Experimental method question - electrochemistry

[rwooduk]

I am looking at a paper (ref below) that uses a method to precipitate calcium carbonate on a steel pipe. However, the paper gives no method because the reference it uses is linked to a thesis that has not been released. The setup is below...

Now, I kind of understand that it uses a method of electrochemical reactions and that the temperature and the voltage can be increased to increase the amount of deposit. But I am confused by several elements:

1. For electrochemists: The steel pipe in the solution is clearly the "working electrode" at -1.6 V, then there's a copper wire at 0 V down the centre of the tube. But what are the red and green wires in the diagram of the working electrode? Are two wires needed to create the -1.6V?

2. For chemists: What is the purpose of the condenser, salt bridge and diffuser?

Any help trying to understand this system would be greatly appreciated.

Ref: Lais, H., et (2019). "Numerical investigation of design parameters for optimization of the in-situ ultrasonic fouling removal technique for pipelines." Ultrasonics Sonochemistry 56: 94-104.al.

 

[Borek]

I don't see any red nor green wires.

Electrochemical setups often use three electrode systems, where the reference electrode serves as a reference only - that is, there is (almost) no current flowing through it, current flows through the other two electrodes. This way the potential of the working electrode can be still controlled, but the reference (which is rather fragile) doesn't have to work under load and is much more reliable/stable/durable.

 

[rwooduk]

I don't see any red nor green wires.

Electrochemical setups often use three electrode systems, where the reference electrode serves as a reference only - that is, there is (almost) no current flowing through it, current flows through the other two electrodes. This way the potential of the working electrode can be still controlled, but the reference (which is rather fragile) doesn't have to work under load and is much more reliable/stable/durable.

Hi Borek, thank you for your reply. Apologies the images were not the best, I have re-uploaded below and circled the red and green wires.

I see, I was not aware of this three electrode system, I will look into this further, thank you for the explanation. I plan to set up a similar experiment on a smaller scale and don't want to electrocute myself / damage equipment / undergraduates who share the lab, so need to consider this aspect carefully.

 

[Borek]

OK, I see these wires - and to be honest I have no idea what role they are intended to play. Unless it is just a connection to the working electrode.

 

[rwooduk]

OK, I see these wires - and to be honest I have no idea what role they are intended to play. Unless it is just a connection to the working electrode.

Yes, I believe those two wires must give the working electrode (the pipe) the -1.6 V bias. Not sure what the green meter thing is (in left image of OP in front of the potentiostat), maybe to get a second reading (2 electrode system). But I can’t see the point in that. Also still a little unsure on the diffuser. Although other experiments I have seen use a rotating working electrode to increase calcium carbonate deposit. They suggest a more turbulent flow increases deposition. So maybe the air from the diffuser has a similar effect.

I would be very interested to know your opinion on the concentration of calcium carbonate solution that you think could be used. Clearly it would have to be supersaturated to achieve a good layer of calcium carbonate deposit on the working electrode (on the inside of the pipe). I need thick scale (> 1 mm) but not sure it will work this well.

 

[Borek]

First of all, I have problems making my mind about what is the chemistry behind scale formation here. The only thing I can think of is that it is pH related, with the reaction being water electrolysis.

Have you tried to contact authors to ask for the original thesis?

 

[rwooduk]

First of all, I have problems making my mind about what is the chemistry behind scale formation here. The only thing I can think of is that it is pH related, with the reaction being water electrolysis.

Have you tried to contact authors to ask for the original thesis?

Here is an excerpt from a paper by Neville et al. which considers a rotating electrode system:

Electrochemical Aspects of Surface/Solution Interactions in Scale Initiation and Growth

Scales also form in cathodic protection systems where, at the cathode of the CP system, the generation of excess OH- ions due to the reduction of oxygen (equation 1 - below) occurs at the metal-electrolyte interface.

$O_{2}+ 2H_{2}O+ 4e^{-}\rightarrow 4(OH)^{-}$

Increases of pH, in the fluid adjacent to a cathodically protected steel surface, to values of greater than 10.9, have been predicted [4] and verified experimentally [5]

In that paper an "index of supersaturation" is given for the solution as 235 (need to figure out how this relates to concentration of the "calcium carbonate solution"). The bias used is -1 V. Further in this paper NaOH is added to solution to maintain pH.

So I am assuming the experiment in the OP works in a similar way. The method is not given in the paper. I have been in contact with the author and she said it is in her thesis which is currently embargoed (she worked with a company). No information is given on the thickness of the scale acquired.

