16 BULLETIN 76, U. S. DEPARTMENT OF AORICULTUEE. 



FIELD METHOD FOR "TOTAL ARSENIC." 



Since, pending further investigation, it is not possible to credit 

 arsenic in the higher form of oxidation (arsenic acid or arsenates) 

 with definite value as a tickicide, the efficiency of dipping baths must 

 be judged for the present solely on the results afforded by the test 

 for actual arsenious oxid. On the other hand, studies of changes in 

 the composition of a number of dipping baths in actual service in the 

 field, together with practical experience, have so far failed to show a 

 clearly defined danger that the average bath, in which the strength 

 of actual arsenious oxid is maintained, is likoly to reach a degree of 

 oxidation which may render it dangerous to cattle. Nevertheless, in 

 view of obvious possibilities in these two directions, it is desirable to 

 possess a field method for the estimation of total arsenic. The method 

 to be described makes use of a chemical reaction recently discovered 

 by the writer, 1 namely, the reduction of arsenic acid to arsenious acid 

 by thiosulphuric acid. After reduction and removal of excess thio- 

 sulphuric acid by iodin in acid solution, sodium bicarbonate may be 

 added and the arsenious oxid, now representing the total arsenic, may 

 be titrated in the usual way. The method is necessarily more com- 

 plicated, more tedious, and. less accurate than the simple method for 

 actual arsenious Oxid. . Nevertheless, rather comprehensive tests by 

 representatives of the bureau in the field indicate that, if conditions 

 demand such a test and no better can be discovered, the one at hand 

 will afford useful results. 



In addition to the outfit and supplies already described, the follow- 

 ing supplies are necessary: 



1. "Red tablets": 



Talcum powder, U. S. P grams. . 10 



Haw potato starch do 30 



Mix, and stir in 0.1 gram Sudan Red III dissolved in sufficient ether to distribute 

 the color, and evaporate off ether at a moderate temperature with frequent stirring. 

 Add: 



Sodium bicarbonate in fine powder grams . . 3 



Special soluble starch do 10 



Mix, and add potassium pyrosulphate powdered to pass a 40-mesh sieve, do 125 



Mix well and compress into tablets of such size that 1 tablet will neutralize 9 to 10 

 c. c. of normal alkali. The mixture, not being granulated, can not be run through the 

 hopper of a power tablet machine, but must be fed by hand into the die from the table 

 of the machine. 



2. "Blue tablets" : 



Ultramarine blue grams. . 1 



Talcum powder, U. S. P do 5 



Sodium thiosulphate, crystallized, ground to pass 20-mesh sieve, and air- 

 dried do. ... 100 



Mix and compress into tablets of such size that 1 tablet will be equivalent to about 

 22 c. c. of twentieth-normal iodin. 2 



1 Journal of Agricultural Research, 1911, vol. I, p. 515. 



2 It is probable that potassium iodid should also be added in making up these tablets in order to prevent 

 some oxidation of arsenious oxid by iodin in acid solution in case "oxidized arsenic" is originally present 

 in amount sufficient to nearly completely use up the thiosulphate. 



