32 Gooch and Peirce — Iodometric Determination of 



iodine necessary to reoxidize the arsenious acid produced, the 

 data were at hand for calculating the amount of chlorate 

 present in the mixture. It was our hope (which proved to be 

 well-founded, as the sequel shows) that selenious acid would 

 behave like a chlorate under similar conditions. 



Pure selenium dioxide was prepared by oxidizing presuma- 

 bly pure selenium in strong nitric acid, evaporating the solu- 

 tion to dryness, dissolving the residue in water, 'treating the 

 solution with barium hydroxide until precipitation ceased, 

 filtering, evaporating the filtrate to dryness, subliming the 

 selenium dioxide from the residue, and resubliming that 

 product in a current of dry oxygen (which we found to be 

 vastly more convenient and effective than dry air) until it was 

 perfectly white and crystalline. From the oxide thus made a 

 standard aqueous solution was prepared, from which portions 

 were measured and (for the sake of greater accuracy) weighed 

 for use in the experiments to be detailed. To each weighed 

 portion of the selenious acid, contained in an Erlenmeyer 

 flask of 300 cm 3 capacity, was added a weighed amount of 

 potassium iodide (somewhat in excess of that theoretically 

 required) prepared in solution of convenient strength and 

 tested as to its reducing power upon arsenic acid under the 

 conditions of the experiments; a solution containing about 

 2 grra. of pure di-hydrogen potassium arseniate was introduced ; 

 and, finally, 20 cm 3 of sulphuric acid of half-strength. Pro- 

 tected from ordinary mechanical loss by a trap (consisting of a 

 two-bulbed drying tube cut short and hung loosely with the wide 

 end downward in the mouth of the flask) and from violent 

 ebullition by the introduction of a few bits of porcelain, the 

 liquid was boiled until the volume decreased according to indi- 

 cating marks on the flask from 100 cm 8 or more to 35 cm 3 — 

 concentration to about this lower limit having been found to 

 be necessary to the completion of the reaction. The residue 

 was cooled, the acid was nearly neutralized with potassium 

 hydroxide, acid potassium carbonate was added until it was 

 present to the amount of 20 cm 3 of its saturated solution in 

 excess of the quantity needed to complete neutralization, and, 

 after the addition of starch, standard iodine was introduced 

 until the starch-blue appeared. The iodine introduced measured 

 the arsenious acid (and so the quantity of iodine set free by the 

 arsenic acid), and the difference between it and the iodine 

 originally present in the form of the iodide represents the 

 amount set free by the selenious acid. 



The following table comprises the details and results of a 

 series of determinations made in the manner outlined : 



