Hello G.S. Uppin,

Here is a common formula used for determining concentrations of mercaptans in the field.  The mercaptan equivalent can be determined by the following calculation:

mg S2-/L=[(A X B)-(C x D)] x 16000 / (mL of Sample)

A = mL of iodine solution
B = normality of iodine solution
C = mL Na2S2O3 solution
D = normality of Na2S2O3 solution

(One millilitre of 0.0250 N iodine solution reacts with 0.4 mg S2-.

Since each mercaptan molecule has one titratable sulphur molecule, the number of titratable sulphur and mercaptan molecules are determined since they are a 1:1 ratio.  From here, your final calculation should be simpler, since you would know the volume of solution titrated and the molecular weight of the type of mercaptan molecule (there are many different types of 'mercaptans').

Note:  If pure hydrogen sulphide gas is present, then this will interfere with your calculated results.

I hope this helps,

Eugene Dakin Ph.D., P.Chem.

Hello Borek,

Chuckle, yes, 1 of my 4 degrees is in Engineering   . I am quite sure that most engineers would not attempt to answer such a question unless they had a background in another area (such as Chemistry or Biology).

This iodometric titration test is so common, that I am sure almost every laboratory has a setup (or the ability) to perform this test.

Unfortunatly, I was unable to determine the chemistry behind the determination... A thorough search was performed in my organic (Solomons), inorganic (C & W), and other various forms of chemical information (CRC), and all to no avail.

The only other standardized information that I could retrieve was in the AWWA standardized test method 4500 (pg 4-127 of the 19th edition) in which the same mathematical formula was provided for Sulphide determination.  Further digging with chlorine determination provided this balanced equation with an acid present (preferably acetic acid):

Cl2 + 2 KI --> I2 + 2 KCl

Iodine solution has a typical short shelf-life.  Usually, I used a 0.0250 N iodine solution.  However, Fisher Scientific has the 0.0250 Normality strength that is stable for an extended period of time (several months).  Although you should restandardize solutions before use, this purchased iodine was quite stable.  I know that keeping the iodine solution away from light assists in its longevity, but I am sure that a 'secret' additive is placed in solution by Fisher Scientific.

A concern that I have about this test method is its practicality.  Samples of water removed from the test site are typically over-saturated with hydrogen sulphide.  Some of this hydrogen sulphide vents and lowers the titratable concentration of H2S.  The reductive tendencies of H2S upon exposure to oxygen when removing the sample also lowers available H2S.  The sample, and titration, would have to be performed in the field at the test site to have a reasonable calculated value.

By the way Borek, I like your website... keep up the great work!!!

Eugene