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The Metrohm 874 Oven Sample Processor can be used for analysing biodiesel and biodiesel blends that may contain additives that cause side reactions with KF reagent.

The temperature ramping facility in Tiamo software provides a heating curve to determine the best temperature for sample determination.

The amount of water in biodiesel determines the calorific value and the shelf life of the fuel.

Biodiesel with high water content has lower oxidation stability.

The lower the oxidation stability, the greater the probability that oxidation products will be formed during long storage periods.

These can cause engine problems due to deposits, particularly with the injection system.

In order to avoid this, DIN EN 14214 limits the maximum water content to 500ppm.

Many biodiesel fuels contain additives that can take part in side reactions during the direct coulometric Karl Fischer titration.

In such cases, Metrohm recommends that the biodiesel sample is not injected directly into the reaction solution.

Instead, the water contained in the biodiesel should be determined with the KF oven method.

The oven is set to 120C, water is then driven off and transferred to the titration cell of the KF Coulometer in a flow of carrier gas (dry air or inert gas).

This process can be fully automated using the 874 Oven Sample Processor.

As working with the oven means that only water and no sample matrix is transferred to the titration cell of the KF Coulometer, it is possible to dispense with the solubility promoter xylene.

However, Metrohm can recommend special coulometric reagents suitable for the oven technique.

In order to define the optimum oven temperature, a temperature gradient or heating curve can be determined using the 874 Oven Sample Processor, 831 KF Coulometer and Tiamo software.

In work carried out by Metrohm, the heating curve was recorded at a heating rate of 2C/min over the temperature range 50 to 250C.

In the Tiamo software, the amount of released water per unit time (ug/min; also known as drift) was plotted as a function of the temperature.

The drift between 50 and 120C is particularly high and it was concluded that the sample releases its water in this temperature range.

At about 120C the drift falls to the initial value of 10 ug/min; no further water is released.

A side reaction occurs in both cases, which results in a renewed increase in the drift.

A side reaction was seen to occur at 150C, so the recommended temperature for oven analysis of biodiesel is 120C.

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