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[girlmark]

The distillation curve test is irrelevant for biodiesel (I'm quoting Iowa State university researchers here). Whaen it is applied to biodiesel (see below), its as a holdout from what the petroleum industry is used to when describing fuels (biodiesel chemistry/engineering is more similar to the oleochemical field than the petrochemical field, but petrochemical engineers dominate the fuels industry for obvious reasons)

Distillation curve is a test that you have to do for petroleum because it is a way to describe the exact chemicals that make up the petrodiesel mixture. Petroleum products can be made of hundreds (I think, or at least dozens) of compounds, and that test exists to tell you what exactly is it that you have in your petrodiesel. It's a way of chemically describing an otherwise rather random mixture that results from the complex process of producing petroleum diesel.

Biodiesel has a simpler composition than petrodiesel. It usually contains a maximum of about 10 different types of biodiesels, based on their fatty acids composition (compared to the many more compounds that can be found in the petrodiesel fraction coming out of a refinery), and those 10 or so fatty acid methyl esters are all the variation you'll find from one biodiesel to another. They all work well in a diesel engine, unlike the different components of petrodiesel of which there are some undesirable fractions.

The reason that 'distillation curve tests' do not work easily for biodiesel , is that all of those fatty acid biodiesels have a similar boiling point (in petrodiesel the boiling points of the compounds tend to be further apart) and also that the boiling point of the possible biodiesels is very very similar to the point at which they all decompose (again, very different situation for petrodiesel compounds). So chemists who are used to working with petroleum diesel where this test is done, are confused about what they are seeing.

The distillation curve is one of the tests that was added to the ASTM D-6751 (biodiesel spec) spec by a member of the petroleum industry who refused to back down on the need for this, though others on the committee didn't agree that it belonged on there. It adds an unneccessary expense to the tests done on commercial biodiesel (it's not easy to do theconventional way) and including it in the ASTM tests was a political move which has been used against us in some legal red tape I don't even want to get into.

Obviously in unwashed homebrew there is some other stuff (soap and glycerol and catalyst) that shouldn't be expected in commercial biodiesel. Regardless of what kind of biodiesel you are testing, though, if you do the distillation curve test, you have to do it under vacuum (not normal) which the lab may or may not expect, and to really control the temperature and watch for all of those different fatty acids to come off the apparatus almost at once. Their boiling points are really close.