ASSAY OF ARSENICAL DIPPING FLUIDS. 7 



usually fades out in a minute. In the case of very dirty baths the 

 color may appear simply as a violet, reddish, or brownish deepening 

 of the naturally dark color of the bath itself. In such cases it is 

 well to have two flasks at hand, both containing the measured por- 

 tions of bath and starch solution. Then by running iodin solution 

 into one of the flasks and constantly comparing the color with the 

 color of the liquid in the other flask, the point at which the change 

 occurs may be more easily distinguished. Since the color is not at 

 all permanent in the case of old and dirty baths, and since slight 

 changes of tint are impossible to distinguish in such baths, it is good 

 practice to add the iodin solution in quantities of 0.5 c. c. at a time 

 when it is suspected that the end point is close at hand, then to mix 

 thoroughly and immediately observe the color. This quantity of 

 iodin solution is usually sufficient to produce a very pronounced 

 change of tint if the end point of the titration really has been reached. 

 The final reading should then be corrected by subtracting 0.25 or 

 0.50 c. c, whichever is judged nearest correct. The number of cubic 

 centimeters of iodin solution needed to produce the blue color, 

 multiplied by the strength of each cubic centimeter in terms of 

 arsenic, and this again by 4, will give the grams of arsenic per 100 c. c. 

 of bath — that is, the percentage. For instance, suppose that each 

 cubic centimeter of iodin solution was found to be equivalent to 

 0.00239 gram arsenious oxid, and that 18.3 c. c. of iodin solution were 

 employed in the titration of 25 c. c. of a bath under examination. 

 Then the bath contains 0.00239X18.3X4 = 0.175 per cent of actual 

 arsenious oxid. 



Theoretically, method "A" can not be applied with perfect accu- 

 racy to dipping baths, on account of the possibility that substances 

 other than arsenious oxid — such as tar and organic matter derived 

 from the cattle — may absorb some iodin and thus lead to false results. 

 Practically, however, this method has been shown by many tests to 

 give useful results on dipping baths of all ages prepared after the 

 "standard formula" recommended by the Bureau of Animal Indus- 

 try. (See p. 1.) That is, the various ingredients of the tar and the 

 organic matter derived from cattle actually do not interfere fatally, 

 but absorb iodin so slowly that the end point with starch is obtainable 

 without difficulty, although it usually fades out in a brief time. 

 Therefore, method "A" is suggested for use in the practical testing 

 for ordinary purposes of baths prepared after the "standard formula." 

 It must not, however, be employed on baths prepared from any 

 proprietary dip unless it is certainly known that the particular dip 

 contains no substance which can interfere. Since it is the present 

 policy of the bureau to permit for use in official dipping only such 

 proprietary preparations as may be satisfactorily tested by means of 

 the field outfit later to be described, and since the field test is practi- 



