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Topic: Measurement of solution electric potential  (Read 466 times)

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Offline Orcio_Dojek

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Measurement of solution electric potential
« on: September 04, 2021, 12:40:10 PM »
For every metal element there is a diagram of its stability in the solution in dependence of solution pH and its potential:

https://www.researchgate.net/publication/320094277/figure/fig1/AS:543823333781505@1506668994825/Potential-pH-diagram-for-V-H2O-system-at-298-K-with-activities-of-dissolved-vanadium-of-1.png

Question is...

The device (pH-meter) measures the voltage of solution, but (when platinum electrode is attached) - electric potential shown be device is the same as in reality  ?

Because....

E = E0 + [RT / zF] ln Cox / Cred

so potential depends on number of electrons (z) changed in time of reaction (for pH meter: 2 H+ + 2 e-  :rarrow: H2), but in reality no one knows WHAT is in the solution (Fe3+, Fe2+, V3+, Cr3+, O2 and so on....).

In my opinion - the device measures potential per se, and "z" (number of electrons) is don't important unless someone want to calculate concentration of oxidizer(s).

I just want to make myself sure..

Offline marquis

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Re: Measurement of solution electric potential
« Reply #1 on: September 04, 2021, 07:05:46 PM »
In most situations, pH measures hydrogen ion concentration.  But are there cases where this doesn't hold true? Sure. Some pH electrodes are bad at pHs above about 11.  The usual reason- alkalai metal error.  The electrode starts reacting to alkalai metals more than pH.  Newer electrodes are better at correcting for this, but the problem is still there. When this was found out, Ion selective electrodes ( ISEs) were developed. I think that's basically what you are asking. 

Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #2 on: September 05, 2021, 03:42:35 AM »
@marquis pH meter (device with a glass electrode) measures solution potential (voltage), and on the basis of reaction "2 H+ + 2 e- ----> H2" calculates concentration of H+ and pH.

Now we take the glass electrode away and put a redox electrode (platinum).

Device still measures potential (voltage), but..

Does the voltage shown by the device is the real voltage or - value is lowered because of Fe3+, V3+, Cr3+, O2 (z = 3, 4 instead of 2) ?

I am asking because I am confused here.

Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #3 on: September 05, 2021, 01:09:28 PM »
Quote
and on the basis of reaction "2 H+ + 2 e- ----> H2" calculates concentration of H+ and pH
I meant - device measures potential on the basis of the reaction: H+(aq) + Na+(glass) ---> Na+(aq) + H+(glass).

Offline marquis

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Re: Measurement of solution electric potential
« Reply #4 on: September 05, 2021, 08:06:06 PM »
A pH meter  is a very high input impedance voltmeter( usually 10 to the 10 th ohms input impedance of higher).  They do the conversion to pH, but some display straight millivolts, so you can calculate concentrations with ion selective electrodes.  The glass electrode requires a counter, reference electrode.  These days, the two electrodes are usually built into the same piece of glass.  The job of these two items is to convert hydrogen ion concentration into an electrical signal.  And there is more to it than just a piece of glass.  It can be an intensive field in itself.  If you replace the glass electrode with platinum, you can do conductivity.  But not pH.

Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #5 on: September 06, 2021, 05:36:31 AM »
Quote
If you replace the glass electrode with platinum, you can do conductivity.  But not pH.
I don't want to measure conductivity, I want to measure potential (voltage) - and I know it is possible.

I just wanted to know - will the outcome will be lowered because of Fe3+, Cr3+, O2.

In my opinion NO. Because electrode measures potential per se and value "z" is don't important unless someone will want to calculate conc. of oxidizer.


Offline marquis

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Re: Measurement of solution electric potential
« Reply #6 on: September 06, 2021, 07:50:59 AM »
You would need to contact the specific electrode makers to be sure.  I've used a number of kinds of pH electrodes, in some pretty stressful situations, and haven't seen much in the way of interferences.  The most common interference is alkali metals error at pH s of 11 and above.  And clay or soil solutions can badly affect pH electrodes life. The specialty electrodes can usually take care of both pretty well (of course you pay for it). As far as measuring high pH (11 and above), most places do dilutions.

Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #7 on: September 15, 2021, 03:14:47 PM »
Now that I have desired electrode I am thinking about some calibration.

My thoughts what can be used:

1. 1 M solution of CuSO4.

Potential of electrode = potential from reduction of Cu2+ to Cu (337 mV) + potential from reduction of atmospheric oxygen to H2O (800 mV at pH = 7) = 1137 mV

(CuSO4 will hydrolyse so potential can be altered by one or two mV due to reduction of H+ to H2)

2. Distilled water (pH = 7,00).

Potential = potential from reduction of O2 to H2O (800 mV at pH = 7) + potential from reduction of H+ to H2 (-277 mV at pH = 7,00 and atmospheric hydrogen pressure = 0,000055 atm) = 523 mV.

I'm just curious - does this have some sense ?

I mean - potentials can be added (Cu2+ and O2 from the air), and..

Potential from the H+ can be measured, because concentration of air hydrogen is very low (0,000055 atm) ?

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Re: Measurement of solution electric potential
« Reply #8 on: September 16, 2021, 03:19:50 AM »
No, potentials in solution are not additive. Potentials are additive when they are in series, in solution they are in parallel.

But: solution has one potential only. If there are several redox pairs with different potentials substances will react till they get to equilibrium.

pH electrode doesn't measure potential related to the H+/H2 reaction. It measures potential related to the reaction of adsoprtion of H+ on the glass electrode surface (note: it is not a redox pair). This is not the same potential you will observe when you put Pt wire in the same solution.
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Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #9 on: September 16, 2021, 03:57:39 PM »
Quote
But: solution has one potential only. If there are several redox pairs with different potentials substances will react till they get to equilibrium.
I am asking because what will be measured with the electrode is the water in closed circulation system.

But - I think that during process of the calibration the air oxygen will increase measured potential.

I know that for calibration people are using K4[Fe(CN)6] / K3[Fe(CN)6] solution (somewhat resistant to the air ?), but I want to use that what I have (Cu2+ / Cu for example).

Any hints how to calculate what part of the potential (Cu2+ / Cu) comes from the air oxygen ?

Quote
pH electrode doesn't measure potential related to the H+/H2 reaction. It measures potential related to the reaction of adsoprtion of H+ on the glass electrode surface (note: it is not a redox pair). This is not the same potential you will observe when you put Pt wire in the same solution.
But isn't true that SHE is the platinum electrode placed in the 1 M HCl solution under 1 atm H2 pressure ?

H+ and H2 is present in water we drink and air we breathe (less than 1x10-7 M and 5 x 10-5 atm) - so IN THEORY - this should lower result of the potential.

In practice - I think that platinum electrode is not sensitive to H+ as the glass electrode, so won't be affected by that low H+ concentration.

Offline Orcio_Dojek

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Re: Measurement of solution electric potential
« Reply #10 on: Yesterday at 03:03:21 PM »
As of Cu2+ / Cu - O2 / H2O system I wrote equation (2 Cu + 4 H+ + O2 ---> 2 Cu2+ + 2 H2O) and calculated potential E = 132 mV mV for pH = 7,00 and O2 pressure = 0,21 atm.

As of O2 / H2O (air - distilled water) system i calculated E = 806 mV.

I hope that this will be sufficient for the calibration.
« Last Edit: Yesterday at 03:16:47 PM by Orcio_Dojek »

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