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Offline La-Lu

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{cathode and anode}
« on: November 16, 2016, 05:22:01 AM »
Hello everyone

I really hope someone can help me solve this thing because it's really making my head hurt  ???

Ok so first let's assume I am not very good at electrochemistry  ::). I have this situation:

I am setting up a galvanic system to monitor pH changes.
I have a carbon rod and a copper wire, each one in a beaker with a salt bridge to connect them.
They are connected to a battery this way: carbon rod to RED wire and copper to BLACK wire.
I turn the battery on and the solution with the carbon rod turns ACIDIC and the one with the copper BASIC.

Now I am going totally nuts over: which one is the "cathode" and which is the "anode"???

I thought the copper would be the anode but then why is the acidic side on the carbon rod and why is the carbon connected to the red wire for this to happen?


I would be so grateful if anyone could help a little on the how and the why. I know this is probably very basic stuff but please don't be rude.

Thank you in any case.... :-\

*MOD Edit -- a useful title*
« Last Edit: November 16, 2016, 05:34:26 AM by Arkcon »

Offline AWK

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Re: {cathode and anode}
« Reply #1 on: November 16, 2016, 05:45:36 AM »
AWK

Offline La-Lu

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Re: {cathode and anode}
« Reply #2 on: November 16, 2016, 07:37:37 AM »
https://www.wikiwand.com/en/Electrolysis_of_water

Thank you so much that was actually quite helpful, it initiated a new search with different keywords so I have now made a nice graph and clarified everything about what's supposed to be going on!

The one thing I still don't get is why am I getting a drop in pH at the carbon rod when it's connected to the red wire??? I am pretty positive it should be the other way round still when I tried it did not work  ???

Maybe I am just losing my mind

Offline La-Lu

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Re: {cathode and anode}
« Reply #3 on: November 16, 2016, 07:38:43 AM »
By the way thank you for the title edit, I think I'm just so desperate I got carried away  ;D

Offline mjc123

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Re: {cathode and anode}
« Reply #4 on: November 16, 2016, 09:01:27 AM »
You have not set up a galvanic cell. You have set up an electrolytic cell. Do you know the difference?
What is the cathode and what is the anode in an electrolytic cell?

Offline La-Lu

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Re: {cathode and anode}
« Reply #5 on: November 17, 2016, 08:55:49 AM »
Oh I see! Thank you for your reply. I am following the work another student did last year and she called it galvanic so I just assumed she was correct. Is it electrolytic because it's not spontaneous?

So cathode > reduction > gain of electrons > acidic environment > carbon rod > BLACK wire?
and anode > oxidation > loss of electrons > basic environment> copper > RED wire?

But when I connect the carbon to the black wire, pH goes up  :-[

Offline La-Lu

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Re: {cathode and anode}
« Reply #6 on: November 17, 2016, 09:18:38 AM »
Oh I see! Thank you for your reply. I am following the work another student did last year and she called it galvanic so I just assumed she was correct. Is it electrolytic because it's not spontaneous?

So cathode > reduction > gain of electrons > acidic environment > carbon rod > BLACK wire?
and anode > oxidation > loss of electrons > basic environment> copper > RED wire?

But when I connect the carbon to the black wire, pH goes up  :-[

So I just checked and the previous work I mentioned above involved a GALVANOSTAT? So I am guessing they were doing two different things, experiments with the battery (electrolytic) and then doing something similar but with the galvanostat (galvanic)? They literally say they use a galvanic cell est up to monitor change in pH and then go on to describe the battery experiment and then the same experiment but with a galvanostat instead.

You wouldn't say I knew anything about chemistry would you  ??? I'm really confused

Sigh

Offline mjc123

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Re: {cathode and anode}
« Reply #7 on: November 17, 2016, 12:24:19 PM »
Quote
Is it electrolytic because it's not spontaneous?
Yes, if the reaction is driven by passing a current from an external source, it's electrolytic. In a galvanic cell the spontaneous chemical reaction causes a current to flow through an external circuit.
The use of a galvanostat doesn't mean it's a galvanic cell, it just means that the current is maintained at a constant value.
What is in your solutions? Water? Acid? Alkali? Non reactive salt? Copper sulfate? This is a rather important point that you haven't mentioned at all!

