# Is it possible to graph the equivalence of p[Cu²⁺] with pH?

In no way I am a chemistry major, so please forgive me if my question seems stupid. I just need a way to find the "equivalent $$\mathrm{pH}$$" of $$\mathrm p[\ce{Cu^2+}]$$ if that makes any sense to you.

I'm making a paper about a transistor that can sense ions (ion-sensitive field-effect transistor); i.e. it's a potentiometric transistor. I was able to simulate voltage values with respect to $$\mathrm{pH}$$ activity in a simulation software named LTspice, but in order for this paper to be passable to whoever shall evaluate it, I must compare my data with a preexisting experimental data.

The only experimental data that I have is from my co-advisee. He measured the voltage values with respect to $$\mathrm p[\ce{Cu^2+}]$$ $$(\ce{CuCl2 · 2 H2O}).$$ I think it wouldn't be proper to compare the voltage vs $$\mathrm{pH}$$ graphs and voltage vs $$\mathrm p[\ce{Cu^2+}].$$

So, I figured that if I can express $$\mathrm p[\ce{Cu^2+}]$$ to its equivalent $$\mathrm{pH}$$ (I don't think this is possible and makes any sense), then I'd be able to compare the two graphs.

• Would it make sense to compare simply on the basis of charge, ie 1 Cu2+ = 2 H+ ? – Andrew Jun 9 at 15:50
• I think so? Then I'll just get the power of both Cu2+ and H+ to form a graph for their equivalence? – TTbulaski Jun 9 at 15:51
• No it wouldn't. Sensitivity to different ions would not be equal. – Mithoron Jun 9 at 19:05

TTbulaski, I gather that you are developing a sensor for copper ions whose output is proportional to -log [Cu(II)]? I believe this is what you are trying to say.

What you can test is whether the output of your sensor gives any linear relationship between actual Cu concentration and the antilog of your sensor signal or not. Plot the Cu ion concentration on the x-axis and the anti-log of your sensor signal on the y-axis? Do you see a linear relationship?

pH and -log [Cu(II)] connection is pretty tricky and it depends what level of depth you want to go to. There is no point in looking for a similarity between the two measurements. People have done calculations to show what species of Cu is present as a function of pH. It is called speciation diagram of copper.

P.S. The most important thing to test any metal ion sensor is something called as "selectivity". No analytical chemistry reviewer will accept the paper until and unless you show how selective is your sensor. For example if you have Zinc or Cd or other transition metals, do you still get a signal for copper in a correct way?

Not long time ago, NASA did a mistake in ion-selective electrodes. They designed or used a nitrate selective electrode and it turned out that is more selective towards perchlorate ion. It was an interesting news.

• Regarding the second paragraph, yes I have both the antilog of the sensor signal and the normal, unaltered graph. The relationship with the Cu2+ is linear at R^2 = .987. Regarding the 3rd paragraph: so I assume you're saying it is not proper to use the experimental data which makes use of copper ions, to verify a simulated circuit which takes pH as "input"? – TTbulaski Jun 10 at 3:08
• Yes, I think so. Do not try to connect Cu(II) and pH by any means. – M. Farooq Jun 10 at 4:19
• Welp, I guess I'll have to go to the university laboratory despite the ongoing lockdown. Thank you for your answer! – TTbulaski Jun 10 at 7:04