Moisture content determination by karl fischer titration

easy way to understand about karl fischer titration in very simple way.

1. Moisture Content Determination By Karl-Fischer Titration Prepared by: Daman Pandey

2. History Introduced in 1935 by Karl-Fischer. Based on Bunson reaction b/w iodine and sulfur dioxidein an aqueous medium. I2+ H2O + SO2 H2SO4 + 2HI Fischer proposed the above reaction to determine the water content, which require a 2:1 molar ratio of water molecule to iodine.

3. Principle: The KF titration principle reax . ROH + SO2 + R’N 2[R’NH]I + [R’NH]SO4R The alcohol react with sulphur dioxide and base to forman intermediate alkylsulfite salt, which further oxidized by iodine to an alkylsulfate salt. In above rex . the water and iodine consume in a 1:1.once the all the water get consumed , the excess of iodine is detected volumetrically by the titrator indicator electrode i.e. end point of titration.

4. Type of K.F. titration: Two-main type of the reaction are. a)Volumetric KFT b) Coulometric KFT a) Volumetric KFT : used to determine water content in the range of 10 ppm to 100%, sample size ̴ 15 and ̴ 0.02g. It dispenses KF titrating reagent containing iodine into the cell using the burette. It detects the endpoint of the titration using the double platinum pin indicator electrode. Water is calculated on the basis of the volume of Karl Fischer reagent consumed.

5. Type of KFT reagent system use in volumetric KFT titration. One-component volumetric KF the titrating reagent contains all of the chemicals needed for the Karl Fischer Reaction, namely iodine, sulfur dioxide, and the base, dissolved in a suitable alcohol (methanol). Easy to handle. Less expensive.

6. 2) Two-component volumetric KF The titrating agent contains only iodine and methanol, while the Solvent containing the other Karl Fischer reaction components is used as the working medium in the titration cell. better long-term stability. faster titration times. costly, and have lower solvent capacity. Advantage of Volumetric KFT Simple to use and cheap. Use to determine high water content. Applicable for solid, liquid and paste samples.

7. b) Coulometric KFT Found in 1959 by Meyer and Boyd as alternative of original volumetric addition of KF reagent. Use to find water content in small amount 1ppm to 5%, sample size ̴ 10g or more and ̴ 0.05g . According to Faraday’s laws, the iodine is produced in proportion to the quantity of electricity. This means that the water content can be determined immediately from the coulombs required for electrolytic oxidation. 1mg of water = 10.71 Coulombs I2 is produced by electrolysis of I- ions and the quantity of electricity used in electrolysis is measured. It generates iodine at the anode of the titration cell, instead of dispensing KF reagent as in volumetric titration and detects the endpoint of the titration using the double platinum pin indicator electrode.

8. calculates the end result based on the total charge passed (Q), in Coulombs, using the onboard microprocessor. Sample must be introduced in liquid or gaseous form through a septum. Type of Coulometric KFT reagent. a). Fritted-cell, coulometric KF b). Fritless -cell coulometric KF a). Fritted-cell, coulometric KF : separates the anode from the cathode that form the electrolytic cell. purpose of the frit is to prevent the iodine generated at the anode from being reduced back to iodide at the cathode instead of reacting with water.

9. b). Fritless -cell, coulometric KF : an innovative cell design is used that through a combination of factors, but without a frit. It make impossible for iodine to reach the cathode and get reduced to iodide instead of reacting with water. Advantages of Fritless -cell :- Use only one reagent, Titration cell easy to clean. More stable background value.

10. Advantage of Coulometric KFT Primarily to determine water in small amount. Absolute method as no titrant is added. Highly accurate. Cheap and fast.

11. SAMPLE WATER CONTENT VOLUMETRIC SAMPLE SIZE COULOMETRIC SAMPLE SIZE 100% 0.02 to 0.05 g NOT RECOMMENDED 50% 0.05 to 0.25 g 0.01 g 10% (100,000 PPM) 0.25 to 0.50 g 0.01 to 0.05 g 5% (50,000 PPM) 0.50 to 2.50 g 0.05 to 0.10 g 1% (10,000 PPM) 2.50 to 5.00 g 0.10 to 0.50 g 0.5% (5,000 PPM) 5.00 to 7.50 g 0.20 to 1.00 g 0.1% (1,000 PPM) 7.50 to 10.0 g 1.00 to 2.00 g 0.01% (100 PPM) 10.0 to 15.0 g 2.00 to 5.00 g 0.001 (10 PPM) 15.0 to 20.0 g 5.00 to 10.0 g 0.0001% (1 PPM) NOT RECOMMENDED 10.0 g OR MORE

12. Karl Fischer reagent: Solution of sulphur dioxide and Iodine in a mixture of methanol and Pyridine. Role of karl Fischer Ingredients: Sulphur dioxide make alkylsulfite salt is then oxidized by iodine to an alkylsulfate salt. Pyridine or imidazole act as buffer. Methanol (Anhydrous) act as solvent.

