10 BULLETIN 76. U. S. DEPARTMENT OF AGRICULTURE. 



to about 200 c. c, nearly discharge the color of iodin by sodium 

 thiosulphate, add a little starch solution, and continue the cautious 

 addition of thiosulphate solution until the blue color disappears. 

 Then add standard iodin solution from a burette until the faintest 

 possible persistent blue remains. Whichever method is used, the 

 solution at this stage should be in a capacious flask, and should be 

 diluted to about 300 c. c. Add a few drops of methyl orange, then 

 render alkaline by the cautious addition with a spatula of anhydrous 

 sodium carbonate. When the solution is clearly alkaline wash down 

 any solid adhering to the neck or sides of the flask and add dilute 

 sulphuric acid until the reaction is clearly acid, taking care that no 

 particles of sodium carbonate remain undissolved. Then add a 

 liberal excess of sodium bicarbonate, 10 c. c. more starch solution, 

 and titrate with standard iodin in the usual manner. -The final 

 titration should of course be corrected for the adsorption of arsenious 

 oxid by blood-charcoal. No additional correction for adsorption of 

 arsenic oxid appears necessary. No extra addition of potassium 

 iodid appears to be necessary in case cresylic acid, etc., is present. 



The above-described methods for actual arsenious oxid and for total 

 arsenic have proved reliable for the examination of all products thus 

 far permitted for use in official dipping, and it is probable that they 

 will be found applicable to any proprietary products which may 

 receive such official recognition in the future, though obviously a 

 positive statement on this point can not be made. The experienced 

 chemist will naturally think of other methods which might be applied. 

 For example, the acidified and filtered bath may be treated in a sepa- 

 rating funnel with ether or some other appropriate organic solvent 

 to extract cresylic acid, fatty acids, etc.; total arsenic may be deter- 

 mined after the destruction of organic matter by appropriate diges- 

 tion or incineration, or it may be separated by distillation; while it is 

 known that some chemists prefer to determine directly arsenic 

 existing as arsenic acid by titration with uranium acetate. 



FIELD METHODS OF ASSAY. 



A field method of assay is generally based upon a laboratory 

 method; apparatus, reagents, etc., being simplified to the extreme 

 in the interests of durability and portability, and the operations 

 being reduced to the fewest and simplest, that they may be success- 

 fully executed by persons wholly unacquainted with chemistry as a 

 science. But a skilled chemist in the laboratory to supervise the 

 iicld tests is even more necessary than if all the work were actually 

 performed in the laboratory itself, for upon his experience and care 

 reliance must be placed to standardize the methods in such a way 

 that useful data may actually result from the manipulation of inade- 

 quate apparatus by unskilled hands. Of course, it is only excep- 



