Compared to petroleum diesel fuel, biodiesel produces fewer emissions and is considered sustainable and environmentally friendly. Biodiesel can be blended with petroleum-based diesel in any ratio. Many countries therefore add biodiesel to conventional diesel fuel between 5% to 15% (v/v). Depending on the type of internal combustion diesel engine, the biodiesel fraction can be much higher or even used as 100% pure biodiesel fuel.
However, the quality of both the biodiesel and its blends needs to be monitored. One of the most important parameters is oxidation stability of fuel. During biodiesel production, the addition and effect of antioxidants (e.g., ascorbyl palmitate) inhibits autoxidation and increases the oxidation stability [1]. The 893 Professional Biodiesel Rancimat is an analytical system for easy and safe determination of the oxidation stability of biodiesel and its blends according to EN 14112, EN 15751, and EN 16568.
This application is demonstrated on various fuel samples which are listed in Table 1.
No sample preparation is required.
The determinations are carried out using an 893 Professional Biodiesel Rancimat (Figure 1).
An appropriate amount of sample is weighed into the reaction vessel, and then the analysis is started.
With the biodiesel Rancimat method, the sample is exposed to an airflow at a constant temperature between 80–150 °C. Highly volatile secondary oxidation products are transferred into the measuring vessel along with the airflow where they are absorbed in the measuring solution.
The conductivity of the measuring solution is continuously registered. The formation of secondary oxidation products leads to an increase in the conductivity. The time until occurrence of this marked conductivity increase is referred to as the «induction time», which is a good indicator for the oxidation stability (Figure 2).
Furthermore, a power regression shows that the blends with the corresponding induction time achieved a coefficient of determination of 0.9996 (Figure 3). This demonstrates the accuracy of the 893 Professional Biodiesel Rancimat.
| Sample (n = 4) | Mean value in h | SD(rel) in % |
|---|---|---|
| 100% Diesel without biodiesel content |
N/A* | N/A* |
| 100% Biodiesel (made from organic waste) | 6.15 | 1.2 |
| Diesel blend with 30% biodiesel (v/v) | 13.55 | 0.9 |
| Diesel blend with 20% biodiesel (v/v) | 17.15 | 1.0 |
| Diesel blend with 10% biodiesel (v/v) | 27.31 | 1.6 |
* 100% diesel is very resistant to oxidation. At the temperature of 110 °C used, there is no induction time within a reasonable period.
The oxidation stability of biodiesel and its blends with diesel fuels is an important parameter listed in a series of standard methods defining the minimum quality requirements for biodiesel marketed as vehicle fuel or heating oil.
By using the Rancimat, diesel oxidation stability can be easily determined for several samples at a time with eight measuring positions in two heating blocks, increasing laboratory throughput. The built-in display shows the status of the instrument and each individual measuring position. A button located at each measuring position on the instrument allows measurements to be started.
The use of practical disposable reaction vessels and dishwasher-safe accessories reduces cleaning to a minimum. This saves time and money and significantly improves accuracy and repeatability.
In this application, diesel, biodiesel, and various blends of both were tested. As diesel fuel itself is very stable against oxidation, no induction period could be found within a reasonable time at 110 °C. However, the determination worked very well for biodiesel and all blends tested.
It was also shown in a power regression that the blends with the corresponding induction time achieved a coefficient of determination of 0.9996, demonstrating the superior accuracy of the 893 Professional Biodiesel Rancimat for this analysis.
- Metrohm AG. Oxidation Stability of Fatty Acid Methyl Esters (FAME, Biodiesel) – Reliable and Accurate Determination of the Oxidation Stability of Biodiesel According to EN 15751; AN-R-009; Metrohm AG: Herisau, Switzerland, 2024.
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