8 BULLETIN 76, U. S. DEPARTMENT OE AGRICULTURE. 



cally identical with method "A," the latter method may be safely 

 employed for the examination of samples known to be prepared 

 from proprietary dips which have received such oflicial recognition. 



METHOD "b" FOR ACTUAL ARSENIOUS OXID. 



Measure 25 c. c. of bath into a small beaker, add 5 c. c. of 10 per- 

 cent sulphuric acid and 0.25 gram of acid- washed blood-charcoal. 

 Stir thoroughly and heat nearly to boiling — best on a steam bath — for 

 five minutes. Then filter and wash with hot water until the filtrate 

 amounts to a little over 100 c. c. If the bath appears to be very 

 heavily loaded with dirt or tar, the addition of about a gram of acid- 

 washed kieselguhr (infusorial, siliceous, or diatomaceous earth), 

 followed by thorough stirring before filtration, will greatly hasten 

 the passage of the liquid through the filter. When the filtrate has 

 been thoroughly cooled to room temperature, sodium bicarbonate 

 is added until 1 or 2 grams are present in excess after effervescence 

 ceases. In case the bath was prepared from a proprietary product, 

 dilute to about 200 c. c. before adding sodium bicarbonate, and 

 lastly also add 2 grams potassium iodid. 1 Finally, the solution is 

 titrated with standard iodin and the result calculated as under 

 method " A." If the greatest possible accuracy is desired a correction 

 must be made for loss of arsenic through the use of blood-charcoal. 

 This substance notably adsorbs arsenious oxid, and even after 

 thorough washing a slight amount remains unaccounted for, which 

 presumably still is retained by the charcoal. The amount of arsenic 

 so lost appears — at least within practical limits — to be independent 

 of the concentration of the bath, but is naturally dependent upon 

 the amount of charcoal employed. Hence the charcoal must be rather 

 carefully weighed or measured, and the amount of the correction for 

 each lot of charcoal must be established by running comparative 

 titrations on two portions of a solution of pure arsenious oxid, one 

 direct, the other after the treatment with charcoal, filtration, and 

 washing, which has been described. 2 The power of the charcoal to 

 produce a water-white filtrate in which the blue end point is sharp 

 and permanent far outweighs its disadvantages if accurate results 

 arc desired. 



Method "B," like method "A," should not be applied to proprie- 

 tary dips unless they are permitted for use in official dipping or are 

 otherwise known to be free from interfering substances. 



1 Dilution and the addition of potassium iodid aids in nullifying the interfering effect of cresylic acid and 

 other substances capable of absorbing iodin which may be present in proprietary dips. Phenols rapidly 

 a bsorb iodin from a solution containing sodium bicarbonate. The presence of free carbonic acid, which of 

 course saturates the liquid during titration, greatly reduces the rate of absorption. Finally, if the solution 

 contains a certain concentration of potassium iodid, and not above a certain concentration of the phenols, 

 the end point comes out perfectly sharp, of good color and permanency, following a titration figure which 

 ars wholly unafTected by the presence of phenols. 



1 The sample of blood-charcoal at present in use in this laboratory demands a correction of 0.2 c. c. of 

 twentieth-normal iodin for 0.25 gram charcoal. 



