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Q & A: where does electrolysis dump heat?

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Most recent answer: 01/13/2015
Q:
A swimming pool salt water chlorinator works by electrolysing salt water to release chlorine gas , with chloride being replenished from added HCl . An interesting observation is that the negative electrode terminal and cable to the power supply is always warmer than the positive cable when the system is working normally and close to maximum of the design . This has been observed over many years and in at least two different brands of manufacture , suggesting this might be a universal phenomenon . With age , the negative cable strands become blackened by oxidation , whereas the positive cable strands remain copper coloured . This must mean the two conductors to the electrodes are unbalanced with more current (Amps) in the negative cable . How is this possible ?The electrodes are well grounded directly in conductive swimming pool water and I have wondered if there could be some kind of ground leak at play here .However , the power pack operates with a fully isolated step-down transformer , so there is no dc leak across to the household ac side , which has a single point p-np bridge to earth of the neutral conductor at the point of entry to the property . The ac side has a Ground Fault Detector Inhibitor safety device in line , which would pick up any imbalance on the ac side to cause a trip-out , which does not happen . The question has stumped many "experts" and I have not yet received a satisfactory explanation . Can you help ? Thanks.
- Dr G A Keen (age 77)
Cape Town South Africa
A:

That's a very interesting question. I can only give a partial, tentative answer. One thing that seems pretty clear is that the different temperatures in the two cables are not due to different currents flowing in the two cables. For the reasons you gave, it's not plausible that one of the cables is consistently partially shorted out by some ground problem. You could always double check via a careful (as in be careful for safety reasons) ammeter measurement of the current in each cable.

So I think that more heat is being generated at one of the electrodes than at the other. Since the copper cables are good heat conductors, that would show up in the cable temperatures. Of course, you'd expect different heat production at each electrode because different chemical reactions are occurring at the two electrodes.

What about the different chemical reactions on the surfaces of the two cables? (I wouldn't assume that it absolutely has to be oxidation, since there are various minor constituents of air that can react with metals.) Since the electrochemical potentials of the two electrodes are different, different reactions will occur on them. Also, most reactions are likely to go a bit quicker on the warmer cable.

Some obvious things to test to help pin down the explanations:

1) the dc current in each cable, as mentioned. Be safe about it!

2) Is the temperature of the cable hotter closer to the unit?

3) Is the surface corrosion faster near the unit?

Mike W.


(published on 01/11/2015)

Follow-Up #1: electrolysis issues solved

Q:
My Fluke clampmeter shows a uniform 21 Amps in each conductor of one electrolysis cell and a uniform 15 Amps in each conductor of a second electrolysis cell , both at about 12 V dc . So , you are quite correct , the conductors' Amps are accurately balanced and the heat must be coming from the cathode. Further evidence is that the cathode plates tend to be more "buckled" over years of use and corrode faster than the anode plates , (which show no corrosion despite producing the Chlorine gas ) . This again suggests that the cathode plates get hot despite the water flow . They also build up with deposited Calcium salts requiring dilute acid to clean them . They wear away to eventually become brittle and just fall to a powder upon touching them (approx 7 years use): time for expensive replacement . Modern units (with different plates ?) automatically reverse the polarity to a regular schedule to self clean the calcium depositions . I think you have fully answered my question . Thank you for your help .
- Dr. Keen
A:

Thanks for the follow up!

Mike W.


(published on 01/13/2015)

Follow-Up #2: electrolysis issues revisited

Q:
You mentioned "With age , the negative cable strands become blackened by oxidation , whereas the positive cable strands remain copper coloured...".If I understood the factors correctly, it seems to me that oxidation of the negative electrode has made it more resistive to electric current than the positive electrode--which, as you mentioned, is copper colored. In turn higher resistance at the negative electrode causes larger voltage drop across it than the positive electrode. This larger voltage drop appears as more heat at the negative electrode. And due to heat conduction, the negative cable attached to the negative electrode also gets warmer.
- Mehran (age 64)
Miami
A:

It sounds like the imbalanced heating starts right away, before either the electrode or the cable has had a chance to degrade much. It's more likely that it's due to the different reactions at the two electrodes, since that difference is always present.

Mike W.


(published on 01/13/2015)

Follow-up on this answer.