Water determination by Karl Fischer Titration (Monograph)
82Pages

Water Determination by Karl Fischer Titration Peter Bruttel Regina Schlink

Water Determination by Karl Fischer Titration Peter Bruttel Regina Schlink All rights reserved, including translation rights. Printed in Switzerland by Metrohm Ltd., CH-9101 Herisau, Switzerland 8.026.5013 – 2006-02 Water Determination by Karl Fischer Titration

1. Introduction. .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . .. . . . . . .. . . . . . .. . .. . .. . . . . .. . . . . . .. . . . . . . . . . 4 2. Reagents . .. .. . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . .. . . . . .. . . . .. . . . . . .. . . . . . .. . . . . . . .. . . . . . .. . . . . . .. . . . . . . . 4 3. Chemical reactions .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . .. . . . . .. . . . . . .. . . . . ....

1. Introduction In addition to the determination of the pH, weighing and acid-base titration, the determination of the water content is one of the most frequently used methods in laboratories around the world. In addition to methods requiring complex apparatus, such as infrared reflection spectrometry, gas chromatography or microwave spectroscopy, two methods in particular have been able to establish themselves: A – Drying methods (drying ovens, infrared lamps and infrared balances). This method is commonly found in various standards, but suffers from the following disadvantages: – In...

Original formulation according to Karl Fischer 254 g iodine is dissolved in 5 liters of anhydrous methanol. This is treated with 790 g pyridine, mixed thoroughly and then 192 g liquid SO2 is added. 1 mL KFR corresponds to approx. 3 mg H2O The titer of this combined reagent was not very stable – titer determinations had to be carried out virtually every day. Help was provided by the use of separate reagents. The titration solution consisted of 30 g iodine in 1 liter methanol. For the reaction solution 60 g SO2 was introduced into a mixture of 300 mL each of methanol and pyridine. The titer...

- Buffer substances or buffer solutions for strongly alkaline or strongly acidic samples. - Certified water standards from 0.1...10 mg/mL H2O with a density of 1 g/mL. This wide range of possibilities allows the user to determine the water content in practically any matrix. It is certainly no longer worth making up the reagents yourself! 3. Chemical reactions Karl Fischer presented the following equation for water determination with his reagent: 2 H2O + SO2 x (C5H5N)2 + I2 + 2 C5H5N ^ (C5H5N)2 x H2SO4 + 2 C5H5N x HI This gave a molar ratio of H2O : I2 : SO2 :...

rate KF reagents are used then the water capacity of the solvent must be taken into consideration, particularly for subsequent titrations. (The water capacity of these solvents is normally about 5...7 mg/mL, for a solvent volume of 25 mL this means max. 125...175 mg H2O.) Influence of the organic solvent Theoretically non-alcoholic KF reagents can also be used, but in this case the stoichiometry also changes. In protic solvents, e.g. methanol or ethanol, the ratio H2O : I2 = 1 : 1, in aprotic solvents, e.g. DMF, the ratio H2O : I2 = 2 : 1. Intermediate values (uneven ratios) have been...

4. Volumetry and coulometry The following are important for both methods: – the KF reagent must be added as precisely, accurately, reproducibly and with as high a resolution as possible, and – the titration cell must be as impervious as possible. It should be penetrated by as little atmospheric water as possible. – The water adhering to the walls of the titration cell (inner water film) must be removed by swirling about the conditioned contents of the titration vessel. Volumetry It is only since the introduction of piston burets by Metrohm in the fifties of the previous century (sounds...

Example for iodine: 2 I – – 2 e – → I2 z = 2, M = 253.8 (I2) 126.9 g iodine is released by 96’485 A in 1 s – or 1.315 mg iodine is generated by 100 mA in 10 s. Requirements for coulometric titration are: – The process must take place with 100% current efficiency. – No side reactions must occur. – Oxidation or reduction must lead to a defined oxidation stage. Metrohm KF Coulometers meet these requirements with modern coulometric reagents. They work according to the galvanostatic coulometry principle, i.e. with a constant current. The same chemical processes take place as in a volumetric KF...

Further disadvantages of these methods are that they can hardly be automated or validated. They fell out of use rapidly as electrometric indication methods became available (toward the end of the nineteen forties). 5.2 Electrometric indication methods In the meantime indication methods using two polarized (polarizable) electrodes have established themselves. Double Pt-wire or double Pt-ring electrodes are commonly used. 5.2.1 Biamperometric indication (Upol or dead stop) In this case a constant voltage of max. 500 mV is applied to the electrodes and the resulting current is measured. This...

5.2.2 Bivoltametric indication (Ipol) In this case a small direct or alternating current is applied between the electrodes (AC is much more sensitive than DC, which is why AC is usually only used for coulometric titrations) and the resulting voltage is measured. The shape of the titration curve is similar to that obtained by biamperometry; however, the jumps are usually more marked and larger. If the sample solution and titrant are electrochemically active then peak-shaped or Vshaped titration curves are obtained. If only one partner is electrochemically active (the KFR with excess iodine)...

6. Instruments 6.1 Volumetric Karl Fischer titration Metrohm offers a large range of instruments for volumetric KF titration, starting with simple instruments for routine water determinations up to high-end instruments with numerous advantages. What all instruments have in common is that they are easy to operate and produce rapid, precise and reproducible results. The Karl Fischer Titrinos and the Titrando with its Karl Fischer mode are automated titration systems that carry out the titration automatically, i.e. the titration rate and volume increments are controlled according to the signal...