154 SOIL BACTERIOLOGY 



(12) Hydrogen Sulphid: 



1. Prepare the following reagents : N/ioo iodin and N/ioo 

 sodium thiosulphate. Standardize the iodin against the 

 thiosulphate solution. One c.c. of the N/ioo iodin solution 

 is equivalent to 0.17 milligram of hydrogen sulphid, using 

 starch as an indicator. 



2. Add to a large glass-stoppered bottle 10 c.c, of the 

 iodin solution and 2 grams of potassium iodid. 



H 2 S + 2! = 2 HI + S 



3. Take 1000 c.c. of the sample to be tested, pour it into 

 the large bottle containing the iodin, shake thoroughly, and 

 allow to stand for some tune. 



4. Titrate the excess iodin with N/ioo thiosulphate. 



5. In the presence of large amounts of hydrogen sulphid 

 use tenth-normal iodin instead of one-hundredth normal. 



Treadwell, Analytical Chemistry, vol. ii, p. 687, 1912. 



