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Name: Chikwendu
Status: Other
Grade: Other
Location: N/A
Country: N/A
Date: July 2006


Question:
Why does the electrical conductivity of some pure hydrocarbon compounds (benzene, octane, 1-octene), decrease with increase in temperature?



Replies:
You need to be sure of the reliability of the data before assigning any "reason" or "cause" to measurements. This is especially the case when measuring a quantity, in this case electrical conductivity, that is very small. In such cases small errors can have major dominant effects. In the case of the compounds you mention here is a short list of factors that have to be taken into account to ensure the data are reliable: In your case: "How pure is pure?" The presence of surfactants (which may desorb off the measuring cell if it has been previously washed) and dissolve in the liquid; impurities, specifically H2O, O2, CO2 from the atmosphere can have complicated effects on the electrical conductivity -- both value and temperature dependence. These atmospheric components, for example have a small, but not negligible solubility in hydrocarbons. The solubility of water in benzene for example is about 0.5%w/w. That is small, but certainly not negligible. The hydrocarbon should be dried with Na metal to remove traces of H2O and CO2. Other gases would have to be removed by freezing/pumping/melting cycling. This makes use of the fact that (usually) gases are not soluble in the solid phase, but this too would have to be verified by infrared or Raman spectral analysis. In addition, depending upon the temperature range, you need to be careful because the density of hydrocarbons is fairly sensitive to temperature. So what you may be "measuring" is the change in the number density of charge carriers as a function of temperature. So density may be the operative variable, convoluted with the temperature dependence of the electrical conductivity. Is the conductivity AC or DC.? This could have a number of effects.

In short: It is difficult to measure "zero" or "infinity" very accurately, because factors that are usually negligible become important. There are some techniques to minimize some of the problems. In the case of electrical conductance for example, you could measure the relative conductivity of two substances in matched cells in a parallel electrical circuit so that at least some of the sources of error cancel, but this too introduces other problems. The advantage is that you only have to make sure of the absolute accuracy of one liquid, not each liquid separately. What you are trying to measure is not an easy determination.

Vince Calder


Hello,

This is the indeed a strange question. The usual question that arises regarding "conductivity" of liquid hydrocarbons in general, is that since they are nearly perfect insulators, what can be done to prevent static buildup that could trigger a fire?

The electrical conductivity of octane is something like a million times worse than pure water (which is itself an effective insulator). My expectation is that 1-Octene (which is just Octane with 2 hydrogen atoms removed and a carbon double-bond in their place), and benzene (a simple ring- shaped hydrocarbon) would display the similar lack of conductivity that octane has. That is, that they would be near-perfect insulators.

Since these liquids fall well into the normal classification of "excellent insulators", their mechanism for conducting what tiny electrical current they may conduct, is likely entirely different than that of normal conductors such as metals or semiconductors. So all this is the long way of saying your question is very puzzling, and I cannot answer it!

Bob Wilson



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