[Orcio_87]

In order to calibrate the ORP electrode i did a few measurements. All measurements were done in presence of the air.

1. Distilled water (pH measured with a glass electrode = 7,00, temperature = 20,7 C).

Reaction:

4 H⁺ + O₂ + 4 e^- ----> 2 H₂O

E calculated = E₀ + [59 / 4] log [H⁺]⁴ x pO₂ = 806 mV

E measured = 278 mV

2. A few pieces of copper wire (about 0.5 g) submerged in about 0.1 M CuSO₄

pH measured with a glass electrode = 1,97 (~2), temperature = 20,5 C.

Reaction:

Cu + 4 H⁺ +O₂ + 2 e^- ----> Cu²⁺ + 2 H₂O

Ionic strength and Cu²⁺ activity calculated here (except of activity H⁺ - taken from the pH):

https://www.lenntech.com/calculators/activity/activity-coefficient.htm

E calculated = E₀ + [59 / 2] log [Cu²⁺] x pO₂ x [H⁺]⁴ = 583 mV

E measured = 587 mV

3. Distilled water with submerged piece of copper wire

pH measured with a glass electrode = 4,50, temperature = 20,7 C.

4 H⁺ + O₂ + 4 e^- ----> 2 H₂O

E measured = 385 mV

4. 0.1 M CuSO₄ solution saturated with a CuCl

pH = 1,97 (I assume that addition of CuCl doesn't changed the pH), temperature 21,0 C.

Reaction:

Cu²⁺ + e^- ----> Cu⁺

Ionic strength and activites calculated here: https://www.lenntech.com/calculators/activity/activity-coefficient.htm

E calculated = E₀ + 59 log (Cu²⁺ / Cu⁺) = 159 mV + 103 mV = 262 mV

Potential need some time to stabilize (it was dropping !), but after all, it comes down to about 159 mV.

- - - - - - - - - - - - - - - -

Are these calculations (point 2. and 4.) correct ?

Why does difference in point 4. (calculated and measured potential) is so big (above 100 mV) ?

Also - experiments with a distilled water (1. and 3.) have any physical sense ?

[Borek]

Measurements in distilled water are always extremally difficult as it has very high specific resistance.

pH of distilled water is rarely 7.00, CO₂ dissolves pretty fast and lowers pH. But see above, I wouldn't trust this number.

[Orcio_87]

To be honest I added some tap water to set pH to 7,00, but OK - high resistance will lower potential.


What about Cu²⁺ / Cu and Cu²⁺ / Cu⁺ system ?

pH of the solution = 1,97 (~ 2) - is it too high to omit 4 H⁺ + O₂ ---> 2 H₂O reaction (E = 1229 mV) ?

Because...

If it is only a reduction of Cu²⁺:

Cu²⁺ + 2 e^- ---->  Cu

then - calculated E (for 0.1 M CuSO₄) is about E = 284 mV (which is smaller than measured E = 587 mV).

But - if it is a reaction:

2 Cu + 4 H⁺ +O₂ ----> 2 Cu²⁺ + 2 H₂O

then - calculated E is:

E = E₀ + 59 / 4 log [Cu²⁺² x pO₂ x H⁺⁴] = 892 - 59/4 x 12,23 = 712 mV

which is bigger, but slightly closer than measured (E = 587 mV).


Again - Cu²⁺ / Cu⁺ system (pH = 1,97).

If this is only a reaction:

Cu²⁺ + e^- ----> Cu⁺

then calculated E is:

E = E₀ + 59 [Cu²⁺ / Cu] = 159 + 59 x 1,742 = 262 mV

but if this is reaction:

4 Cu⁺ + 4 H⁺ +O2 ----> 4 Cu²⁺ + 2 H₂O

then calculated E is:

E = E₀ + 59/4 log [Cu²⁺⁴ x pO₂ x H⁺⁴ / Cu⁺⁴] = 1070 + 59/4 x 5,36 = 1149 mV

while measured E has value about 160 - 180 mV.



I know that there are ORP standards for sale (Fe³⁺ / Fe²⁺ or quinhydrone) but...

What will be measured with the electrode is the water in closed circulation system, and - one day someone may ask - does the electrode was ever checked with some external standard ?

And - for that person it will be nice to see, that THERE IS a difference - between the field measurement and - measurement of a standard in an opened beaker.



And EVEN if I assume that air (oxygen) is don't important then still - why does Cu²⁺ / Cu system measured potential is too big, while Cu²⁺ / Cu⁺ system - too small in comparison with the calculations ?

