336 F. A. Gooch and P. E. Browning — Determination 



is obvious that such errors as 0*003 to 0*006 grm., which 

 Messrs. Friedheim and Meyer found even in the absence of 

 bromides and chlorides, are not explicable by the action of the 

 salts which we used. Our errors ranged under like conditions 

 from 0*0009 grm. — to 0*0003 grm. + , with a mean error in nine 

 determinations of - 0002 grm. — . 



Everybody knows that the starch iodide test is most delicate 

 in acid solutions and in presence of combined iodine, but 

 Mohr's method of titrating arsenious oxide and iodine against 

 one another in alkaline solution is sufficiently delicate for very 

 exact work provided only that the alkali in excess is in the 

 form of the bicarbonate, that the starch emulsion is used in 

 abundance, and that the volumes of solutions titrated are reg- 

 ulated to low and uniform measure. In many determina- 

 tions of iodine made by our method at different times and with 

 different materials it has never been our ill-fortune to chance 

 upon results so extraordinary as those of Messrs. Friedheim 

 and Meyer, though we have met in the course of our work 

 with potassium arseniate so contaminated with nitrates as to be 

 unfit for use and with alkaline hydroxides too impure to em- 

 ploy. Most analytical processes depend for their exactness 

 upon the use of proper materials : ours is no exception to the 

 rule in this regard. 



As to the correctness of the main reaction there appears to 

 be no difference of opinion between Messrs. Friedheim and 

 ourselves. We haveii therefore, taken the pains, perhaps un- 

 necessarily, to make experiments in which the estimation of 

 the iodine of the same identical portions is effected both in the 

 distillate and in the residue, in order that the two modes of 

 estimation may be brought into direct comparison. It is 

 scarcely needful to add that we took care to work with pure 

 reagents. The potassium iodide, like that which we employed 

 in our former investigation, was prepared by acting with re- 

 sublimed iodine upon an excess of iron wire, pouring off the 

 solution from the iron when the color of iodine had vanished, 

 adding iodine equal to one-third the amount of that originally 

 used, pouring the filtered liquid into a boiling solution of the 

 calculated equivalent of potassium carbonate (from the bicar- 

 bonate), and filtering off the precipitated magnetic oxide of 

 iron. The slightly alkaline solution thus made, containing ap- 

 proximately 2 grm. of potassium iodide in 100 cm 3 , and free 

 from chlorine and bromine, was standardized by precipitating 

 the iodine from weighed portions in the form of silver iodide 

 and weighing upon asbestos. The other reagents — the sul- 

 phuric acid, the sodium hydroxide, the acid potassium carbon- 

 ate, the dihydrogen potassium arseniate — when present in the 

 proportions used in our process, and mixed with 5 cm 3 of clear 



