ASSAY OF ARSENICAL DIPPING FLUIDS. 13 



one-hundredth of 1 percent of arsenious oxid in the bath under test. 

 Therefore its strength should be originally fixed by standardizing it 

 with the field apparatus against an average sample of used bath from 

 the field in which the percentage (adjusted if necessary by the addi- 

 tion of a little concentrated solution of arsenious oxid in sodium car- 

 bonate) of actual arsenious oxid is close to 0.20 per cent and is accu- 

 rately known through laboratory analysis. The true strength of this 

 empirically standardized iodin solution should then be ascertained by 

 titration against a strictly tenth-normal or twentieth-normal solution 

 of arsenious oxid, and the result obtained may thereafter serve as the 

 basis for the preparation of subsequent lots of test fluid. Obviously 

 the true strength of the iodin solution will be influenced to some extent 

 by the method of graduation of the cylinder, whether graduated "to 

 contain" or "to deliver," and also by the depth of the meniscus, 

 which in turn is influenced by the diameter of the cylinder. Hence, 

 all field outfits under the supervision of a single laboratory should be 

 fitted with cylinders of uniform model. Reserve supplies of test fluid 

 should be kept in small, well-filled, tightly closed glass-stoppered 

 bottles, and in a cool, dark place. 



In addition to the test fluid, starch and sodium bicarbonate, or 

 some equivalent substance, are of course necessary. In fact, the 

 practical preparation of a satisfactory form of starch has been 

 the greatest difficulty attached to the whole process, though at the 

 same time the key to its success. 



It has been known for many years that by the use of alcohol starch 

 may be precipitated in water-soluble form, also that high-percentage, 

 yet mobile, starch solutions may be obtained through proper treat- 

 ment with hydrochloric acid, but the working out of a practical 

 process for the preparation in quantity of a dry starch readily soluble 

 in cold water and appropriate for use as an indicator appears not to 

 be recorded. 



Into a 5-liter round flask with a long neck is brought 400 grams potato 

 starch, 2,300 c. c. distilled water, and, lastly, 80 c. c. of normal hydro- 

 chloric acid. The flask is well shaken to thoroughly wet and distrib- 

 ute the starch and is floated in a kettle of water previously brought 

 to vigorous boiling. The neck of the flask conveniently rests on the 

 side of the kettle at an angle of about 45°, and as soon as the 

 flask is brought into the bath it is gently but continuously rotated 

 about its longitudinal axis. As the flask becomes hot the starch 

 forms an evenly distributed, uniform jelly, which in about 7 minutes 

 from the time of starting begins to liquefy and to fall away from the 

 wall of the flask. When this stage is reached the mouth of the flask 

 is loosely closed with an inverted beaker and the flask left in the boil- 

 ing bath with an occasional rotation until the liquid becomes mobile 



