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We continually perform research into how our products behave in application and investigate new ways to analyse product per formance. Resistance to oxidation is important in many applications of specialty oils and so we have invested a large amount of effort in developing tools to study and analyse oxidation phenomena.

One such tool is the newly developed method for the determination of peroxide levels in o i l . Th is method has many advantages over other methods aimed at judging the level of oxidation. The method

Measuring peroxides in oilby Chatleen Karlsson and Dr. Per Wikland, Nynas

A new method for the determination of peroxides in oil developed by researchers at Nynas promises to become a useful tool for both product development and analysis. Able to detect the very first signs of oxidative ageing, the INPOX method is versatile, accurate, clean, and easy to use.

is especially attractive as it is designed to detect the very first oxidation products: peroxides.

Perox ides play a central ro le in the degradation of oil. Like all organic matter, oil is constantly under threat of degradation by peroxidation, i.e. auto-oxidation by atmospher ic oxygen brought about by radical chain reactions of organic peroxides. It is important to control and prevent this destructive effect in both the technical applications of hydrocarbon liquids and in food preparations containing lipids of biological origin.

The process of hydrocarbon auto-oxidation (see Fig. 1) starts with thermal generation of a hydrocarbon radical, which quickly reacts with available oxygen to form a peroxy radical. This radical in turn abstracts a hydrogen from another hydrocarbon molecule (generating another radical) to form a hydroperoxide. This is the core of the auto-oxidation chain reaction and it is only limited by the availability of oxygen once it has started.

Hydroperox ides are the in i t ia l semi-stable products of hydrocarbon auto-oxidation. The radical chain reactions of hydroperoxides and peroxides that inevitably follow ultimately lead to familiar oxygenated hydrocarbon derivat ives such as alcohols, keto-compounds and carboxylic acid derivatives (and carbon dioxide, CO2) via radical terminations and subsequent radical and/or non-radical pathways. To date oil oxidation stability and the effects of antioxidants have generally only been judged against one of the ultimate products of hydrocarbon oxidation, i.e. carboxylic acids. The reason for this is the ease with which one can determine acidity by acid/base titration. However, for many applications it is of potential interest to determine the very earliest signs of oxidation, i.e. peroxides or, more specifically, hydroperoxides.

There are a variety of redox (titration or voltametry) methodologies (see Fig. 2) for the determination of hydroperoxides that have been applied for vegetable oi ls, biofuels and jet fuel as wel l as transformer oil. In the most established method (ASTM D3703-99) hydroperoxides are allowed to oxidise iodide ions to iodine, which is then determined by color imetr ic t i t rat ion with th iosul fate using starch as the indicator. However, from an environmental point of view, this method is problematic as it requires the use of large volumes of Freon (originally) or other halogenated solvents. It is also very labour intensive and requires prior knowledge of the expected amount of peroxides in the sample.

The technique developed involves adding a reagent selective for hydroperoxides to the oil sample. The peroxide level can then be determined by a modern gas chromatography-mass spectrometry (GC-MS)-based determination method. This method is applicable for all light mineral oils and vegetable oils used for electrical insulation (transformer oil), but can equally well be applied to liquid fuels or lubricating oils.

Fig. 1: Elementary steps in hydrocarbon auto-oxidation (simplified).

Fig. 2: Different methods can be used to measure oxidation products in oil.

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TRANSMISSION AND DISTRIBUTION

The dynamic range of the method is so wide that only one concentration of the reagent solution is necessary to cover all the ranges of peroxide content that are of interest – from new to very severely aged oils. The latter is also the main reason for utilization of GC-MS (nowadays a standard inst rument) as s ingle ion monitoring (SIM) enables good detection and quantif ication, even in the ver y complex matrixes of aged mineral oils.

Furthermore, the method is based on the internal standard (IS) technique, as this provides much better reproducibility over time. Our method can also simultaneously determine the oxidation inhibitor content of the oil, which is why we have named it Inpox.

Compared to other analys is methods rout ine ly used to judge ox idat ion in t ransformer oi l, the method has clear advantages. The normal methods of

analys is of t ransformer oi l resul t in values for total acid number (TAN), interfacial tension (IFT) and dielectric dissipation factor (DDF). Of these only TAN values are unambiguously linked to oil oxidation. Low IFT and high DDF values can be caused by the effects not only of oxidation products but also of contaminants from other sources. Oil samples with intermediate to high TAN values can have very different IFT and DDF values.

The problem with TAN is that it only provides a measurement of oxidation that has already happened. On the other hand, peroxides indicate ongoing oxidation, as they are the ver y f i rst oxidation products. Early indication of oxidation is important, not least because it is l ikely that peroxides, l ike acids, can damage the solid insulation as well as contribute to copper corrosion/ dissolution.

From studies on used transformer oil, peroxide levels appear to be reasonably stable in oil samples stored at room temperature (in the dark). If oil samples (even inhibited grades) are stored in glass bott les openly in the lab, the peroxide content can become very high. Sampling and storage routines are therefore important.

In general, oils that showed significant TAN values had low peroxide content, showing that oxidation had already happened. In ful l accordance with how inhibitors function, there were no significant peroxide levels in inhibited oil as long as there was some inhibitor left. For uninhibited grades, the peroxide leve l c lea r l y shows when the o i l s have exhausted the natural inhibitors, something which is not poss ible to measure any other way.

The method also measures levels of dibutyl para-cresol (DBPC) through direct detection of the molecules themselves. In contrast, the generally used infrared (IR) method measures only an absorption, which in principle can arise from other molecules. In general the IR method appears to work just as well as the more reliable Inpox method for oil samples with low TAN values. However, at higher TAN values there is evidence of disturbance from oil oxidation products in the IR method.

The method will be a useful tool both in the development of new products in cooperation with out customers, and in analysis of field samples of oils already in service. We also believe that this versatile method can be utilised to further the scientific and technical knowledge of how hydrocarbon fluid age under different conditions.

Contact Alistair Meyer, Nynas , Tel 011 675-1774, alistair.meyer@nynas.com

Fig. 3: During oxidative ageing, peroxide levels soar long before acidity develops.