It was chosen as the zero point of the scale because of the availability of very high purity n-heptane, unmixed with other isomers of heptane or other alkanes, distilled from the resin of Jeffrey Pine and from the fruit of Pittosporum resiniferum.
The oil can also be distilled into a very pure form of n-Heptane.
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The petroleum nut, Pittosporum resiniferum, is a tree that grows in the Philippines and Malaysia, particularly in the wilderness surrounding the Mayon Volcano and in the Cordillera of the Philippines and Mount Kinabalu of Sabah, Malaysia.