Offline La-Lu

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Re: {cathode and anode}
« Reply #8 on: November 17, 2016, 04:03:40 PM »
Quote
Is it electrolytic because it's not spontaneous?
Yes, if the reaction is driven by passing a current from an external source, it's electrolytic. In a galvanic cell the spontaneous chemical reaction causes a current to flow through an external circuit.
The use of a galvanostat doesn't mean it's a galvanic cell, it just means that the current is maintained at a constant value.
What is in your solutions? Water? Acid? Alkali? Non reactive salt? Copper sulfate? This is a rather important point that you haven't mentioned at all!

Hi thank you for replying again. I see the point now. It's definitely electrolytic  :D I might get the courage and ask their supervisor some clarification on the terms that they used hoping I don't pass as a complete idiot!

I am using KCl 0.1M on both sides. Should I also dare ask her why the flow only appears to be going the right way if I switch the wire colours? (as mentioned solution with cathode will go acidic when connected to red wire which I think is the wrong colour?)

Thank you so much for helping this has already gone such a long way for me  :-\

Offline mjc123

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Re: {cathode and anode}
« Reply #9 on: November 18, 2016, 07:50:38 AM »
Quote
as mentioned solution with cathode will go acidic when connected to red wire which I think is the wrong colour?
I repeat, what is the cathode in an electrolytic cell?

Offline La-Lu

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Re: {cathode and anode}
« Reply #10 on: November 18, 2016, 08:33:06 AM »
Quote
as mentioned solution with cathode will go acidic when connected to red wire which I think is the wrong colour?
I repeat, what is the cathode in an electrolytic cell?

Is it not where electrons are gained?the carbon rod? so negative side and black wire?

I just talked with the supervisor and she literally said to my face this is a galvanic cell because it uses a galvanic system. Don't know what to think anymore  :'(

Offline mjc123

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Re: {cathode and anode}
« Reply #11 on: November 18, 2016, 10:48:31 AM »
Why do you assume the carbon rod? The cathode is the negative electrode - whichever electrode is connected to the negative terminal of the battery (presumably black wire). Whether that's the carbon or the copper, that's the cathode, and that's where reduction will take place.
I don't know what "uses a galvanic system" means. It may just mean that it's done under galvanostatic (constant current) rather than potentiostatic (constant voltage) conditions. That wouldn't make it a galvanic cell. The dissociation of water is not spontaneous.

Offline La-Lu

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Re: {cathode and anode}
« Reply #12 on: November 21, 2016, 04:07:14 AM »
Why do you assume the carbon rod? The cathode is the negative electrode - whichever electrode is connected to the negative terminal of the battery (presumably black wire). Whether that's the carbon or the copper, that's the cathode, and that's where reduction will take place.
I don't know what "uses a galvanic system" means. It may just mean that it's done under galvanostatic (constant current) rather than potentiostatic (constant voltage) conditions. That wouldn't make it a galvanic cell. The dissociation of water is not spontaneous.

Hi sorry for disappearing I know you are helping and I should reply more promptly.

I assume the carbon rod because that's what we want, which is the carbon to be the cathode. So more than assuming is choosing I think? But that should be ok if as you say it's just a matter of deciding?

Ok I guess she said it's galvanic because later in the experiment we will be using a galvanostat instead of a battery. But still I don't know what I'll write in my report. Whether with the battery is electrolytic and THEN with the galvanostat is galvanic. I will figure something out!

I am back in the lab tomorrow and I will try and make something work. I bought a new battery and I will be armed with all the notes that you guys helped me put down. Hopefully I'll manage to make sense of this and update  :) thank you so much again for replying.

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