Application of the 875 Gas Analyzer to Measure Water Content in Gases and Liquefied Gases

The 875 KF Gas Analyzer is a tiamo™-controlled system for determination of the water content in gases and liquefied gases. The analyzer module comprises a pipe system that includes four software-controlled magnetic valves, which control the sample gas and the rinsing gas flow. The module also features a mass flow controller (MFC), an oven, a coulometer, a generator electrode with diaphragm, and the titration vessel with indicator.

Experimental Procedure

The 875 KF Gas Analyzer does not require any sample preparation. Prior to performing sample measurements, it is necessary to rinse the tubes to remove any traces of water by a nitrogen supply. After sample measurements, the remaining water in the tubes is flushed into the titration vessel using nitrogen.

Pre-drying the nitrogen before application is recommended as the capacity of the cartridge in the 875 KF Gas Analyzer is not large enough to absorb water. The oven prevents ice formation and condensation at the precision control valve. Since the MFC utilized for measuring the gas’ mass flow is calibrated for nitrogen, the measured value is accurate for nitrogen only. For that reason, it is necessary to perform a calibration for each new sample.

The sample has to be fed in its liquid form into the 875 KF Gas Analyzer for measurement of water content in the liquefied gas. Hence, the sample container needs to be positioned in a manner to allow the sample to flow into the tube system.

However, the position of the sample container does not affect the result in case of the analysis of a permanent gas. The container is always coupled with the steel capillary to the 875 KF Gas Analyzer for sample measurements. Figure 1 shows the sequence of the predefined method for sample measurement.

Figure 1. Method sequence of a water determination in gas (l or g).

The least amount of sample to be fed is defined in the sample table. As soon as the feeding of the sample begins, the MFC starts measuring the gas volume. After the addition of the minimal amount of sample, the sample inlet valve is closed. The gas is still flowing into the titration vessel because the remaining sample evaporates. When the gas flow is 0mL/min, the total volume of the sample passed through the MFC is recorded.

The nitrogen is used to flush the residual water in the tubes into the titration vessel. Figures 2 and 3 depict a typical titration curve, where there are increasing drift points to the presence of water in the tubes. Flushing the water with nitrogen ensures that the complete water present in the sample is measured.

Figure 2. Mass flow and drift curve; oven temperature 50°C; PCV half turn open; minimal amount of sample 4000mg.

From the titration curve, a decrease in the drift values can be observed especially during the decrease of gas flow due to the closing of the sample inlet valve. The titration may stop at this point without an extraction time. Hence, a lengthy extraction time is crucial to obtain the correct values. The extraction time is automatically estimated at the start of each sample measurement.

Figure 3. Mass flow and drift curve; oven temperature 50°C; PCV open five graduation lines; minimal amount of sample 4000mg.

Around 78 hours were taken for the long-term test of the 875 KF Gas Analyzer that involved 360 water determinations. Although the anolyte and the catholyte were not exchanged completely, their volumes were kept constant. Water accumulation in the catholyte is avoided by drying the catholyte compartment with the addition of 2mL of Titrant 5 every day.

Experimental Results

The 875 KF Gas Analyzer showed excellent reproducibility of the results with the mean value of the 360 determinations of 26.5pg/g and a standard deviation (SD) of 0.8pg/g (rel. SD: 3.2%) (Figure 4). The measurements were performed at 50-80°C to assess the influence of the oven temperature. Additionally, the PCV was opened fully after 42 measurements at 50°C to check whether its position has an influence on the measurements.

Figure 4. Measured water contents during long-term test.

The gas flow through the MFC is maintained at 5L/min as higher flow rates can direct the liquid to flow into the MFC and destroy it. To prevent this damage, the sample inlet valve is automatically closed by the predefined methods when the gas flow is greater than a defined limit.

The two MFCs assessed the stability of the mass flow measurement (Figures 5 and 6) with nitrogen as sample. There is no considerable difference observed in the two slopes of the characteristic curves (98.6% before and 98.3% after the long-term test).

Figure 5. Reference flow versus measured flow – before long-term test

Figure 6. Reference flow versus measured flow – after long-term test.

The drift relies on the anolyte level in the titration vessel, i.e., the lower the level compared to catholyte level, the higher will be the outflow of catholyte (Figure 7). During conditioning, the catholyte is not dried, resulting in the accumulation of the decomposition products of the reagent and water.

There is a considerable decrease in the drift following the addition of the iodine-containing methanol (titrant 5). Nevertheless, there is no relation between the start drift and the measured water content (Figure 8).

Figure 7. Drift progression; the fluctuation correlates with the level of the anolyte, which leads to a different flow of catholyte into the titration vessel.

Figure 8. Start drift values and measured water contents after exchanging reagents (first 30 measurements).

The oven temperature does not have any influence on the sample fed into the titration vessel. Conversely, the variance of the amount of sample introduced into the vessel is much higher when the PCV is completely open. Hence, the sample inlet valve is opened and closed to control the gas flow (Figure 9).

Figure 9. Introduced amount of sample at different temperatures and PCV positions. The sample inlet valve was closed after addition of 4000mg of sample.

Conclusion

The gas flow is never 0mL as the tube is not completely closed by the PCV. It is necessary to treat PCV carefully as it can be damaged easily. The complete system can be checked by a gas with certified water content. Additional information, particularly on the calibration method, can be found in the manual of the 875 KF Gas Analyzer.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.