Karl Fischer titration,principle,apparatus, titration types,Endpoint detection

Karl Fischer titration
principle
apparatus
titration types
Endpoint detection
Hybrid titrations

1. Karl Fischer titration   Titration  method in analytical chemistry  that uses   coulometric or volumetric titration to determine trace amounts of  water in a sample .   Invented in 1935 by the German chemist  Karl Fischer .

2. The Karl Fischer Titration is based on an iodine / iodide reaction : The water reacts with iodine . The endpoint of the titration is reached when all the water is consumed  The process uses an organic base (B), sulphur dioxide, iodine and an alcohol. The original Karl Fischer method used pyridine as organic base and methanol as alcohol. Since then the reagents have been improved. Nowadays Karl Fischer reagents are available which are less toxic (no more pyridine but imidazole as organic base, ethanol instead of methanol) and which provide a faster reaction. During the titration, iodine is added to sample and the amount of iodine used to consume all the water contained in the sample is measured. karl fischer titration principle

3. K arl fischer titration apparatus

4. Coulometric titration Volumetric titration K arl fischer titration  Volumetric Karl Fischer Titration , a solution with an exactly known concentration of iodine is added to sample by means of an electric burette. The amount of iodine added to the sample is calculated from the volume of iodine solution used.

5. In the   Coulometric Karl Fischer Titration the iodine is electrolytically generated. The amount of idodine added to the sample is determined by measuring the current needed for the electrochemical generation of the iodine.  When reacting with water, the brown iodine is reduced to the colourless iodide. Hybrid Karl Fischer titration  It is basically a combination of both, the coulometric and the volumetric method: The iodine is electrolytically generated and –if the moisture content of the sample exceeds a certain level – a solution with an exactly know concentration of iodine is added at the same time.

6. SO 2  + CH 3 OH + B ↔ CH 3 SO 3- + HB+  CH 3 SO 3- + H2O + I 2  + 2B → CH 3 SO 4 -+ 2HB++ 2I - Karl Fischer titration Reaction Base(B):-Pyridine Imidazole Alcohol:-Methanol Ethanol

7. Coulometric Titration The main compartment of the titration cell contains the  anode  solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B),  SO 2  and I 2 . A typical alcohol that may be used is  ethanol or diethylene glycol monoethyl ether, and a common base is imidazole. The titration cell also consists of a smaller compartment with a   cathode  immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane . The Pt anode generates I 2  when current is provided through the electric circuit. The net reaction as shown below is oxidation of SO 2  by I 2 . One mole of I 2  is consumed for each mole of H 2 O. In other words, 2 moles of electrons are consumed per mole of water. B·I 2  + B·SO 2  + B + H 2 O → 2BH + I −  + BSO 3 BSO 3  + ROH → BH + ROSO 3 − The end point is detected most commonly by a  bipotentiometric  method . A second pair of Pt electrodes are immersed in the anode solution. The detector circuit maintains a constant current between the two detector electrodes during titration. Prior to the equivalence point, the solution contains I −  but little I 2 . At the equivalence point, excess I 2  appears and an abrupt voltage drop marks the end point. The amount of charge needed to generate I 2  and reach the end point can then be used to calculate the amount of water in the original sample.

8. Volumetric titration The volumetric titration is based on the same principles as the coulometric titration except that the anode solution above now is used as the titrant solution . The titrant consists of an alcohol (ROH), base (B), SO 2  and a known concentration of I 2 . Pyridine has been used as the base in this case. One mole of I 2  is consumed for each mole of H 2 O. The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above .

9. Endpoint detection When reacting with water, the brown iodine is reduced to the colourless iodide. At the endpoint of the titration when all the water is consumed the colour of the solution turns increasingly from yellow to brown. As there is no sharp colour change and the coloration differs in nonpolar solvents (such as DMF) and polar solvents (as e.g. methanol) , it is not easy to determine the endpoint of the titration visually. For this reason, the endpoint of the titration is usually determined electrometrically with a double platinum wire electrode . There are two ways to electrometrically detect the endpoint: Biamperometric indication Bivoltametric indication

10. Biamperometric indication A constant voltage of approximately 500 mV is applied to the wires of the electrode and the resulting current is measured. As long as there is water in the sample, no free iodine is present in the solution. When the endpoint of the titration has been reached, the following reactions occur at the wires of the electrode : Cathode: I 2  + 2e -  → 2I - Anode: 2I - - 2e -  → I 2 When the endpoint has been reached the current thus increases from almost nil to a few μA.

11. Bivoltametric indication A small current ( normally in the range of 1 … 50 μA ) is applied between the electrodes and the voltage required to maintain this current is measured. Normally alternating current is used (AC) as it yields a higher sensitivity of the electrode than direct current (DC). The voltage required to maintain the current is in the range of several 100 mV as long as an excess of water is present in the sample. When the endpoint of the titration is reached, free iodine is available in the solution and the voltage drops to 100 mV or less.  Normally, the endpoint potential level (switch off voltage) must be selected according to the type of solvent being used and/or the type of sample being titrated. The ideal switch off voltage depends on the type of sample and solvent used. With normal Karl Fischer Titrators it must be determined experimentally: If the switch off voltage is too low , too much iodine is added before the endpoint is detected, the water contents yielded are too high. If the switch off voltage is too high , the titration does not start automatically as no free iodine is required for this voltage to be achieved .

