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Introduction
The Karl Fischer technique has become the standard method for quantifying water in chemical substances. When samples undergo secondary reactions with the components of the KF reagents, false results are generated. Reagent manufacturers have offered solutions to deal with some of these interferences or secondary reactions. These solutions include K-reagents for aldehydes and ketones, buffer solutions to offset pH problems etc. However, there are samples that just cannot be introduced directly into the KF titration vessel because of extensive side reactions or because they release water very slowly. One alternative for analyzing such materials is heating them in an oven and flushing the released water vapor into the KF titration vessel with dry air or nitrogen. Critical parameters are temperature and gas flow. Temperature should be optimized to release water quickly without decomposing the sample.
The carrier gas must flush the water vapor from the system quickly, but there must be sufficient residence time in the KF vessel solution to efficiently extract and titrate the water.
Brinkmann/Metrohm® offers a single-stage oven, the 832 KF Thermoprep, for this application. The 832 KF Thermoprep offers the additional advantage of containing hazardous samples in the vial, thus reducing the level of exposure for operators.
The 832 KF Thermoprep can be used with volumetric or coulometric KF titrators, depending on the water content of the sample. It is supplied with head-space sample vials and a crimping tool as well as built-in controls for temperature and air flow. In standard operation, moisture content of three blank vials is determined and the average value subtracted from each sample determination. This requires two methods in the KF titrator, one for blank and one for standard.
Instrumentation and reagent
- KF titration system (coulometric or volumetric)
- 832 KF Thermoprep
- KF oven standard (e.g., Hydranal® oven standard 5.55% H2O)
- Analytical balance
Performance check
Procedure for blanks
- Recall the blank method in the KF titrator.
- Prepare the KF vessel and press “Start.”
- Wait until the conditioning light is stable and the display shows “Ready.”
- Gently swirl the solution around the vessel walls and wait for the “Ready” display. Repeat the swirling action until the cell shows a low, stable drift.
- Set the temperature on the 832 KF Thermoprep to 220°C.
- Place an empty, sealed (conditioning) vial in the oven, lower the needle assembly so that the inner and outer needles are both inserted below the septum of the vial cap.
- Turn on the air pump (or N2 control valve) and adjust gas flow to ~70 ml/min. As air/N2 flows through the titration vessel, conditioning will be reactivated. Wait for the temperature and the cell to stabilize.
- Select three empty vials at random and secure caps with the crimping tool.
- Press “Start” on the titrator, raise the needle assembly, replace the conditioning vial with a blank vial and then lower the needle assembly as described above in No. 6.
- Any moisture in the vial will be titrated and the result displayed (and/or printed).
- After each blank is run, place the conditioning vial in the oven and wait until the system stabilizes.
- The average of the three blanks will be stored and automatically subtracted from each sample run.
Procedure for standard
- Recall the standard method in the titrator.
- Place a conditioning vial in the oven, lower the needle assembly and press “Start” on the titrator.
- Wait for the system to stabilize.
- Weigh accurately about 50 mg–100 mg of standard into a clean sample vial, secure the cap with the crimping tool.
- Press “Start” on the titrator, raise the needle assembly, replace the conditioning vial with the sample vial and then lower the needle assembly as described above in Procedure for blanks No. 6.
- Type in the sample weight and press “Enter.”
- The water released will be titrated and the result displayed (and/or printed).
- After each sample is run, place the conditioning vial in the oven and wait until the system stabilizes.
- Acceptable values on the Hydranal oven standard are 5.55 ± 0.2%.
Troubleshooting
If the value for the standard is out of specifications:
- First verify that the KF titrator (without the oven) works properly by running liquid standards directly in the vessel
- Ensure that the caps on the sample vials are securely crimped; you should not be able to turn the cap by hand
- Verify the oven temperature; rerun the sample at a slightly higher temperature (225°C)
- Check airflow, change inlet filter if necessary
- Verify the accuracy and stability of the balance
- Make sure that the standard is properly stored, and it is not contaminated. If in doubt, try a new bottle
Automated 774 Oven System
For labs that have high sample throughput, Brinkmann/Metrohm offers the 774 Oven Sample Processor that can handle 33 samples and three blanks per rack. It operates under the same basic principle as the 832 KF Thermoprep with options for automatic reagent exchange. Optional Tiamo™ titration software allows complete system control and data management from a connected computer.