13. Formula used in Calculations:- To determine water equivalancy factor, F: F= W/V Where, W is the weight of water in mg. V is the volume in mL of KF reagent consumed Water content determination in sample:- water content (%)= SF x 100 / mg. of test preparation. Where, S is the volume in mL of KF reagent consumed F is the water equivalancy factor

14. Water determination of Ketone and aldehyde group Aldehyde and Ketone both form acetals and ketals respectively in the presence of conventional reagents. The reactions create water, which is also titrated, resulting in vanishing endpoints and cause high water content calculations. With aldehydes a second side reaction, the bisulfite addition, can also occur. This consumes water and leads to false low moisture content measurements

15. The reactivity of aliphatic ketones decreases with increasing chain length whereas aromatic ketones are altogether less inclined to react in the presence of reagents. The formation of acetals and ketals can be suppressed by replacing methanol in the titrating agent with another solvent. A best solvent is one that assures Stoichiometry of KF reaction and dissolve both Reactant and product of the reaction. E.g. 2-methoxyethanol, ethanol, propanol , Chloroform, Formamide

16. Various factors affect The Karl Fischer titration :- the atmospheric humidity (drift determination) Close all openings in the titration stand, Condition the titration cell before use . Protect the titration cell with a desiccant the working medium proper cleaning of cell Rinse the burette 2 times into a waste bottle before you determine the concentration. the pH of the sample the side reactions between the sample and the Karl Fischer reagent ( rex . With ketone and aldehyde )

17. Effect of pH on KFT :- The rate of the reaction depends on the pH value of the solvent, or working medium. The optimum pH range of the sample solution for efficient Karl Fischer titration is between pH 5.5 and 8 When the pH is greater than 8.5, the reaction rate increases due to chemical side reactions This results in a more sluggish endpoint and higher iodine consumption, which will affect results. When the pH is less than 5,the reaction is slow. Highly acidic or basic samples need to be buffered to bring the overall pH into that optimal (5-8) range.

18. Advantages of Karl Fischer titration:- Extreme accuracy Easy to handle. Specificity for water determination because it does not detect the loss of any other volatile substances  Wide range of moisture determination. rapid method and requires minimal sample preparation suitable for water determination in solids, liquids and gases .

19. General Consideration:- Changing the molecular sieves every 6 weeks is appropriate in the presence of medium humidity. Easy to handle. Molecular sieve can be regenerated at 300 degree C for 24h. Clean the platinum pins on the indicator electrode with a soft paper tissue, afterwards , flush it with ethanol or methanol.   If drift vlaues is very high during pre-titration change the molecualr sives , check leakge , ensure no side reaction or pH change has occurred. The cell solution should be replaced after approximately 30 mL of titrant have been used in the case of cell solutions with 20... 25 mL of solvent

20. Water Determination as per USP Method I ( Titrimetric ) Determine the water by Method Ia , unless otherwise spec ified in the individual monograph. Method Ia (Direct Titration ) Principle -The titrimetric determination of water is based upon the quantitative reaction of water with an anhhydrous solution of sulfur dioxide and iodine in the presence of a buffer that reacts with hydrogen ions. For determination of trace amounts of water (less than 1%), it is preferable to use a Reagent with a water equivalency factor of not more than 2.0 .

21. Water Determination as per Ph. Eur. Method I Principle -The semi-micro determination of water is based upon the qunatitative reaction of water with sulfur dioxide and iodine in a suitable anh medium in the presence of a base with sufficient buffering capacity. Verification of accuracy Between two successive sample titrations, introduce an accurately weighed amount of water in the same order of magnitude as the amount of water in the sample. The recovery rate is within the range from 97.5 percent to 102.5 per cent for an addition of 1000 μg of water and in the range from 90.0 per cent to 110.0 per cent for the addition of 100 μg of water.

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