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Electrolysis for dummies

Discussion in 'Electronic Basics' started by Michael, Jun 24, 2006.

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  1. Michael

    Michael Guest

    This is a very basic question but I'd like to fully understand what's
    happening.....but then I know nothing about chemistry.

    Why is it exactly that there isn't an electrolysis problem when you use an
    AC voltage on probes in water?

    Does the negative half of the cycle negate any electrolysis effect that the
    positive half does? ....why?

    --stupified
     
  2. Tim Williams

    Tim Williams Guest

    Does the negative half of the cycle negate any electrolysis effect that
    Ya, for the most part. Because reactions are reversible, and 8.3ms isn't
    long enough for the atoms to spread out.

    Some things can still be dissolved. For instance, I'm told 6VAC and
    hydrochloric acid will "dissolve anything" (including platinum).

    Tim
     
  3. Michael

    Michael Guest

    So, (for a water level sensor), do you think that rather than AC, I could
    use DC on my probes and just "sample" every 5 seconds or so for a very short
    duration?

    A very very short pulse every 5 seconds or so?
     
  4. Mike Monett

    Mike Monett Guest

    Not exactly. Electrolysis still occurs with most metals. The process is as
    follows:

    With silver-plated electrodes and pure distilled water, the positive
    electrode releases silver ions (Ag+) and the negative electrode releases
    hydroxyl ions (OH-). The equations are

    anode: 2Ag - 2e(-) --> 2Ag(+)
    cathode: 2H2O + 2e(-) --> H2(g) + 2OH(-)

    When the polarity reverses, these ions are now in the vicinity of the
    electrode, and can combine in a thin layer next to the electrode called the
    Nernst Diffusion layer. This produces silver hydroxide, AgOH, which is
    inert and insoluble and precipitates out of solution, or forms a dark black
    or brown film on the electrode. The equation is

    Ag(+) + OH(-) --> AgOH

    At the same time, some ions manage to escape into the solution. These move
    under the influence of the applied field and increase the conductance of
    the solution.

    A similar reaction occurs with zinc and copper electrodes, except the
    copper ion has an ionization state of 2, so twice as many hydroxyl ions are
    formed at the cathode. The zinc and copper ions do not form hydroxides, but
    plate out at the cathode as pure metal.

    The increasing conductance of the solution shows that free ions are
    liberated and electrolysis is taking place. This can be prevented by using
    platinum or carbon electrodes which release oxygen at the anode and
    hydrogen at the cathode.

    The hydroxyl and hydronium ions recombine so there is no net increase in
    charge carriers, and the conductance remains the same. This feature is used
    in conductivity meters, such as pure water testers (PWT) or total dissolved
    solids (TDS) meters.

    If you use DC instead of AC, ions from contaminants in the water migrate to
    their respective electrodes and form a cloud around the electrode. This
    acts similar to the space charge in vacuum tubes and changes the apparent
    conductance. Changing the distance between the probes changes the
    conductance in strange and unexpected ways that are not related to the
    probe separation.

    You can avoid this effect by swishing the probes around to disrupt
    the cloud and minimize this effect. This makes a poor man's conductance
    meter and is adequate for non-critical tests.

    Regards,

    Mike Monett
     
  5. Mike Monett

    Mike Monett Guest

    You may run into problems with contamination after a while. Bacteria and
    other life forms can grow and cover the electrodes.

    If you just want to detect water level, why not do the same as a clothes
    washer? Go to your local appliance store and find where they put the scrap
    appliances. If you ask nice and leave things neat and clean, they will
    probably let you retrieve some parts. The water level sensor is a flexible
    tube going to the drum and connected to a diaphram switch. Most are
    adjustable, or you can change the water level by changing the amount of
    tubing immersed in water.

    Regards,

    Mike Monett
     
  6. Michael

    Michael Guest

    Very impressive Mike...I read it slowly.....you're very fluent on the
    subject.

    I think what I'm going to do is just get a glass of water and breadboard
    different arrangements and look at the results.

    I want to use the input of an optoisolator to sense the water level which
    will control a pump.....so it's not like the probes will be in the water
    long....and just a short random pulse would work.....plus the opto input
    takes very little current to "turn on".

    I'll have the output of the opto trigger my PIC microcontroller.

    I'm just wondering with such little contact with water whether there will be
    a problem. (probably)
     
  7. BFoelsch

    BFoelsch Guest

    A problem bigger than electrolysis will in all likelihood be a
    build-up of minerals on the probes. Each time the probes become wet
    and then dry out as the level falls any dissolved solids in the water
    will remain on the probes. Depending on the nature of your water, this
    may or may not be a major issue, but it is why most modern level
    control systems do not use conductivity probes, although they were
    quite popular in the past.
     
  8. Mike Monett

    Mike Monett Guest

    Thanks for the compliment. Yes, there will likely be many problems trying
    to use conductance. Did you see my other post on using the washing machine
    solution? You probably couldn't find water more contaminated than in a
    clothes washer, and the diaphram method can work for decades without
    failure.

