128 EXPERIMENT STATION. [Jan. 



centratioii lias a certain influence on titration, it is advisable to 

 maintain approximately the same volume in every case. The 

 tendency of some solutions to become muddy on titration can 

 often be relieved by additional bicarbonate, though the condi- 

 tions involved seem to have no appreciable influence on the 

 results. The quality of all reagents employed in the determina- 

 tion should be proved by blank tests, which should not exceed 

 .10 of a cubic centimeter iodine solution for the amounts em- 

 ployed. Some lots of bicarbonate have been found unfit for 

 such work. Due recognition should be given the blank in calcu- 

 lating results. 



Considerable trouble is often experienced in determining in- 

 soluble matter with hydrochloric acid, due to the splitting off of 

 Avhite arsenic, especially with Paris gi'een. To offset the diffi- 

 culty it was found advisable to combine the determination with 

 that of preparing the arsenic solution by simply filtering off the 

 residue. The points noted above may be briefly summarized : — 



Transfer 2 grams of finely ground sample, together with 50 

 cubic centimeters of sodium acetate (1-2), to a 500 cubic centi- 

 meter graduated flask, and boil five minutes. Cool under tap, 

 add about 00 cubic centimeters of hydrochloric acid (1-3), and 

 shake until solution is effected. Make to volume and filter. 

 Pipette 25 or 50 cubic centimeters into an Erlenmeyer flask, 

 neutralize with dry sodium bicarbonate, add 25 cubic centimeters 

 of sodium potassium tartrate ^ (1-10), to redissolve precipitated 

 copper, approximately 3 grams of sodium bicarbonate, water 

 sufficient to make a volume of 100 cubic centimeters, 2 cubic 

 centimeters starch paste (1-200), and titrate with 1^/20 iodine 

 to a permanent blue color. Toward the end of the reaction cork 

 the flask and shake vigorously, to insure proper end point. Cal- 

 culate results as arsenous oxide. The residue in the graduated 

 flask is brought onto the filter, well washed, calcined in a porce- 

 lain crucible and weighed as insoluble matter. 



The above process has given excellent results with copper 

 aceto-arsenite, copper arsenite and calcium arsenite. Sodium 

 acetate does not prevent hydrolysis of copper and calcium arse- 

 nites, as in the case of Paris green, but serves to take up free 



' Used only with the copper arsenites. 



