Half-baked idea

Discussion in 'General Electronics Chat' started by plc_user1973, Jun 25, 2010.

  1. plc_user1973

    Thread Starter New Member

    Apr 12, 2009
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    I know the knee jerk response to what I am about to ask is, "How ridiculous!", but let me ask anyway.

    I think from my research that pure salt water has better conductivity than copper, why not place some stainless steel electrodes in the ends of 1/2" PVC tubing and use saltwater as conductors? I think the resistance of 1000' would be way less than 1000' of 1/2 solid copper.

    I understand the breakdown voltage is way lower, but would that be an issue? As the water diminished, I would just fill it back up.

    Like I said, half baked, but would it work?
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    I think you may have your figures wrong. AFIK copper has far higher conductivity than salt water. You would also produce chlorine by electrolysis, which would be nasty.
     
  3. kellys_eye

    New Member

    Apr 24, 2006
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    Best check your research......
     
  4. kubeek

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    Sep 20, 2005
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    If that was true it would be used for a long time now. And the chlorine production is sadly a big problem too, as it is aggresive and poisonous.
     
  5. dsp_redux

    Active Member

    Apr 11, 2009
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    I have yet to be attacked by chlorinated water ;).
     
  6. kubeek

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    Sep 20, 2005
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    Just wait for it :cool:

    Anyway, there is quite a big difference between amount of chlorine from dissolving lots of salt (NaCl) and from chlorinated water.
     
  7. jpanhalt

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    Jan 18, 2008
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    In fact, there is no (or almost no) "chlorine" in salted water, regardless of how much NaCl you dissolve. Chlorinated water contains chlorine (Cl_{2}). The difference is, of course, the difference between chlorine and chloride.

    While in this case the distinction is irrelevant to considering the ridiculous proposition of the OP that salt water is a better conductor than copper, in the case of the health benefits of fluoridated water, the distinction between fluorine and fluoride does make a difference.

    John
     
  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    http://en.wikipedia.org/wiki/Resistivity

    But you mentioned research. It seems I heard that a supersaturated solution at extremely high pressure - basically wet crystal salt squeezed into a diamond anvil - was some of the best conducting material this side of superconductors?

    You can't be talking about salt water as most people think about it.

    Ionic conduction of that type is all about the high pressures closing the material space so that the electrons orbitals of adjacent atoms are overlapped. I will take a chunk of copper over a PVC pipe (NO CRAPPING WAY!) filled with damp salt at a few million atmospheres pressure (more pressure than the blast wave of an atomic bomb.)
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Not chlorinated water, think hydrochloric acid. There is quite a difference. Any metal fixtures would dissolve.

    This was one of the experiments I did when I was a preteen. It was pretty dramatic how fast the metal dissolves.
     
  10. kubeek

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    Sep 20, 2005
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    I think you misunderstood me, the electrolysis dissolves NaCl into NaOH, Cl2 and H2, see this.
     
  11. jpanhalt

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    Jan 18, 2008
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    It had been a long day, and I had had my usual relaxant. I was being a little silly.

    You do bring up some interesting chemistry. Electrolysis of NaCl in water can produce NaOH, Cl_{2}, and H_{2}. The reaction of Cl_{2} and H_{2} to form HCl is quite slow in the dark at room temperature, but can occur with explosive rapidity in light or by ignition. It used to be taught as a classic example of the difference between thermodynamic equilibrium and kinetics.

    John
     
  12. kubeek

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    Yes, also I think that CL2 alone eats metals quite happily.
     
  13. jpanhalt

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    Yes, but the action on stainless steel is usually attributed to the presence of high concentrations of chloride. Thus, ferric chloride etches SS, but ferric sulfate does not or is much less aggressive. Aqua regia is often described as being made from nitric acid and HCl. Again, it is the high chloride concentration that HCl contributes, not its acidity per se that contributes to the action of aqua regia.

    The lesson, if there is one, is that chlorides are often much more corrosive than other salts, such as sulfates.

    John
     
  14. someonesdad

    Senior Member

    Jul 7, 2009
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    Let's turn back to the OP's original question; we'll avoid any knee-jerk responses :p and let's put some actual numbers down on "paper". Correct me where I'm wrong, hopefully politely :). I think his idea merits a serious response.

    I'm going to make some numerical electrical estimates and compare a conductor made of PVC pipe with an electrolyte with a chunk of copper wire. My reason for responding is that I know from casual measurements I've made that electrolytes can be pretty good conductors. The concern about generating noxious/corrosive gases is warranted, but I'm going to leave it for a chemist to expound on actual numbers.

    Everyone should be familiar (from their basic high school physics class) with the calculation of the resistance of a rectangular chunk of matter:

    R = \rho \frac{L}{A}

    where ρ is the resistivity of the material, L is the length of the material and A is its cross-sectional area. Conductance σ is the inverse of resistivity and conductivity is conductance per unit length. You may want to look at the wikipedia article on conductivity.

    From here, 0.746 g of KCl in 1 liter of water results in a 0.01N solution with a conductance of 1413 S.

    Since I use 3/4" PVC pipe a lot around the house, let's use a 100 m chunk of PVC pipe filled with an electrolyte. The inside diameter of schedule 40 3/4" pipe is about 20 mm. That means the cross-sectional area is 0.0013 square meters and the volume is 0.13 cubic meters or 130 liters. Thus, we'll have to mix in about 100 g of KCl with 130 liters of water to fill this pipe with a 0.01N solution.

    The pipe's resistance will be

     R = (\frac{1}{1413 S} ) \frac{100 m}{0.0013 m^2} = 54 \Omega

    This is the same resistance as about 100 m of 32 gauge (AWG) copper wire. The above link for the KCl says that Kohlrausch found experimentally that the conductance varied by the square root of the molar concentration for many solutions (look specifically at figure 4), so one can see that adding a bunch more salt probably won't be changing the resistance drastically -- adding 10 times as much KCl would only bring the resistance down by about a factor of 3. Thus, you have a quickie answer for why you haven't seen tubes of ionic solutions being used for electrical conduction.

    Let's say for sake of argument that a 10 foot piece of 3/4" PVC pipe sells for $1.5 and the unions are $0.25 each. To cover 100 m, you'd need 33 pieces of 10' each. Unless you planned to drive conductive rods in the ground to use ground as a return conductor, then you'd need another 33 pieces for the return conductor. Ignore the cost of glue, installation, salt, water, and metal conductors at the ends of each pipe. You'd thus need 66*1.5 + 64*0.25 or $115 just for the plastic pipe and fittings to run a 100 m line. This doesn't compare well to the convenience of running two (small) insulated copper conductors. I'm also ignoring that you'll need to paint the PVC to make sure it doesn't sun rot. Plus, you'll need to maintain it on a regular basis.

    Thus, I'm not running out and using this method anytime soon... :)
     
  15. jpanhalt

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    Jan 18, 2008
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    Interesting calculations. Note that: 1) The OP has not been back; and 2) The OP asserted originally:
    That is patently incorrect and doesn't require calculations, it requires DATA before I would waste the time and paper to do a calculation.

    Thanks for doing the calculation anyway. I'd still like to see the OP's data.
    John
     
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