Automatic Karl Fischer Water Determination with the 774 Oven Sample Processor

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Based on Metrohm^® Application Bulletin 280/1 e

Introduction

In principle, the 774 Oven Sample Processor can be used with all samples that release water when heat is applied. However, the KF oven method is essential whenever direct volumetric or coulometric Karl Fischer titration is impossible because the sample contains interfering components or, owing to its consistency, is difficult to place in the titration vessel.

The combination of the 774 Oven Sample Processor with a Metrohm coulometer is ideal for samples with a low water content. Food, pharmaceutical products, plastics or petrochemical products can be analyzed automatically and extremely accurately. On the other hand, a Metrohm volumetric Karl Fischer titrator is preferred for a sample with a very high water content.

The water is driven out from the heated sample by a stream of dry carrier gas and transferred to the titration vessel, where the water is determined by a KF titration. For temperature-sensitive samples, e.g., food, the water can be released gently at lower temperatures by simultaneous extraction with methanol. In this way, it is possible to prevent any water released by decomposition or any water of crystallization that may split off from being determined and therefore produce incorrect results.

This Application Bulletin uses examples from the food, pharmaceutical, plastics and petrochemical industries to describe automatic Karl Fischer water determination using the 774 Oven Sample Processor and the 756 KF Coulometer. Information applying to the combination of the Oven Sample Processor with a volumetric KF titrator (758 KFD Titrino or 841 Titrando^®) is given in brackets.

Instrumentation and accessories

Instrumentation

774 Coulometric System

• 774 KF Oven Processor (Brinkmann Catalog No. 020816058)
• 756 KF Coulometer (Brinkmann Catalog No 020815051)
• Test tool for oven temperature
• Calibrated reference (Brinkmann Catalog No. 020978708)

774/841 Volumetric System

• 774 KF Oven Processor (Brinkmann Catalog No. 020816031)
• 841 KF Titrando (Brinkmann Catalog No. 020840005)
• Dosino^® with 10 ml buret (Brinkmann Catalog No. 020763124)
• Titrando to PC connecting cable (Brinkmann Catalog No. 020241004)
• Test tool for oven temperature
• Calibrated reference (Brinkmann Catalog No. 020978708)

Reagents

• Hydranal^® Coulomat AG Oven, Riedel-de Haën No. 34739 (for coulometric KF titration)
• Hydranal Methanol Dry, Riedel-de Haën No. 34741 (as additional extraction agent or for volumetric KF titration)
• Hydranal Composite 5, Riedel-de Haën No. 34805 (for volumetric KF titration)
• Hydranal Standard sodium tartrate dihydrate, Riedel-de Haën No. 34803
• Hydranal Water Standard KF Oven (potassium citrate monohydrate), Riedel-de Haën No. 34748
• Molecular sieve, pore size 0.3 nm, Brinkmann Catalog No. 020689536
• Carrier gas (air or N₂)

Sample preparation

The sample is thoroughly mixed under conditions that are as dry as possible. The sample weight depends on the water content. Sample weights that are too low have a negative effect on the measuring accuracy (weighing error); the maximum sample weight is limited by the reagent capacity (1,000 mg H₂O per 100 ml reagent for Hydranal Coulomat AG Oven). The absolute amount of water transferred to the titration vessel for analysis should be in the range 300 µg –1,000 µg. We also recommend that the KF reagent is changed once per week under «normal» operation, more often if necessary.

The appropriate amounts of sample, which should previously have been conditioned for at least 24 h in the surrounding air, are weighed into the sample vessels. The vessels are then sealed with PTFE-coated septa, which have also been conditioned. Gloves should be worn when working to avoid contaminating the samples and prevent contact with skin moisture.

Table 2 clearly displays the relationship between the amount of sample used in the analysis and its water content.

Instrument preparation

On the 774 Oven Sample Processor, the lift positions for the conditioning vessel (rinsing position) and the sample vessels (working position in the oven) as well as the maximum lift path are configured. The previously programmed methods for the Oven Sample Processor and the KF Coulometer (or the KF Titrino) are called up by Tiamo™ titration software or directly on the instruments themselves and, if necessary, modified to suit the samples to be analyzed.

