KF Coulometry

Volumetric / Coulometric Titration Volumetric Karl Fischer Iodine is added by burette during titration. Water as a major component: 100 ppm - 100 % Coulometric Karl Fischer Iodine is generated electrochemically during titration. Water in trace amounts: 1 ppm - 5 % + - 2018/11/9 AA

Titration Cell + – Generator electrode Diaphragm Anolyte Anode Cathode + – Generator electrode Diaphragm Sensor electrode double platinum pin electrode Anolyte Anolyte: sulfur dioxide, imidazol, iodide, different solvent for different applications: methonol or ethanol with chloroform, octanol, etylenglycol Catholyte: similar (or modified) solution Catholyte 2018/11/9 AA

2 I- I2 + 2 e- Coulometry H O + I + SO + RN + ROH Ù (RNH)SO Based on the same reaction as volumetric Karl Fischer Titration: H 2 O + I + SO + RN + ROH Ù (RNH)SO 4 R + 2(RNH)I - Iodine reacts with water 1:1 - The solvent methanol is involved in the reaction. - A suitable base keeps the pH 5 - 7 But iodine will be produced just in time from iodide: 2 I- I2 + 2 e- Anodic Oxidation 2018/11/9 AA

2 I- I2 + 2 e- H2 2 H+ + 2 e- Iodine Production + – I- I Anode Iodine production by oxidation Cathode Hydrogen production by reduction H2 2 H+ + 2 e- H+ - H Side reaction: Reduction of sulfur components. After 1 - 2 weeks, smells like mercaptans.  Change catholyte every week! 2018/11/9 AA

Coulometry Theory Definition One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s). 1 C = 1 A • 1 s To produce one mol of a chemical compound, using one electron, 96484 C are required. Charles Augustin de Coulomb 14.6.1736 - 23.8.1806 2018/11/9 AA

Absolute method, no standardization! Coulometry Theory Definition One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s). 1 C = 1 A • 1 s Two iodide ions react to iodine, which in turn reacts with water: 2 I–  I2 with H2O Therefore 1 mol water (18 g) is equivalent to 2 x 96484 C or 10.72 C / mg water. To produce one mol of a chemical compound, using one electron, 96484 C are required. Absolute method, no standardization! 2018/11/9 AA

Iodine Production Speed The iodine production speed depends on: + – I- - I H+ H surface of the electrode voltage at the generator electrode the conductivity of the electrolyte Influence to conductivity: Samples and additional solvent as chloroform, etc. Warning: Low conductivity Normal conductivity  high current = 400 mA  2100 µg H2O/min Very low conductivity  low current = 200 mA  1050 µg H2O/min 2018/11/9 AA

Resolution and Detection Limit + - Resolution: 0.1 µg water Detection limit: 5 µg water for 5 g sample  1 ppm Measuring range: 10 µg - 100 mg water/sample 1 ppm - 5 % water 2018/11/9 AA

Repeatability coulometry volumetry srel < 0.5 % srel < 0.5 % Not suitable for volumetry srel > 5 % srel 5 - 0.5 % srel < 0.5 % volumetry 1 ppm 10 ppm 100 ppm 1000 ppm 1 % 10 % 100 % Not suitable for coulometry srel < 0.5 % srel 5 - 0.5 % srel > 5 % 2018/11/9 AA

Filling Titration Cell Anode + – Cathode Catholyte: Fill in 5 mL catholyte. Anolyte: Fill in ~ 100 mL anolyte The level of the anolyte should be 3 - 5 mm higher than the level of catholyte so that the flow is from the anolyte compartment to catholyte compartment.  Low drift value Catholyte With stirring the level difference of anolyte and catholyte will be stable. Anolyte 2018/11/9 AA

Filling Titration Cell Anode + – Cathode Catholyte always contains water! If the catholyte level is higher or at the same level as the anolyte, there is a flow of moisture into the anolyte compartment. Catholyte  High drift value Anolyte 2018/11/9 AA

With or Without Diaphragm + – + – What are the differences? 2018/11/9 AA

+ – + – With or Without Diaphragm With Diaphragm Without Diaphragm I- It is possible that iodine can go to the cathode and convert to iodide. I- - I Iodine is only in the anode compartment and reacts with water. 2018/11/9 AA

