March 29, 2024, 02:28:09 AM
Forum Rules: Read This Before Posting


Topic: ANALYSIS OF THIOLS  (Read 13054 times)

0 Members and 1 Guest are viewing this topic.

G.S. UPPIN

  • Guest
ANALYSIS OF THIOLS
« on: March 18, 2005, 04:00:36 AM »
HOW IS THE MERCAPTAN EQUIVALENT OF A SULFUR COMPOUND DETERMINED BY USING IODINE?

  WT. OF SUBSTANCE*1000/ML. OF IODINE*NORMALITY

how is this calculation done?

Offline eugenedakin

  • Oilfield Consulting Chemist
  • Retired Staff
  • Full Member
  • *
  • Posts: 658
  • Mole Snacks: +88/-2
  • Gender: Male
  • My desk agrees with the law of entropy
    • Personal Website
Re:ANALYSIS OF THIOLS
« Reply #1 on: May 08, 2005, 12:58:21 AM »
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.
« Last Edit: May 08, 2005, 01:03:18 AM by eugenedakin »
There are 10 kinds of people in this world: Those who understand binary, and those that do not.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:ANALYSIS OF THIOLS
« Reply #2 on: May 08, 2005, 05:01:28 AM »
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)

Eugene, to post such an equation you have to be an engineer :)

Could you please show the chemistry behind the determination?

What iodine solution do you use - I thought they are not stable enough to be used as standard slutions so iodine have to be generated in situ (or its concentration should be checked soon before the determination).
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline eugenedakin

  • Oilfield Consulting Chemist
  • Retired Staff
  • Full Member
  • *
  • Posts: 658
  • Mole Snacks: +88/-2
  • Gender: Male
  • My desk agrees with the law of entropy
    • Personal Website
Re:ANALYSIS OF THIOLS
« Reply #3 on: May 09, 2005, 01:24:17 AM »
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
There are 10 kinds of people in this world: Those who understand binary, and those that do not.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:ANALYSIS OF THIOLS
« Reply #4 on: May 09, 2005, 06:07:05 AM »
OK, I found it:

H2S + I2 -> S(s) + 2I- + H2O

Excess I2 is titrated with thiosulfate as usual in iodometric methods.

Quote
Further digging with chlorine determination provided this balanced equation with an acid present (preferably acetic acid):

Cl2 + 2 KI --> I2 + 2 KCl

I would expect Cl2 to react with acetic acit to produce chloroacetic acid, but perhaps it is so slow process that it doesn't interfere with the determination.

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

I have seen your screenshots... And I have a question about flow calculations that you may be able to put some light on, but I will post it in General Chemistry Help Forum, as it has nothing to do with analytical chemistry and I will not litter the forum I am moderating :)
« Last Edit: May 09, 2005, 06:14:21 AM by Borek »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links