The flow rate of the carrier gas (air or N2) should lie between 40 ml/min–60 ml/min. For solid samples, 40 ml/min is normally used; for liquid samples, 60 ml/min is set. The gas flow must be large enough to transfer the released moisture as quickly as possible to the titration vessel, and small enough to guarantee the complete absorption of the water in the titration solution.

The vessels for conditioning, system preparation, determination of the blank value (see under «Titration sequence») and the sample vessels are placed on the rack of the 774 Oven Sample Processor. For coulometric KF titration, the titration vessel is filled with 150 ml Hydranal Coulomat AG Oven (for volumetric KF titration with approx. 30 ml–40 ml Hydranal Methanol Dry) and then conditioned, i.e., titrated to dryness.

Temperature ramps

For samples whose temperature behavior is unknown, a so-called temperature ramp is run (available temperature range 50°C–250°C). The heating rate should be selected so that it can be guaranteed that the temperature can actually be achieved inside the sample. Figure 1 shows a temperature ramp in which the sample is heated from 50°C to 250°C in 100 min, i.e., the heating rate is 2°C/min.

Conditioning

Conditioning, i.e., titrating the titration vessel to dryness, is carried out by stirring without increasing the temperature (oven temperature = initial temperature) with the 774 Oven Sample Processor in the so-called conditioning position. This conditioning step must be carried out before every determination. The oven is then heated up to the selected analysis temperature. Only now is the water driven off from the sample by a combination of a dry stream of carrier gas and the increased temperature of the oven and transferred to the KF titration vessel, where the determination by a Karl Fischer titration takes place. This titration may be coulometric (with the 756 KF Coulometer) or—for samples with a high water content (>50%)—volumetric (with the 758 KFD Titrino or 841 Titrando). The latter method should be used if very small sample weights and the associated large measuring inaccuracies are to be avoided.

The double hollow needle of the 774 Oven Sample Processor can be lowered almost to the base of the sample vessel and in this way the carrier gas can be passed through the heated sample. With liquid samples this achieves an additional extraction effect. For the analysis of solid samples, a shortened penetration needle is normally used [headspace technique; this requires the use of special adapters (Brinkmann Catalog No. 020000201)]; this only penetrates the sample vessel a few millimeters deeper than the exhaust needle. In this way turbulent mixing of the sample and contamination of the following samples or the conditioning vessel by substances adhering to the needle tip can be avoided. This also prevents the penetration needle from being bent, as this could happen if it comes into contact with light, voluminous samples, such as granulated plastics.

Karl Fischer titration

For a coulometric Karl Fischer titration, a modified KF reagent containing iodide is used. During the determination the iodide is oxidized anodically (generator electrode) to iodine, which then reacts with the water. As this is an absolute method, no titer adjustment is necessary. As in volumetric Karl Fischer titration, the endpoint indication is bi-voltametric using a double Pt-wire electrode (indicator electrode) to which a constant alternating current is applied. This creates a potential difference (voltage) between the two platinum wires. If even very small amounts of free iodine are present, the voltage drops suddenly; this indicates the endpoint of the titration.

In volumetric Karl Fischer titration, the titer of the titrant must first be determined by using a certified water standard. In combination with the 774 Oven Sample Processor, this is done by driving off the water from the «KF Oven Standards» sodium tartrate dihydrate at 160°C or potassium citrate monohydrate at 220°C.

Titration sequence

The water content determined by the gas extraction with subsequent KF titration is made up as follows:

Water content_absolute = water content_sample + blank value + drift

In conditioning, during which the needle is located in the conditioning vessel, any moisture contained in the system is removed until a constant low drift in the range 10 µg/min–20 µg/min is achieved. The conditioning step must be carried out before each new analysis, and also before system preparation and the determination of the blank value. In combination with Tiamo titration software, the start of the sample series takes place via the <COND> button; this starts conditioning both the Oven Sample Processor and also the KF Coulometer or KF Titrino. If Tiamo software is not used, the analysis is started with the <START> key of the 774 Oven Sample Processor. At this point the KF Coulometer or the KF Titrino must be in the STOP mode. With automatic drift correction, the drift value measured immediately before the determination will be used for determining the water content of the sample (subtracted).

