Refractive Index of Biodiesel Fuels

In the esterification of oil to make biodiesel glycerol initially present in the unprocessed plant oil, being denser than the product ester, separates and is taken off at the bottom of the reactor. In refractive indices of soybean derived material of various proportions of glycerol were reported. The untreated oil had a glycerol content of about 10%, and the totally esterified oil a glycerol content of zero. The refractive index of the latter was 1.457 and that of the latter 1.473 and the rise in refractive index with glycerol content was linear. The refractive index of methyl oleate is 1.454, very close to the value for the totally esterified oil in the work under discussion.


In biodiesels containing different proportions of methyl oleate and methyl linoleate were investigated for refractive index. The molecular formula of the former is C19 H36 O2 and that of the latter C19 H34 O2, there being one double bond in the carbon structure of methyl oleate and two in the carbon structure of methyl linoleate. The biodiesels with a preponderance of the linoleate had lower refractive indices than those with a preponderance of the oleate, indicating that unsaturation raises the refractive index.