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Author: Subject: Zinc voltammetry: some problems
blogfast25
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[*] posted on 2-2-2023 at 05:32
Zinc voltammetry: some problems


I want to develop a method for voltammetric determination of a well known metal. I started off using the Daniell cell as a model:

$$Zn(s) | Zn^{2+}(aq) || Cu^{2+}(aq,sat) | Cu(s)$$

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:

EMF LR.png - 15kB

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]




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[*] posted on 2-2-2023 at 05:35


Cell and electrodes:


IMG_20230202_115551_273 (1).jpg - 59kB IMG_20230202_110156_968 (1).jpg - 54kB



IMG_20230202_110156_968 (1).jpg - 54kB

[Edited on 2-2-2023 by blogfast25]




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[*] posted on 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.





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[*] posted on 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.





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Cathoderay
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[*] posted on 7-2-2023 at 12:25


What is your separator?
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