INTRODUCTION
The chemical stability of biodiesel determines how long it can safely be stored and how it might break down under extreme conditions. How is the chemical stability of biodiesel measured? To discuss the answer to this question a review is provided of the chemical structure of biodiesel. This is followed by a discussion of the most commonly used stability value, the "Iodine Value" (IV). Finally, alternative stability measurements "Oil Stability Index" (OSI), and bis-allylic/allylic position equivalents (APE / BAPE) are mentioned. (Any lack of stability due to bacterial degradation or water contamination is not discussed here).

BIODIESEL CHEMISTRY
An understanding of biodiesel stability requires an understanding of the chemical make up of biodiesel and its parent, vegetable oil. Fats/Oils contain a glycerol molecule bonded to three fatty acid chains. This structure can also be called a triester or triglyceride.

Typical Oil: Trilinoleic Ester of Glycerol

Different fats/oils contain different types of fatty acid chains. Also, multiple types of these triesters will be present in any fat/oil. These chains differ in the number of carbon atoms and the number of carbon-carbon "double bonds" in the chain. For example, in soybean oil, there are 4 types of chains that contain 18 carbon atoms. A double bond normally introduces a "kink" in the chain. These double bonds play an important part in the stability of biodiesel. (Double bonds are represented with two parallel lines in the chemical formula diagram.). Note that vegetable oils and biodiesel are not hydrocarbons because oxygen atoms are present in the structure while gasoline and petro-diesel are true hydrocarbons as they contain molecules like iso-octane and cetane respectively. All these fuels are a relatively efficient energy store and release energy during combustion with Oxygen in a gasoline or diesel engine.

Fats, which tend to be solid at room temperature, tend to have fewer double bonds, which leads to straighter chains allowing the nice packing a solid requires. Oils, which tend to be a liquid at room temperature, tend to have more double bonds, with corresponding kinks in their fatty acid chains leading to a liquid state. It is possible to "hydrogenate" an oil to remove double bonds and make it more solid at room temperature. The opposite is also possible.

Glycerol, an alcohol

TRANSESTERIFICATION
For many years it has been known that vegetable oil can be converted to "Biodiesel". This transformation process is called transesterification. This process replaces one type of alcohol (glycerol) with another (in biodiesel ethanol or methanol is used). To convert vegetable oil into biodiesel it is combined with ethanol or methanol in the presence of a catalyst (sodium/potassium hydroxide). In the resulting transesterification reaction, the triglyceride structure is "broken" and three ethanol/methanol

Methanol, an alcohol

molecules replace the glycerol molecule. The result is three separate fatty acid chains and a waste byproduct of glycerine (that glycerol molecule has to go somewhere). A specific example of a fatty acid found in biodiesel is linoleic acid which has 18 carbon atoms, two of which have double bonds.

An examination of soybean oil, and biodiesel made from it thru transesterification, reveals 5 variations of fatty acid chains, in approximately this mix:

Composition of soy oil
8% with 16 carbon atoms (aka "Palmitic Acid")
3% with 18 carbon atoms (aka "Stearic Acid")
25% with 18 carbon atoms and 1 double bond (aka "Oleic Acid")
55% with 18 carbon atoms and 2 double bonds (aka "Linoleic Acid")
8% with 18 carbon atoms and 3 double bonds (aka "Linolenic Acid")

Linoleic Acid, a common component of Soy biodiesel

Biodiesel produced from different source oils (termed "Feedstocks") will contain different proportions and types of fatty acid chains. This is why SME - Soy Methel Ester (biodiesel produced from soybean oil using methanol during transesterification) does not have the identical chemical properties of RME - Rapeseed Methel Ester (biodiesel produced from rapeseed oil).