[Borek]

Ignoring everything else for now - I would expect Cu⁺ to reduce dissolved oxygen, assuming there is still still CuCl present potential should be dictated by the Cu²⁺/Cu⁺ system (at least initially, in the long run and in contact with air oxygen is in a huge excess).

[Orcio_87]

I tried it once again. I made a solution of 0.1 M CuSO₄ (this time it had pH = 1,74) saturated with CuCl.

Measured potential was 154 mV (previously 159 mV). After some time (5- 10 min.) potential was increasing to 155 - 160 mV, BUT - after shaking the flask it was dropping to the previous level - 154 mV.

I calculated potential for a reaction (Cu²⁺ + e^- ----> Cu⁺) to be 262 mV (activity of Cu²⁺ = 0,0168, activity of Cu⁺ = 0,000304), so there IS a difference (about 110 mV), but I believe that this is the right method and right way of thinking.

Of course I will think about something more, because calibration on the basis on one reaction is uncertain.

Thanks for the help

[Borek]

Measured potential was 154 mV (previously 159 mV).

What do you use for reference?

[Orcio_87]

If you ask about reference electrode - the electrode has internal silver chloride electrode as a reference.

But the device displays value (mV) calculated vs standard hydrogen electrode.

[Orcio_87]

Is it possible that solubility of CuCl increases with lowered pH ?

Because - concentration of CuSO₄ in both experiments was meant to be the same (0.1 M), but - second time pH was lower (pH = 1,74 vs 1,97).

If so - this means 5 mV difference on every 0,23 pH unit, which gives about 268 mV for pH = 7,00 vs. E calculated = 282 mV.

But - is it so ? I think that second time pH was lower because CuSO₄ crystals were more dry, and thus concentration of CuSO₄ was a bit bigger.

[Orcio_87]

I think that low pH (pH not HCl or other chlorides) increases solubility of CuCl. It is because CuOH is very weak base (pKa = 14) and hydrolysis of CuCl is very strong, so H⁺ will shift hydrolysis (or rather - dissolution of CuOH) to the left:

CuCl + H₂O ----> CuOH_(s) + HCl

Overall - electrode is OK.

[Borek]

On the second thought initial concentration of oxygen (assuming saturated solution in contact with atmosphere) is larger than the concentration of Cu⁺ in saturated solution in contact with CuCl (that's ignoring all side reactions). Can you try to do the measurements using oxygen free solution? At least use freshly boiled water and limit its contact with the air as much as possible.

[Orcio_87]

Is it larger ? Solubility of CuCl = about 0,47 mmol / L. Solubility of O₂ (temp 25 C) = about 7 mg / L --->  0,22 mmol / L.

In my opinion everything was OK. Measured potential was about 154 mV. Even if it started rising by 5- 10 mV I shaked the flask and in a few seconds the potential falls back to 154 mV.

Method is OK.

I'm just curious - is it possible that low pH increases solubility of CuCl.

Because the first time I got the result 159 mV for 0.1 M CuSO₄ / CuCl system (pH = 1,97), and - few days later - 154 mV for the same system (pH = 1,74).

I think it's because the CuSO₄ crystals were more dry.

[Borek]

Nah, you are right, 40 mg is for 1 bar, not for 0.21 bar.

Still, these are close enough to add problems.

I have just run 0.1M CuSO₄/2e-4 M CuCl through an equilibria program (ignoring ionic strengths/activities, they can change the result, but not by over 2.5 pH unit), your pH looks surprisingly low.

[Orcio_87]

I made CuSO4 from 99 % copper wire and moderate H₂SO₄ (2,3 H₂SO₄ : 1 Cu), but - the reaction was very, very slow (even after heating) so I had to add some 30 % H₂O₂ (which means that some of the H₂SO₄ was left unused).

After cooling down (when CuSO₄ x 5 H₂O crystals drops out from the solution) all of the solution was poured away, and crystals left to dry.

Maybe small portion of H₂SO₄ was trapped inside the crystals, during the cooldown ?


But what about Cu²⁺ concentration (0,0188 M) ? Is it true that 80 % of CuSO₄ is undissociated ?

[Borek]

Different sources give different values of the stability constants for Cu²⁺/SO₄^2- system, they do agree that they are surprisingly high though.

[Orcio_87]

Interesting. I was asking because I expected degree of dissociation to be 90- 95 %, and this is merely 19 %.