The Karl Fischer oven method of water determination, using the Metrohm 774 Oven Sample Processor, measures water content and avoids many of the normal problems encountered
Many active pharmaceutical ingredients and adjuvants contain water in an adsorbed form (surface water) or bound as a hydrate (water of crystallisation). The water content of medicaments strongly influences their quality, shelf life and stability as well as the release of the active substances. The determination of water therefore assumes great importance in pharmaceutical analysis.

The European Pharmacopoeia, 4th Edition (2002), describes various methods for determining the water content of pharmaceuticals.

By far the most important method is the Karl Fischer titration.

Normally the titration is carried out volumetrically (semi-micro determination).

For substances with a very low water content a coulometric KF titration (micro-determination) is performed.

The KF oven method is used for difficult samples.

Many substances release their water only slowly or at high temperatures.

They are therefore not suitable for a direct Karl Fischer titration.

An additional problem is the low solubility of certain samples in alcohols.

In these cases traditional methods recommend the use of toxic solvents to promote dissolution or alternatively extensive sample preparation procedures.

Other substances undergo side reactions with the KF reagents, thus falsifying the result.

The European Pharmacopoeia specifies that these types of pharmaceuticals are not to be analysed by Karl Fischer titration but by determining the loss on drying in a drying cabinet or desiccator (under vacuum if necessary).

However, with this method all volatile components released at the particular temperature (e.g impurities) are determined and not specifically the water content of the substance.

By using the KF oven method the above-mentioned problems can be avoided.

The substance under investigation is heated in a tube oven and the released water is transferred by a carrier gas to the titration cell where it is determined by Karl Fischer titration.

As only the water enters the KF cell and the sample itself does not come into contact with the KF reagent, this means that unwanted side reactions and matrix effects are eliminated.

Automation of this process brings clear advantages.

The 774 Oven Sample Processor allows automation of the KF oven method.

In contrast to the conventional Karl Fischer drying oven, the samples are no longer introduced into the oven by means of a sample boat, but the vial technique is applied instead.

The substances to be analysed are weighed directly into sample vials, which are then sealed tightly and placed in the rack of the Oven Sample Processor.

For the analysis the sample vessel is moved by the turntable to the appropriate position above the oven and then lowered automatically into the heating block.

At the same time a double hollow needle pierces the septum of the vial.

Via the inlet needle a stream of dry carrier gas (air or inert gas) is passed through the heated sample.

The carrier gas, loaded with the released moisture, then flows through the outlet needle and a heated transfer tube directly into the titration cell, where the Karl Fischer water determination takes place.

Depending on the sample water content, the determination is carried out either volumetrically or, at the trace level, coulometrically.

The automation of the Karl Fischer water determination using the 774 Oven Sample Processor brings decisive advantages.

Strictly reproducible analysis conditions for all samples as demonstrated by the significantly improved repeatability of the results.

Considerably increased sample throughput and therefore improved efficiency.

Manual sample preparation is reduced to a minimum.

Considerable savings in time.

No contamination of the oven and titration cell; consequently there are no carryover and memory effects.

Much lower reagent consumption as the titration solution only requires changing at infrequent intervals.

Improved water release from the sample as the carrier gas does not just pass over the sample but directly through it.

In addition, the Oven Sample Processor allows temperature gradients to be run.

Using the recorded water-release curve, it is possible to determine the optimum analysis temperature for the particular sample.

The curve also allows statements to be made about the kinetics of water release as a function of temperature.

Some typical results are as follows.

Using the KF oven method, we analysed about 40 pharmaceuticals from the European Pharmacopoeia.

The analyses were carried out with the 774 Oven Sample Processor in combination with a 756 KF Coulometer.

The investigated pharmaceuticals were substances with a defined water content, some of which undergo side reactions with the KF reagents and therefore cannot be analysed by direct Karl Fischer titration.

As stated above, according to the Pharmacopoeia the water content of such substances must be determined by loss on drying in a drying cabinet or desiccator (under vacuum if necessary).

We used the following equipment.

774 Oven Sample Processor.

756 KF Coulometer, including KF cell without diaphragm.

728 Magnetic Stirrer.

6.5617.000 complementary equipment for automatic reagent exchange.

700 Dosino.

PC with VESUV 3.0 Metrodata software for data acquisition, storage and reprocessing.

