Karl Fischer titration sample analysis

The main objective of the lEA PYRA Round Robin was to determine the interlaboratory precision and methods applied for elemental composition, water, pyrolytic lignin and main cotiqjounds. Two poplar oils were analysed by the lEA PYRA participants. It was concluded that the precision of carbon and hydrogen was very good, liquid sample handling plays a very important role in the C, H analysis, water by Karl-Fischer titration was acceptable, but should be checked carefully, and the method for the determination of pyrolytic lignin should be improved (3). 

Water content is much easier to measure than water activity. Most often the sample is dried at 100-105 °C for 24 h or until constant weight is attained. Some other methods as vacuum-oven drying, moisture evolution analysis, Karl Fischer titration, gas chromatography, near infrared analysis, and nuclear magnetic resonance could be also apphed. Oven drying is the simplest and least expensive. 

Water analysis of liquids was by Karl Fischer titration. Liquids were analyzed by GC for qualitative or semi-quantitative characterization. Quantitative analysis of selected liquid components was performed by HPLC. The sample was prepared by diluting it with two parts water to cause phase separation of pyrolytic lignin. Simple sugars, anhydrosugars, and other lower molecular weight components such as acetic acid and hydroxyacetaldehyde remain in the aqueous phase. 

Karl Fischer titrations are frequently used for the analysis of water content in such samples as food materials, pharmaceuticals, and solvents used in industrial applications, or for the determination of hydration water in crystals. Dedicated units are commercially available and many automatic titra-tors or pH meters feature a connector for current biased voltage measurements labeled Karl-Fischer or simply KF . 

On a heat transfer option for moisture analysis by KF, the sample is heated at a temperature greater than 100°C in an attached oven for a specified amount of time under a flow of dry nitrogen. The water evolved is carried by the flowing gas and concentrated by bubbling into the Karl Fischer titration vessel containing methanol. Karl Fischer reagent is used, and water reacts with the titrant as indicated above. 

Near-infrared spectra were collected on powdered samples, obtained by cmshing five tablets and underdosing or overdosing the samples as described. A Technicon InlraAlyzer 450 filter-based instrument was used for the analysis. Tablet drug content was determined by HPLC and water content by Karl Fischer titration. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

The approved variations [14] in the Karl Fischer method include volumetric titration methods to either a visual (excess iodine or addition of an indicator) or volta-metric endpoint detection method. The visual or voltametric endpoint methods usually require 30-40 mg of sample for analysis for freeze-dried biological products containing from 1.0% to 3.0% residual moisture. Coulometric Karl Fischer instruments generate the iodine from potassium iodide for water titration at the electrodes. Only 10-20 mg of freeze-dried sample is required for accurate analysis. 

Coulometry is, in addition to gravimetry, a primary standard analysis technique. Coulometry is used for the determination of the thickness of metallic coatings by measuring the quantity of electricity needed to dissolve the coating. Coulo-metric titration is applied to determine the concentration of water on the order of milligrams per liter in different samples when Karl Fischer reaction is used. 

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