364 AGRICULTURAL ANALYSIS 



the hot mixture in small quantities at a time, and beating up the 

 precipitate well with a thin glass rod after each addition. The 

 pump is kept in action all the time, but to keep out dust during 

 the washing, the cover is only removed from the crucible when 

 the fluid is to be added. 



Put the vessels containing the precipitate aside, return the wash- 

 ings once through the asbestos so as to obtain them quite clear, 

 remove them from the receiver, and set aside to recover the excess 

 of silver. Rinse the receiver and complete the washing of the pre- 

 cipitate with about 200 cubic centimeters of cold water. Half 

 of this is used to wash by decantation and the remainder to 

 transfer the precipitate to the crucible with the aid of a trimmed 

 feather. Finish washing in the crucible, the lumps of silver 

 chlorid being broken down with a glass rod. Remove the 

 second filtrate from the receiver and pass about 20 cubic centi- 

 meters of 98 per cent, alcohol through the precipitate. Dry at 

 from 140 to 150. Exposure for half an hour is found more 

 than sufficient, at this temperature, to dry the precipitate thor- 

 oughly. It has been proposed to modify this process somewhat 

 by directing that the precipitate be washed several times by de- 

 cantation instead of with 200 cubic centimeters of water, this 

 quantity not being considered sufficient in all cases. 



The above is the old method of standardizing the hydrochloric 

 acid. The method now in use is as follows : 



By means of a preliminary test with silver-nitrate solution, 

 to be measured from a burette, with excess of calcium carbon- 

 ate to neutralize free acid and potassium chromate as indicator, 

 determine exactly the amount of nitrate required to precipitate 

 all the hydrochloric acid. To a measured and also weighed por- 

 tion of the standard acid add from a burette one drop more of 

 silver-nitrate solution than is required to precipitate the hydro- 

 chloric acid. Heat to boiling, cover from the light, and allow 

 to stand until the precipitate is granular. Then wash with hot 

 water through a gooch crucible, testing the filtrate to prove ex- 

 cess of silver nitrate. Dry the silver chlorid at 140 to 150 C. 



(6) Standard sulfuric acid, the absolute strength of which has 

 9 Bureau of Chemistry, Bulletin 107, 1907 : 5. 



