EXPERIMENT STATION BULLETINS. 



375 



or hydrogen peroxide method. The former is disagreeable to use and 

 much time is consumed in evaporating off the excess bromine. The 

 latter requires the use of hydrogen peroxide solution free from sulfates. 

 Since it is very difficult to secure such a solution in the market, it must 

 be prepared by allowing the solution to stand for some time in contact 

 with barium carbonate, or the amount of sulfur in the hydrogen per- 

 oxide must be determined and deducted from the final result. Also 

 when hydrogen peroxide is used some time must be spent in heating 

 the mixture before acidifying with hydrochloric acid and precipitating 

 with barium chloride. Since sodium peroxide free from sulfates is 

 easily obtainable, the above method avoids all the inconveniences in- 

 cident to the use of bromine water and hydrogen peroxide. 



Comparative results for total sulfur using sodium peroxide, hydrogen 

 peroxide and bromine as oxidizing agents: 



TABLE I. 



Monosulfide sulfur. — The monosulfide sulfur was determined in this 

 laboratory at first by the ammoniacal zinc chloride method, the only 

 modification being the method of drawing test portions for determining 

 the end point. Thase test portions were filtered by means of the ap 

 paratus devised by Knorr* for work in sugar analysis. The apparatus 

 consists of a piece of glass tubing closed at one end by a piece of per- 

 forated platinum foil sealed into the glass. An asbestos pad is sucked 

 into this and the test portion drawn through the pad. By inverting the 

 tube a drop of the filtered solution may be added to the nickel sulfate 

 on the tile. Even with this rapid method of filtering, it Avas found to be 

 a long and tedious process to accurately determine the end point. There- 

 fore an attempt was made to find some other method for determining 

 the monosulfide sulfur. In titrating the solution of lime sulfur with 

 iodine for the purpose of determining the thiosulfate sulfur according to 

 the method described below, it was noticed that the yellow color of the 

 solution disappeared when the same number of cc. of N/10 iodine had 

 been used as were required of the decinormal zinc chloride solution to 

 completely remove the sulfide from solution. This indicates that the 

 reaction CaSx-|-l2=Cal24-^x goes to completion before the reaction be- 

 tw^een the iodine and calcium thiosulfate commences. This principle 

 was therefore made use of in determining the monosulfide sulfur and 

 the method was tried on a large number of samples containing varying 



♦Described in Principles and Practice of Agricultural Analysis. Wiley, Vol. Ill P. 130. 



