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Reagents for Karl Fischer Oven Titration Available Online:
The Use of Ovens:
Pre-titration Some examples of suitable carrier gases are air or nitrogen. Nitrogen is preferred when the substance being investigated is sensitive to oxidization at process temperatures of 100-300°C. Commercially available pumps can be used for air and are available already equipped with dryers. Nitrogen may be taken from a pressurized container. We recommend the use of molecular sieves to dry the carrier gas. Residual water content of less than 10 µg H2O/liter is required to eliminate potential problems in the subsequent water determination. Carrier gases containing high amounts of water should be pre-dried using silica gel. KF reagents are not intended as drying agents as the solvents evaporate in the drying tube and can cause side reactions through dissociation of the components at higher temperatures. The flow of gas is set between 100-500 mL/min. Smaller amounts of gas reduce the amount of moisture present in the carrier gas, and improve the absorption in the KF reagent. Larger amounts of gas are preferred when large quantities of moisture are being transferred. Adjustments of Oven The moisture driven off from the sample in the drying oven is transferred into the titration cell by the carrier gas. As the substances investigated often contain only trace amounts of moisture, it is preferred to combine the drying oven with a coulometer. A somewhat larger oven, with a sample capacity of approx. 10 g, should be used for volumetric titrations. The sample size depends on the water content of the sample and the type of equipment used for KF titration. We recommend 500-3000 µg water amounts for coulometric titration, and 1-10 mg of water for volumetric titration. Lesser amounts of water respectively increase the standard deviation of the determinations; larger water amounts can lead to condensation in the pipes. 
Titration The flow of gas is set to the desired rate and conducted through the KF cell. The system must be brought to a stable condition before adding the sample. The drift must remain constant for at least 10 minutes, as this drift value is used later for automatic correction of the subsequent water determination. The baseline conditions should remain constant through the conditioning of the system and for subsequent determinations. Once the drift has stabilized, the previously weighed sample is added. The oven has to be opened for this. The combustion boat is quickly placed in the cold zone of the tubular oven. The oven is then immediately closed. This critical step should be practiced and optimized with dummy runs. The titrator is set so that it does not automatically shut off prematurely; it should be programmed to run for at least 10 minutes. End of Titration We recommend a stable period of 20 seconds for the end point of a coulometric determination. The working conditions must be optimized for each product analyzed. In particular, the optimum oven temperature to remove the water must be evaluated. The temperature must be high enough to drive off the moisture in the sample within 10-15 minutes. The upper temperature is limited by the possibility of substance decomposition. We determine the optimum temperature for a substance empirically, by graphically evaluating the course of the titration over time. Water in the titration cell is not detected in the first minute (initiate phase). The curve then steepens as the water is released from the sample. The curve levels off after about 10-20 minutes as the last traces of moisture are driven off and the carrier gas purges the oven dry again. The end of the titration should be reached after another 3-10 minutes. |
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