Hydrocarbon Conductivity and Temperature

name         Chikwendu
status       other
grade        other
location     N/A

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