8 BULLETIN 76, U. S. DEPARTMENT OF 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 official 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 Meselguhr (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 
are 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 
absorb 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 
appears wholly unaffected by the presence of phenols. 
2 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. 
