376 



STATE BOARD OF AGRICULTURE. 



amounts of sulfide and thiosulfate sulfur, comparing it carefully with 

 the results obtained using the ammoniacal zinc chloride method. The 

 two methods were found to check nicely. With a little practice the 

 end point can be determined more accurately with the iodine solution 

 than with the ammoniacal zinc chloride. The white milk of sulfur 

 separating out forms a good background for determining the point at 

 which the yellow color disappears. This method avoids the labori- 

 ous process of filtering test portions to determine the end point by 

 means of an external indicator. Tarter^ and Bradley endeavored to use 

 the disappearance of color as an end point reaction with ammoniacal 

 zinc chloride as the titrating agent but because of the yellow color of the 

 zinc polysulfide, found it to be very unsatisfactoi'y. 



The following table shows a number of comparative analyses by the 

 two methods, each analysis being given in duplicate. In each of these 

 tests a 10 cc. sample of the diluted solution was used. 



TABLE II. 



Sample No. 



1. (a), 

 (b). 



2. (a), 

 (b) 



3. (a). 

 (b) 



4. (a), 

 (b). 



5. (a), 

 (b). 



By ammoniacal zinc chloride. 



c. used. 



21.75 

 21.35 

 13.30 

 13.15 

 23.40 

 23.70 

 21.55 

 21.70 

 31.60 

 31.80 



Per cent 



monosulfide 



sulfur. 



3.48 

 3.42 

 2.13 

 2.10 

 3.74 

 3.79 

 3.45 

 3.47 

 5 06 

 5.09 



Bv N-10 iodine. 



. used. 



21.30 

 21.35 

 13.10 

 13.20 

 23.50 

 23.60 

 21.45 

 21.55 

 31.60 

 31.60 



Per cent 

 monosulfide 



sulfur. 



3.41 

 3.42 

 2.10 

 2.11 

 3.76 

 3.78 

 3.43 

 3.45 

 5.06 

 5.06 



From the above table it will be seen that the general results by the 

 two methods are in quite close agreement but the duplicates obtained 

 using the iodine give better checks than those in which the decinormal 

 zinc chloride was used. Where the sample is to be used for the thio- 

 sulfate sulfur determination, it is better to take 25 cc. of the diluted 

 sample instead of 10 cc. 



Thiosulfate sulfur. — The thiosulfate sulfur may be determined by us- 

 ing the same sample that has been used for determining the mono- 

 sulfide sulfur. Therefore, after the end-i>oint for the monosulfide has 

 been reached we continue to add N/10 iodine until the characteristic 

 blue color is obtained, starch solution being added as an indicator. 

 This additional amount of iodine determines the amount of thiosulfate 

 sulfur. In case this same sample is to be used for the determination 

 of the sulfate and sulfite sulfur, it is better to leave out the starch solu- 

 tion and determine the end point by means of the colorizing effect of 



(1) Journal of Ind. and Eng. Chemblry, Vel. 2, p. 271. 



