Biodiesel does not seem to be regulated very well in the UK. Be careful what you buy and never assume it's good quality fuel. There are two tests to put your mind at rest. The Wanquist test measures the purity: dissolve 3ml of biodiesel into 27ml methanol and shake, leave to stand. Biodiesel is soluble in methanol, any drop out is unconverted vegetable oil.
The second test is easier: add 50% water and 50% biodiesel. Shake and leave to stand. The bottom water layer should be clear, cloudy water indicates soap contamination.
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