System preparation means that the whole system will be adjusted to the selected conditions. An empty sample vessel will be treated in exactly the same way as the following samples, but the value obtained will not be taken into account. We recommend that this step—which at the same time is used for checking that the analysis system is working properly—is carried out before each new sample series.

Apart from the water in the sample, the sample vessel also contains atmospheric humidity; this makes a blank value determination necessary. If methanol is used as an additional extraction agent then its water content must also be determined, i.e., included in the blank value. A threefold determination of the blank value is normally sufficient. The mean value is stored as a Common Variable and taken into account in the calculation of the water content (subtracted).

The system preparation and the blank value determinations must be carried out under exactly the same conditions as the analysis of the samples. In order to do this, an empty system preparation vessel and three empty (or filled with solvent) blank value vessels are placed on the rack of the 774 Oven Sample Processors and processed before the samples.

Instrument settings

Table 1: Control and titration parameters

Troubleshooting

Procedure for poor precision (reproducibility):

• Optimize the titration and control parameters
• Check whether the sample vessels are tightly sealed
• Clean electrodes: Clean indicator electrode mechanically with moist polishing powder (aluminum oxide or toothpaste), rinse and dry; treat with concentrated HNO₃ or change the electrode; align Pt-pins parallel; clean generator electrode with concentrated HNO₃
• Drift too high: Check titration cell, septum and/or seals leaks; molecular sieve exhausted; poorly conditioned cell; ensure thorough mixing
• KF reagent contaminated/used up: Change the solution; use a different batch number if necessary
• Check electrical contacts
• Check oven temperature
• Balance: Too inexact, drafts, temperature influences, temperature equilibrium not reached; sample weight not optimal/too low Possibly carry out a validation of the analysis system

Validation of the analysis system

GLP (Good Laboratory Practice) requires, among other things, that the correctness and precision of analytical instruments is checked at regular intervals by using SOPs (Standard Operating Procedures). We recommend validating the analytical system as a whole by carrying out a series of determinations with certified standards (see Metrohm Application Bulletins No. 255 and 273).

The validation of the 774/756 and 774/758 or 774/841 Systems is carried out by using sodium tartrate dihydrate or potassium citrate monohydrate. These KF standards are supplied with analysis certificates in which their exact water content is given. A tenfold determination is carried out with different sample weights. The relative standard deviation should be less than ≤1.0%.

In addition to the start-up check routines of the instruments, the use of the built-in diagnosis programs and regular service, we also recommend checking the electronic components of the instruments (756 KF Coulometer or 758 KFD Titrino or 841 Titrando) by using the 767 Calibrated Reference.

A measuring insert for the heating block with a built-in temperature sensor is used in combination with a calibrated temperature measuring instrument as an oven temperature test tool for the 774 Oven Sample Processor. The measuring setup is inserted into the oven block in the same way as a sample vessel and fixed in position. When the set temperature has been reached, the inner temperature measured by the test tool is compared with the display of the Oven Sample Processor. If the measured temperature varies greatly from the oven temperature displayed (more than ±4°C), please contact Metrohm Service.

In order to check the gas flow of the Oven Sample Processor, a calibrated flowmeter is included in the gas flow of the system and the flow rate measured by the test tool is compared with the set value. The permissible tolerance of the flowmeter (with microbridge flow sensor) built into the 774 Oven Sample Processor is ±20%. The external flowmeter cannot be purchased from Metrohm AG.

Examples of applications

The following table provides an overview of samples analyzed by the 774 Oven Sample Processor. In each case the Karl Fischer water determination was carried out in combination with the 756 KF Coulometer using N2 as the carrier gas (flow rate 40 ml/min). The water content as well as the sample weights and analysis temperatures are given.

Table 2: Water content of samples from various branches together with their analysis parameters

Schematic diagram of the 774 Oven Sample Processor and 756 KF Coulometer Analysis System

Method for validating the 774/756 System using sodium tartrate dihydrate (created with 756 KF Coulometer)

Method for validating the 774/756 System using potassium citrate monohydrate (created with 756 KF Coulometer)