The reagents.used were as follows.

Hydranal Coulomat AG Oven, Riedel-de Ha‰n nr34739.

Hydranal Water Standard KF Oven (potassium citrate monohydrate), Riedel-de Ha‰n nr34748.

Nitrogen as inert carrier gas.

The analytical procedure was as follows.

Between 15 and 30 mg of the pharmaceuticals to be analysed is weighed into the sample vials, which are then hermetically sealed with PTFE-coated septa.

At least a threefold determination is carried out on each substance.

Prior to each determination the complete system is conditioned until a constant low drift (approx 10 æg/min) is attained.

During this procedure the needle is located in a special conditioning vessel on the rack of the Oven Sample Processor.

In order to obtain correct results, the blank of the sample vials - i.e the moisture adhering to the vessel walls, vial cap and septum - must be determined and taken into account when calculating the water content of the samples.

This is done by analysing three empty vials at the oven temperature required for the particular sample.

The complete system is checked at regular intervals with a certified KF standard (Hydranal Water Standard KF Oven).

When selecting the temperature to be used for driving off the water, the thermal stability (instability) of the particular pharmaceutical must be taken into account as well as the fact that water is only released at a sufficiently rapid rate at temperatures above 100 deg C.

This means that the oven temperature should be chosen as high as possible to ensure short determination times, but still be 20 to 30 deg C below the decomposition temperature.

The analysis temperatures are determined on the basis of the water-release curves that were recorded for all the investigated pharmaceuticals in the temperature range 50 - 250 deg C.

In addition, all the pharmaceuticals were examined by means of a Kofler microscope and their melting points were determined.

This instrument allows the substance to be closely observed during the heating-up and melting phases; any alterations such as colour changes, sublimation or decomposition reactions can be easily recognised.

Metamizole sodium melts at 220 to 221 deg C with decomposition.

Water determination by direct volumetric or coulometric KF titration is not possible as the substance is oxidised completely or partially by iodine.

The water-release curve was recorded using a heating rate of 2 deg C/min, ie metamizole sodium was heated from 50 to 250 deg C in 100 min (= 6000 s).

The curve corresponds to the absolute amount of water released, the curve showing the associated drift.

Both the surface moisture and the water of crystallisation are released within the time interval 0 - 1600 s (50 - 103 deg C); this is indicated by the continuous increase of the red curve in this region as well as by the occurrence of the "drift peak".

The drift then falls to its original value of approx 10 æg/min and remains virtually constant for 3800 s.

Starting at 5400 s (230 deg C) both curves show a steep increase.

Evidently water is released by decomposition from this temperature onward.

A temperature from the central region of the plateau of the red curve (150 deg C) was selected as the oven temperature for determining the water content of metamizole sodium.

This ensures that the water is released quickly and completely without decomposition.

In conclusion we can say that the water contents determined with the 774 Oven Sample Processor and 756 KF Coulometer all lie within the ranges specified in the European Pharmacopoeia.

The Pharmacopoeia usually gives a very wide recovery range for the loss on drying.

In the case of quinine hydrochloride, for example, a range between 66.2 and 110.3% is specified, based on the theoretical (calculated) water content.

In contrast, the oven system yields an excellent recovery of 96.8% for this substance.

When all the investigated pharmaceuticals are considered, the recovery using the KF oven method always lies between 90 and 110%.

The repeatability of the values obtained with the oven system is also excellent.

This can be seen from the relative standard deviations, which all lie between 0.30 and a maximum of 2.0%.

When compared with the drying cabinet method, the Karl Fischer water determination using the 774 Oven Sample Processor and 756 KF Coulometer offers additional clear advantages: determinations that normally take several hours can be performed in 10 to 12 min.

In addition, the analytical procedure is completely automated.

A further bonus point is the specificity of the described method, in which only the water released by the substance is determined and not all the other volatile components released at the particular temperature.

Finally the small amount of substance required is also an advantage, this aspect being particularly important when analysing very expensive pharmaceuticals.

Whereas the determination of the loss on drying normally requires sample weights of 1 g, the KF oven method in combination with the KF Coulometer requires only 15 to 30 mg.

Moreover, as the substance is not destroyed during the water determination it can also be used in further investigations.