For example, soybean oil has a melting point of -16C, rapeseed oil melts at -10C and palm oil melts at 35C. More information on chemical makeup of various types of oils/fats here

Fatty acids that have no double bonds are termed "saturated". These chains contain the maximum number possible of hydrogen atoms per carbon atom. Stearic Acid is saturated. Fatty acids that have double bonds are "unsaturated". These chains do not contain the maximum number of hydrogen atoms possible due to the double bond(s) present on some carbon atoms. Linoleic Acid is unsaturated. (One double bond is termed "mono-unsaturated, more than one double bond is termed "poly-unsaturated"). There are some good diagrams and more information here. The location and number of double bonds are important because they influence reactions that can occur to destabilize the fatty acid chain. (Paper on chemical reactions on fatty acids.) The interaction of oxygen molecules with the fatty acid chain, called "oxidation" is the chemical mechanism that destabilizes oil/biodiesel. Table 3 in this offsite resource shows the relative oxidation rates of oleic, linoleic, and linolenic fatty acids with oxygen as 1, 27 times, and 77 times respectively. After oxidation, hydroperoxides (one hydrogen atom and 2 oxygen atoms) are attached to the fatty acid chain. In a food oil this leads to rancidity. In biodiesel these degraded chains can polymerize, hooking together into various substances including insoluble gums that clog up parts.

STABILITY MEASUREMENTS
To compare the chemical stability properties of different biodiesel fuels, it is desirable to have a measurement for the stability of the fuel against such oxidation as described above. Currently the most common method for doing this, and the one specified in many of the biodiesel fuel specifications is called the Iodine Number or Iodine Value. The Iodine Value is not determined by measuring the stability of the fuel, rather it is determined by measuring the number of double bonds in the mixture of fatty acid chains in the fuel by introducing iodine into 100 grams of the sample under test and measuring how many grams of that iodine are absorbed. Iodine absorption occurs at double bond positions - thus a higher IV number indicates a higher quantity of double bonds in the sample. Numbers range from 10 for Coconut oil, 94-120 for Rapeseed oil, 117-143 for Soybean oil, up to 185 for Sardine oil. Biodiesel from these oils have Iodine values something like 97 for Rapeseed Methyl Ester, 100 for Rapeseed Ethyl Ester, 123 for Soy Ethyl Ester and 133 for Soy Methyl Ester.

The Iodine Value can be important because many Biodiesel fuel standards specify an upper limit for fuel that meets the specification. For example, Europe's EN14214 specification allows a maximum of 120 for the Iodine number, Germany's DIN 51606 tops out at 115. The USA ASTM D6751 does not specify an Iodine value. It might be noted that the Euopean and German specifications result in a defacto ban on Soy based biodiesel.

The Iodine value (IV) does not necessarily make the best measurement for stability as it does not take into account the positions of the double bonds available for oxidation. In some cases this can lead to IV values that are misleading. This study, states: "The IV did not correlate well with oxidative stability."

Other measurements of stability are available which do take into account double bond position. One is termed "Oil Stability Index" or OSI and is measured in hours by looking at the conductivity in water of the degraded fatty acids at a specific temperature. A detailed paper describing OSI is here. Another stability specification is known as "APE" and "BAPE" for "allylic position equivalents" and "bis-allylic position equivalents" which takes into account both the number and position of double bonds in the fatty acid chains. See this paper for more information on APE/BAPE.

It should be noted that APE and BAPE, as well as OSI, differentiate between hypothetical mixtures of fatty acids that all have the same IV, but differ in actual stability.

How well does Iodine Value correlate to chemical stability? Testing has been performed on biodiesel fuels with differing IV values to determine this. Rapeseed Methyl Ester, Sunflower Methyl Ester, and Camelina Methyl Ester which have respective IV's of 107, 132, and 150 were compared in engine use. This study concluded in part that: "The engines were dismantled before and after the tests. No unusual deposits could be found in the cylinder liner, the combustion chamber, the injector and the valves. The experiences have shown that it is possible to operate an engine with a methyl ester containing more than 30% of unsaturated acids." This study, along with 100's of thousands of user driven miles on Soy Methyl Ester in the U.S. would seem to indicate that higher IV values do not necessarily indicate an unsuitable stability property for biodiesel.

CONCLUSION
To try and wrap things up, it is clear that both vegetable oils and biodiesel can eventually degrade thru oxidation. The commonly used measurement for this property, Iodine Value, should be reasonably understood as only a rough guide to a process that is better understood by examining the chemical makeup and breakdown process involved. Alternative properties, specifically OSI and APE/BAPE, more accurately reflect the chemical stability of biodiesel.

For specific biodiesel handling guidelines regarding storage and use stability the reader is referred to the extremely well done US DOE BioDiesel Handling and Use Handbook. See section 3.6 "B100 Stability".

Other nice pics referenced above:

Reference: Iso-Octane, typical of gasoline

Reference: Cetane, typical of petro-diesel