I built a Daniell cell with a CuSO4/Cu reference electrode (about 7 mL electrolyte), 50 mL anodic compartment and used a plate of 99% Zn as
anode. Stock solutions (but not standardised) of ZnSO4 were prepared as 'analytes'.
Problems with reproducibility and time to equilibrium forced me to scale everything down, with a RE of about 1.5 mL and an anodic compartment volume
of about 5 mL: [see next post]
I also tried stirring and flushing with Ar welding gas but neither made any difference.
With this configuration I measured the EMF (at 19 C) for five Zn2+ stock solutions and their replica.
With Nernst we get:
$$\color{red} {\underbrace{E=E° -\dfrac{0.059}{n} \log_{10} \gamma [Zn^{2+}]}_{\text{Applicable at only 298K}}} \label{Nernst Short}$$
The following picture says more than a thousand words:
The problems are:
* poor reproducibility
* long equilibrium times: 10 mins or longer.
Both the intercept and the slope are in the right ballpark.
[Edited on 2-2-2023 by blogfast25]blogfast25 - 2-2-2023 at 05:35
Cell and electrodes:
[Edited on 2-2-2023 by blogfast25]RU_KLO - 7-2-2023 at 11:28
It is possible that the zinc plate is covered on oxide and should be polished? (from your pictures)
(once I tried to check for zinc on a zinc plate with CuSO4 solution (it contained another reagent - cannot recall which was. The test for zinc,is to
get a black spot when you add a drop of the solution on zinc)
I know for sure that it was Zinc but the test did not work (no black spot)
When I polished the Zinc, the test was succesful (black spot)
so maybe oxide maybe is interfering? (wearing of on each test, so different results)
Note: I do not have knowledge of Voltammetry.
RU_KLO - 7-2-2023 at 11:28
It is possible that the zinc plate is covered on oxide and should be polished? (from your pictures)
(once I tried to check for zinc on a zinc plate with CuSO4 solution (it contained another reagent - cannot recall which was. The test for zinc,is to
get a black spot when you add a drop of the solution on zinc)
I know for sure that it was Zinc but the test did not work (no black spot)
When I polished the Zinc, the test was succesful (black spot)
so maybe oxide maybe is interfering? (wearing of on each test, so different results)
Note: I do not have knowledge of Voltammetry.
Cathoderay - 7-2-2023 at 12:25
What is your separator? Bedlasky - 11-2-2023 at 05:43
Are you sure that this is voltammetry? It look like potentiometry to me. If I understand it correctly, you just measured how potential changed with
decreasing Zn2+ concentration. If you want to do voltammetry, you must apply changing pontential and measure change in current.
Are you sure that this is voltammetry? It look like potentiometry to me.
Nice experiment though. Do you consider use similar setup for potentiometric titration?
Voltammetry/potentiometry... you know?
No, it's potentiometry, definitely.
I wanted to use the setup for standard addition and titration methods but with such poor reproducibility that's out of the question.
I'm now waiting for an Ag/AgCl ref. electrode, maybe it will give better results?
[Edited on 11-2-2023 by blogfast25]blogfast25 - 11-2-2023 at 09:03
An Aga agar gel, 1 M NaCl, at the bottom of the Cu ref. electrode.blogfast25 - 24-2-2023 at 10:52
The new Ag/AgCl reference electrode has arrived:
So the experiment above was repeated, albeit with a slightly better grade of ZnSO4.
The results were very much similar to the first run:
* reproducibility was slightly better but not brilliant
* equilibrium times remained long: 5 mins and longer
The slope was -8.2+/-2.2 mV when acc. Nernst (see above) it should be about 28 mV. I can't explain the discrepancy. The lower slope
means of course reduced sensitivity.