Without Diaphragm It is possible that iodine can go to the cathode and convert to iodide. – + Prevention: Small cathode surface  less chance to contact iodine - H+ H high iodine production speed  hydrogen protects cathode high stirrer speed  iodine reacts faster with water I- - I Only a little less accurate for samples with very low water content. bigger sample  error has no effect 2018/11/9 AA

– + Without Diaphragm The hydrogen produced at the cathode is a very good reducing agent. – + R-NO2 R-NH2 + H2O Easily reducible samples (nitro compounds) get reduced, which produces water. H+ H - I- - I  wrong result (too high value) Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc. 2018/11/9 AA

Without Diaphragm Titration cell easier to clean. Long-term drift value more stable. Only one reagent. Automation of emptying and refilling electrolyte. A little bit less accuracy for very small water content (< 50 µg/sample) Not recommended: for easily reducible samples: nitro compounds, unsaturated fatty acids, etc. 2018/11/9 AA

With and Without Diaphragm Application With and Without Diaphragm With out diaphragm: a little bit less accuracy for very small water content (< 50 µg/sample) Examples: Transformer oil Mean n srel µg water /sample with or without diaphragm 16.3 ppm 6 1.5 % 34 - 40 with diaphragm 19.6 ppm 6 5.7 % 39 - 43 without diaphragm 2018/11/9 AA

– + Without Diaphragm Titration cell without diaphragm is ideal for: Hydrocarbons Halogenated hydrocarbons Alcohols Esters Ethers Acetamides Mineral oils Edible oils Ethereal oils + – For this applications the titration cell without diaphragm is recommended. 2018/11/9 AA

Different anolyte for different applications Analyte For a complete water determination the sample must be completely dissolved in the anolyte. Anolyte + - Sample not dissolved, emulsion:  Too low result Different anolyte for different applications 2018/11/9 AA

Analyte for samples easy to dissolve alcohols, ethers, esters, hydrocarbons, halogenated hydrocarbons, nitro components, etc. For cell with diaphragm  with methanol (HYDRANAL Coulomat AG) (apura - combiCoulomat frit)  with ethanol (HYDARANAL Coulomat E) Anolyte + - For cell without diaphragm  with methanol (HYDRANAL Coulomat AD) (apura - combiCoulomat fritless) 2018/11/9 AA

Analyte for samples not easy to dissolve edible oils, ethereal oils, ointments, etc.  with methanol and octanol For cell with diaphragm (HYDRANAL Coulomat AG-H) (with 20 % hexanol) (apura - combiCoulomat fritless) add up to 40 % octanol or decanol Anolyte + - For cell without diaphragm (HYDRANAL Coulomat AD) (apura - combiCoulomat fritless) add up to 20 % octanol or decanol 2018/11/9 AA

Conductivity influences Generation of iodine Indication of the endpoint Conductivity of electrolyte decreases during determination long chained alcohols (hexanol, octanol, decanol), xylene or chloroform can be added. without diaphragm addition of max. 20 % to CombiCoulomat fritless high current at generator electrode limit 5 - 6 mS/cm before current breaks down with diaphragm addition of max. 40 % to CombiCoulomat frit higher robustness 2018/11/9 AA

Analyte for samples difficult to dissolve mineral oils, transformer oil, silicon oils, etc  with methanol and chloroform For cell with diaphragm (HYDRANAL Coulomat A) (with 20 % chloroform) (HYDRANAL Coulomat AG) (without chloroform) (apura - combiCoulomat frit) (without chloroform) add chloroform (maximum 50 %) Anolyte + - For cell without diaphragm (HYDRANAL Coulomat AD) (apura - combiCoulomat fritless) add up to 30 % chloroform 2018/11/9 AA

Analyte for Ketons and Aldehydes ketones and aldehydes react with methanol ketal and acetal formation + 1 H2O Anolyte + -  special reagent for ketones For cell with diaphragm (HYDRANAL Coulomat AK and CG-K) with a long chain alcohol instead of methanol For cell without diaphragm (HYDRANAL Coulomat AK) Caution with aldehydes! Short chain aldehydes (for example acetaldehyde) will be oxidized at the anode. + 1 H2O Long chain aldehydes (for example benzaldehyde) are no problem! 2018/11/9 AA