6 BULLETIN 76, U. S. DEPARTMENT OF AGRICULTURE. 



sulphuric acid until the solution is slightly acid to litmus paper or 

 methyl orange, or, best of all, phenolphthalein. When thoroughly 

 cooled to room temperature, make up the solution to exactly 1,000 

 c. c. Each cubic centimeter will then contain 0.0025 gram arse- 

 nious oxid. The chemist 1 will recognize this as approximately 

 a "twentieth-normal" solution. If well stoppered, the solution 

 will keep a year or more. 



(4) Standard iodin solution. — Weigh out about 6.5 grams of iodin 

 and 20 grams of potassium iodid. Cover with about 100 c. c. of water 

 and leave, occasionally mixing, until all iodin is dissolved. Then 

 make up to 1 liter. To obtain the exact strength of the iodin solution, 

 measure exactly 25 c. c. of the standard arsenious oxid solution into 

 a 200 c. c. flask or wide-mouth bottle, add about 25 c. c. of water, 

 about 2 grams of sodium bicarbonate, and a few cubic centimeters of 

 starch solution. Then, while shaking the flask, run in the solution 

 of iodin from a burette 2 until the blue color of iodized starch just 

 remains permanent. Twenty-five cubic centimeters of the standard 

 arsenious oxid solution contain exactly 0.0625 gram of arsenious oxid. 

 Therefore, the number 0.0625 divided by the number of cubic centi- 

 meters of iodin solution necessary to just produce a permanent blue 

 color, gives a quotient which represents the exact weight of arsenious 

 oxid to which each cubic centimeter of iodin solution is equivalent. 

 For example, suppose 26.2 c. c. of iodin to be required. Then 

 0.0625^-26.2 = 0.00239 gram arsenious oxid per each cubic centime- 

 ter of iodin. The standard iodin solution should be kept only in glass- 

 stoppered bottles, as it will be weakened by contact with rubber or 

 cork. If preserved in tightly stoppered and well-filled bottles in a 

 cool, dark place, it retains its strength a considerable time, but in 

 practice it should be standardized against the standard solution of 

 arsenious oxid every week or two. 



For the actual analysis two methods will be described. If the 

 sample is a concentrated preparation, it should always be reduced 

 by dilution with water to the strength at which it is intended for use 

 in actual dipping. 



METHOD "A" FOR ACTUAL ARSENIOUS OXID. 



Measure 25 c. c. of bath into a beaker, flask, or bottle of convenient 

 size, add about a gram of sodium bicarbonate and about 10 c. c. of 

 starch solution. Run in standard iodin solution until the blue color 

 appears just as in standardizing the iodin solution, though here it 



i The experienced chemist will undoubtedly employ solutions of definite normality. The directions 

 here given are intended, as simply as possible, to meet the needs of those who may be without much 

 chemical knowledge or a highly accurate analytical balance, perhaps lacking even burettes and pipettes. 



*A 25 c. c. measuring cylinder may be employed if a burette is lacking. 



