours at 250° C. in a current of 

 dry air. In these experiments, before passing through the drying 

 towers the air had first been passed over hot copper oxide in order to 

 oxidize any organic matter it might contain. Furthermore, the con- 

 centrated sulphuric acid in the drying towers had been heated with 

 a small quantity of potassium dichromate. One end of the hard glass 

 tube was connected to the apparatus for supplying pure air, by means 

 of a well-fitting ground joint upon which no lubricant was used. The 



10 Zeit. physik. Chem., 46, 194 (1903). 