    Regards,

    Mike Monett
     
  9. http://www.tinaja.com/glib/muse153.pdf


    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  10. Mike Monett

    Mike Monett Guest

    Hi Don,

    I did not write the above, as implied in your quoted text, and I do not
    normally sign my posts with the word "stupidified". I was replying to the
    OP's question.

    Thanks for your url. However, in another post, the OP tells the reason for
    wanting to learn about electrolysis. It has nothing to do with generating
    hydrogen.

    Regards,

    Mike Monett
     
  11. Mike Monett

    Mike Monett Guest

    Hi - using the neat XPat feature of XNews, I was able to retrieve the
    parent and see that you were responding to the OP's post and not mine.

    Sorry for the confused response. Anyway, he wants to measure water level,
    not make hydrogen gas.

    Regards,

    Mike Monett
     
  12. pom

    pom Guest

    Mike Monett a écrit :
    Yes, there will likely be many problems trying
    Your proposed solution of the level-sensing problem with a washing
    machine sensor is rather failproof.
    But all the different drawbacks of a conduction based sensor made me
    think of capacity change. This can be enormous, given the very high
    value of the dielectric constant of water.
    And as the original poster seems to want to go rather far,
    technologically speaking, by using a microcontroller...
    So long
    Peter
     
  13. You just missed the thread on capacitance based level sensors. With
    these, the electrodes are not in direct contact with the water so
    electrolysis is not an issue.
     
  14. Another thing that works well (if the OP has the room for it in his
    tank) is a float switch. There is a type which is a tilt switch encased
    in a float that hangs from its cable. The float is buoyant cable end
    down, so when the fluid reaches it, it tips to one side, closing the
    circuit.

    These are used in pumped sewage systems, which are even more
    contaminated than most washing machines.
     
  15. Mike Monett

    Mike Monett Guest

    Don't most sump pumps come with one already installed? They are a bit of a
    pain to adjust, and I have had problems with reliability. Either not
    shutting off, or failing to turn on. Sometimes the switch breaks or jams.
    Sometimes the float sinks. I think these are fine for sewage plants, where
    someone is constantly available to maintain them. But I have a suspicion
    the diaphram may be much more reliable.

    Of course, you really need two. One to trun the pump off when the level
    falls below the setting, the other to turn the pump on whe the level rises.

    Now comes the tricky part. It needs some form of memory to realize that it
    has turned on because the water level rose, but it shouldn't run off until
    it clears the lower sensor. This could be a simple relay, but now we are
    adding complexity and more failure modes.

    So perhaps there is a simpler method, for example using fluid logic, that
    might be more reliable?

    Regards,

    Mike Monett
     
  16. Mike Monett

    Mike Monett Guest

    [...]
    Maybe there is. Only needs one switch. This concept is for a sump pump
    mounted in the basement.

    Make sure the pump has an anti-siphon valve at the exit. Most do - it is
    silly to try to pump water out of the basement and have it return as soon
    as the pump stops.

    Connect the tube for the diaphram switch at the pump housing near the
    exit and anti-siphon valve.

    Adjust the diaphram switch to turn on at the desired water level.

    When the pump turns on, the water pressure will keep the switch closed.

    When the pump runs out of water, the anti-siphon valve will prevent water
    from flowing back into the pump. The pressure will drop, opening the
    diaphram switch. The pump turns off and the system waits for the next
    cycle.

    Of course, there may be some additional details that require solution, or
    the OP may have a completely different need in mind.

    But this eliminates the adjustment problems and unreliability of the float
    and level switch.

    Regards,

    Mike Monett
     
  17. This is the kind of float switch I was thinking about. They are pretty
    reliable, since they have no external adjustments or moving parts:

    http://www.deanbennett.com/zoellerfloatswitches.htm

    The only disadvantage these have is in an environment where they might
    get tangled up with something.
     
  18. Mike Monett

    Mike Monett Guest

    I've had different problems with sump pumps all my life. Rarely the same
    thing twice. Anything with wires, rods, levers, floats, plugs that come
    loose, corroded switches, and so forth. I'd minimize the number of moving
    parts and keep them protected inside the housing so nothing can get at
    them.

    Regards,

    Mike Monett
     
  19. jasen

    jasen Guest

    my chemistry teacher put two stainless steel electrodes in a jar of dilute
    sodium hydroxide, connected 12V AC (50Hz) and produced a mixture of hydrogen
    and oxygen gas (Which He trapped in soap bubbles and ignited)
    possibly it does some of it <<1% , but I don't think it happens fast
    enough to negate most of it.

    Bye.
    Jasen
     
  20. jasen

    jasen Guest

    wahtever you do the positive electrodes will dissolve if it contacts the
    electrolyte. stainless steel is much better than copper

    if you want to avoid this you could use insulated probes and measure the
    capacitance. - earth one and measure the impedance of the other.
     
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