 

[ZapperZ]

I am looking at a paper (ref below) that uses a method to precipitate calcium carbonate on a steel pipe. However, the paper gives no method because the reference it uses is linked to a thesis that has not been released. The setup is below...

View attachment 251884

Now, I kind of understand that it uses a method of electrochemical reactions and that the temperature and the voltage can be increased to increase the amount of deposit. But I am confused by several elements:

1. For electrochemists: The steel pipe in the solution is clearly the "working electrode" at -1.6 V, then there's a copper wire at 0 V down the centre of the tube. But what are the red and green wires in the diagram of the working electrode? Are two wires needed to create the -1.6V?

2. For chemists: What is the purpose of the condenser, salt bridge and diffuser?

Any help trying to understand this system would be greatly appreciated.

Ref: Lais, H., et (2019). "Numerical investigation of design parameters for optimization of the in-situ ultrasonic fouling removal technique for pipelines." Ultrasonics Sonochemistry 56: 94-104.al.

Have you tried contacting the authors or corresponding author directly? Might as well get it directly from the horse's mouth.

Most authors would be thrilled that someone is reading their papers and wants to know more. So your e-mail asking about their setup would be taken as a compliment.

Zz.

 

[Borek]

Have you tried contacting the authors or corresponding author directly?

I have been in contact with the author and she said it is in her thesis which is currently embargoed (she worked with a company).

 

[ZapperZ]

I read about the thesis being under "embargoed", which makes no sense in terms of academic openness. However, I find it hard to believe that she, or other authors (especially her supervisor if she was the graduate student) can't answer the questions being asked about the paper.

Zz.

 

[rwooduk]

I read about the thesis being under "embargoed", which makes no sense in terms of academic openness. However, I find it hard to believe that she, or other authors (especially her supervisor if she was the graduate student) can't answer the questions being asked about the paper.

Zz.

To be honest they have been very helpful in answering some of my questions, but I think they are keen for a collaborative effort, so they are not telling me everything. Which I suppose is fair, if I had spent 10 years on a project perhaps I would like more back. Unfortunately this is not my decision to make and my hands are somewhat tied. But certainly, if the situation was reversed and if it didn't go against the company I was working for, I would give them all I could. Science works best when we all work together! But the higher I go the more I see the politics :-/

btw one of my colleagues made a comment "Why is the calcium carbonate solution blue? Perhaps there is copper in there?", I find different perspectives fascinating!

Anyway, I will figure it out! Thanks for the help!

 

[Borek]

Copper counter electrode, it has to corrode, this part is rather obvious :)

 

[rwooduk]

Copper counter electrode, it has to corrode, this part is rather obvious :)

Ahh, of course!

 

[rwooduk]

I'll update this for completion and because I have one more question. So we have:

$O_{2}+ 2H_{2}O + 4e^{-} \rightarrow 4OH^{-}$

$2H_{2}O + 2e^{-} \rightarrow 2OH^{-} +H_{2}$

The hydroxide ions at the liquid metal interface increases the pH and leads to precipitation of $CaCO_{3}$ on the electrode surface:

$HCO_{3}^{-}+ OH^{-}\rightarrow CO_{3}^{2-} + H_{2}O$

$Ca^{2+}+ CO_{3}^{2-}\rightarrow CaCO_{3(S)}$

Now, I thought the diffuser of the experiment was to pump more oxygen into the solution so there could be more reduction of oxygen by the electrons and more hydroxide ions formed. But (according to the paper of the above equations) $CO_{2}$ is used to dissolve the calcium carbonate into solution i.e. to form the calcium carbonate solution. When the calcium carbonate is fully dissolved the solution is left to rest until a pH of 7 is reached (too low for spontaneous nucleation).

Anyone have any thoughts on if the diffuser is used for $O_{2}$ or $CO_{2}$? If more $O_{2}$ was dissolved into solution would this cause more calcium carbonate production, as I originally thought?

 

[Borek]

My bet would be on CO₂. While there is plenty of oxygen in any experimental setup, concentration/partial pressure of CO₂ is (even if we - as in "civilization" - do our best to make it higher) still too low for substantial decrease of pH, necessary to substantially increase CaCO₃ solubility.

You can get a reasonable estimate of the equilibrium concentrations before electrolysis by taking into account all involved equilibria (water autodossiciation, Ksp for CaCO₃, two dissociation steps for carbonic acid, one of the using partial pressure of CO₂), all mass balances and charge balance, then solving this system of equations. Quite a standard procedure, rather nasty to do by hand, but numerically trivial. Actually you should also take ionic strength and activities into account, makes calculations a bit harder (still, not that difficult from the numerical POV).