12. Which method should be used, coulometric or volumetric ? In general it can be said that the method has to be chosen depending on the water content of the samples to be measured: the coulometric method is suitable for samples with a low water content (10 μg … 100 mg) the volumetric method is suitable for samples with a higher water content (0.1 … 500 mg).

13. Which sample size should be used ? The suitable sample size depends on the desired degree of accuracy the method used (coulometric, volumetric or hybrid) the titer (water equivalent) or the Karl Fischer Reagent (for volumetric and hybrid titrations ) When working with a  Hybrid Titrator , the recommended sample sizes are the same as for coulometric titrations for low water contents (0.001 … 0.1 %)  the same as for volumetric titraitons for higher water contents (> 0.01%), as it is advisable to work with bigger sample sizes in order to avoid weighing errors.

14. Water content Volumetric Coulometric [%] [ppm] WE = 2 WE = 5   0.001 10 > 25 g not recommended 5 … 10 g 0.01 100 > 20 g not recommended 1 … 5 g  0.1 1000 2 … 9 g  5 … 22.5 g 100 mg … 1 g 1 10000 0.2 … 0.9 g  0.5 … 2.25 g 10 mg … 100 mg  5 50000 40 … 180 mg  100 … 450 mg < 50 mg 10 100000 20 … 90 mg 50 … 225 mg < 50 mg  50 500000        not recommended < 50 mg not recommended The table below gives a rough indication for the sample size to be used in volumetric and coulometric titrations based on its estimated water content.

15. Volumetric titrations To ensure maximum accuracy of the results the amount of titrant used should (ideally) be between 20% and 90% of the burette volume. All volumetric Karl Fischer Titrators from KEM are equipped with 10 mL burettes. The samples should thus ideally contain between 4 and 18 mg of H 2 O if the titer of the reagent is 2 mg/mL or between 10 and 45 mg of water it the titer of the reagent is 5 mg/m l . Coulometric titrations The amount of water contained in the samples should be small for two reasons: Many samples can be titrated without needing to replace the Karl Fischer reagent. Short analysis times. For these reasons, the sample should ideally contain between 100 and 1000 μg of H 2 O .  To ensure reliable results, the samples should contain at least 50 μg of H 2 O as always small traces of water can enter into the titration cell.

16. Hybrid titrations The sample size should be such that the samples contain between 50 μg and 18 mg of H 2 O if the titer of the reagent is 2 mg/L 50 μg and 45 mg of H 2 O if the titer of the reagent is 5 mg/L Reagents for hybrid titrations For hybrid titrations, three reagents are required: Anolyte (solvent in the titration cell) Catholyte (cathode compartment of the generator electrode ) Volumetric titration reagent

17. Reagents for volumetric titrations Single component reagents This type of reagent contains all reactants ( iodine, sulfur dioxide, imidazole and diethyleneglycol monoethyl ether ). Since methanol has been replaced by diethyleneglycol monoethyl ether, the titer of one component reagents became quite stable. The loss of titer is approximately 5% per year. This type of reagent is suitable for most volumetric Karl Fischer titrations, including the analysis of ketones Two component reagents The reactants are in two separate solutions . The solvent is normally a solution of sulphur dioxide and imidazole in methanol. The titrant is a solution of iodine dissolved in a suitable solvent. Faster titration speed. More accurate results for samples with low water contents. Exact titre with a high stability. Higher buffer capacity. Two component reagents contain methanol and are thus not suitable to analyse samples which contain aldehydes and ketones with short chain lengths. Aldehydes and Ketones react with the methanol contained in the Karl Fischer reagents: They form acetals and water which leads to erroneously high results. .

18. Special reagents for aldehydes and ketones During the titration of aldehydes, a side reaction, the so called bisulfite addition can occur. This side reaction consumes water and leads to erroneously low results. For the determination of water in aldhydes and in certain reactive ketones it is thus required to use this type of reagent . There are two types of coulometric Karl Fischer reagents: Anolytes and Catholytes . When working with a coulometer equipped with a generator electrode with diaphragm (1) , the Anolyte (3) is filled into the titration vessel whereas the Catholyte (2) is filled into the cathode compartment of the generator electrode . For coulometers with a diphragmless generator electrode, only one single reagent is required. It is important to make sure that only reagents intended for diaphragmless electrodes are used. Reagents for coulometric titrations

19. CALCULATION FORMULA %Water(w/w)= B.R. F. 100 Wt.100 B.R=KFR reagent vol. consumed in ml F= KFR factor in mg/ml Wt.=weight of sample in mg