31 Gooch and Perkins — Determination of Free Iodin 



e. 



The procedure was simple. The standard N/10 iodine solu- 

 tion was drawn from a burette into a 250 cm3 Erlenmeyer fiask 

 containing a weighed amount of finely divided silver. The 

 flask, properly trapped and attached to a mechanical shaker 

 adjusted to give the liquid a rapid rotary motion, was shaken 

 until the iodine color had vanished. The liquid, usually 50 cm3 in 

 volume, was diluted to about 100 cm3 and the residue of silver 

 and silver iodide, collected in a perforated crucible fitted with 

 asbestos felt, was washed, dried at 130° to 140°, and weighed. 

 The difference between the weight of silver taken and that of 

 the residue of silver and silver iodide should, according to the 

 theory of action, be the measure of the free iodine. The 

 accompanying cut shows the mechanical shaker and the adjust- 

 ment of apparatus used throughout the work. The flask at 

 one side, fitted with a bulb-trap held in place by an outer 

 rubber band, was used in the experiments of Tables I and II. 

 The flask mounted upon the shaker was used for the operations 

 carried out in hydrogen and recorded in Tables III and IV. 



In Table I are given the results of experiments made with 

 silver reduced in the wet way, by the action of zinc upon silver 

 chloride (A), silver nitrate (B), or silver iodide (C) ; and, in a 

 dry way, by the action of hydrogen upon silver sulphide (D), 

 or upon silver oxide (E). In the first set of experiments of each 

 sort the reduced silver was dried and used without special 

 previous treatment ; in the second set of each sort the reduced 

 silver was shaken with a solution of potassium iodide, washed, 

 and dried before being used to absorb the iodine. The object 

 of shaking the reduced silver with potassium iodide was to con- 

 vert to silver iodide any incompletely reduced silver chloride, 

 nitrate, or sulphide, and this treatment does reduce considerably 

 the very large error noted in all of the experiments with the 

 untreated silver ; but the similar, if less marked, effect upon 

 silver reduced from the iodide suggested that a part of the 

 unfavorable effects in the case of the untreated silver might 

 be due to action between potassium iodide and metallic com- 

 ponents of the zinc. Even in those experiments in which the 

 reduced silver was previously treated with potassium iodide 

 the errors are too large and too variable for a good analytical 

 process. 



