4 BULLETIN 76, U. S. DEPARTMENT OF AGRICULTURE. 



the strength of arsenical solutions prepared after the ''standard 

 formula." 



It is necessary therefore to describe (1) methods for the determina- 

 tion of actual arsenious oxid, and (2) methods for the determination 

 of " total arsenic/' that is, methods which shall include arsenic 

 present in the oxidized form as well as that existing as actual arse- 

 nious oxid. It is necessary also to describe different variations or 

 modifications of processes for these two determinations, respectively 

 appropriate for use by (1) trained chemists with abundant laboratory 

 facilities, (2) persons possessing but slight chemical training and 

 equipment, and (3) persons in the field possessing no chemical 

 knowledge or training whatever, who obtain their results by manipula- 

 tion of an "outfit" prepared by a trained chemist. 



LABORATORY METHODS. 



It is to be distinctly understood that the methods here described 

 are not in all cases adapted or intended to reach a high degree of 

 accuracy from a chemist's viewpoint, though as a matter of fact 

 most of them are very accurate under especially favorable conditions. 

 A certain variation must necessarily be allowed in the composition 

 of baths prepared in the field. Again, experience has shown that the 

 percentage of arsenic in such baths may vary within certain limits 

 without perceptible effect upon either their effectiveness or their 

 safety. Therefore the purpose of assay is not primarily to determine 

 the exact percentage of arsenic — though the nearer this result is ap- 

 proached the better — but to insure that the composition of the baths 

 used shall fall within certain limits. Extreme accuracy, which is not 

 necessary, must be sacrificed to simplicity and convenience, which 

 are necesssary to permit the execution of very frequent tests. 



METHODS FOR ACTUAL ARSENIOUS OXID. 



Whatever method may be employed for the determination of actual 

 arsenious oxid, there are certain precautions connected with the tak- 

 ing and the storage of samples which can not be omitted if the results 

 are to be of any value whatever. As already noted, the arsenic in 

 used arsenical baths tends to change its degree of oxidation, 

 mainly through the action of microorganisms. Other tilings being 

 equal, the rapidity with which this change takes place is greatly 

 influenced by temperature, so much so that a sample contained in an 

 ordinary cork-stoppered bottle and exposed to summer heat may show 

 more change after one day than it would show after a week in the 

 original vat in its comparatively cool location underground. The rate 

 of the change can accordingly be retarded by keeping the sample cold, 

 as by storage in an ice box after receipt; but if the sample has trav- 



