# Electrolysis for dummies

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

1. ### MichaelGuest

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 WilliamsGuest

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. ### MichaelGuest

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 MonettGuest

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 MonettGuest

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. ### MichaelGuest

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. ### BFoelschGuest

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 MonettGuest

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. ### Don LancasterGuest

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

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

10. ### Mike MonettGuest

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 MonettGuest

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. ### pomGuest

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. ### Paul Hovnanian P.E.Guest

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. ### Paul Hovnanian P.E.Guest

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 MonettGuest

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 MonettGuest

[...]
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. ### Paul Hovnanian P.E.Guest

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 MonettGuest

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. ### jasenGuest

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. ### jasenGuest

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.