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So, the lab work is this. I made an AgCl electrode of a silver wire by coating it with AgCl. This was done by electrolysis by immersing the wire in HCl and using platinum as a counter electrode and applying current to the system. After that, the AgCl electrode was immersed into HCl dilutions of different molalities and the cell potential was measured against a hydrogen gas electrode (normal hydrogen electrode NHE).

The measurements were done at temperature of 25 °C. The results of the measurements are as follows:

\begin{array}{llc} \hline c\,(\pu{mol/kg}) &\sqrt{c}\, (\sqrt{\pu{mol/kg}}) & \mathrm{E\,(V)} \\ \hline 0.001 & 0.032 & 0.6322 \\ 0.002 & 0.045 & 0.6815 \\ 0.004 & 0.063 & 0.6497 \\ 0.006 & 0.077 & 0.6280 \\ 0.008 & 0.089 & 0.6153 \\ 0.010 & 0.100 & 0.6049 \\ 0.020 & 0.141 & 0.5700 \\ 0.050 & 0.224 & 0.5030 \\ \hline \end{array}

Because the cell potential can be calculated by (as the pressure gradient of the hydrogen electrode was 0): $$ E(m)= E^0 + \frac{2ART}{F} \sqrt{\frac{m_{\text{HCl}}}{m^0}} $$

By plotting the table values we get (excluding the first data point as an outlier):
enter image description here

We can see from the equation that the $E^0$ would be about 0,708 V. However, literature references say the standard potential of the AgCl electrode is about 0,222 V. Any idea what I am doing wrong here?

Then I also should plot the HCl mean activity coefficient values calculated from the formula: $$ E(m) = E^0 - \frac{2RT}{F} \ln \left[ \gamma_{\text{HCl}} \left( \frac{m_{\text{HCl}}}{1\ \text{mol/kg}} \right) \right] \\ \Leftrightarrow \ \gamma_{\text{HCl}} = e^{ \frac{(E^0-E)F}{2RT} } / \frac{m_{\text{HCl}}}{1\ \text{mol/kg}} $$

Thus I get values:

\begin{array}{cc} \hline m & \gamma_\ce{HCl} \\ \pu{mol/kg} \\ \hline 0.001 & 4371.98 \\ 0.002 & 837.44 \\ 0.004 & 777.51 \\ 0.006 & 790.73 \\ 0.008 & 759.33 \\ 0.01 & 743.74 \\ 0.02 & 733.46 \\ 0.05 & 1080.77 \\ \hline \end{array}

But literature references (CRC Handbook of Chemistry and Physics - 97th Edition, p. 5-101) gives greatly different values:

\begin{array}{cc} \hline m & \gamma_\ce{HCl} \\ \pu{mol/kg} \\ \hline 0.001 & 0.965 \\ 0.002 & 0.952 \\ 0.004 & 0.929 \\ 0.01 & 0.905 \\ 0.02 & 0.876 \\ 0.05 & 0.832 \\ \hline \end{array}

Any help would be greatly appreciated.

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    $\begingroup$ You say hydrogen electrode. Is is a standard hydrogen electrode (SHE)? I have only ever met one person in my life, the late Prof. Roger Bates, who used an actual SHE, at the former NBS no less. They are notoriously finicky: freshly platinized platinum black on platinum, etc. Maybe try the Ag/AgCl electrode versus something easier and not finicky? $\endgroup$
    – Ed V
    May 16 '21 at 13:06
  • $\begingroup$ I used normal hydrogen electrode (NHE). $\endgroup$
    – nh3
    May 16 '21 at 13:11
  • $\begingroup$ Apparently your measurements tend to the potential of silver and not of AgCl. $\endgroup$
    – Maurice
    May 16 '21 at 13:13
  • $\begingroup$ That is actually true, that the values tend to point in the direction of a silver electrode (standard potential of 0.800 V). But how could that be, the electrolyte used in the lab work was hydrochloric acid (HCl). It does not contain any silver cations to be reduced to silver metal. $\endgroup$
    – nh3
    May 16 '21 at 13:21
  • $\begingroup$ May be your coating of $\ce{AgCl}$ is not perfect. It may slowly dissolve to give enough $\ce{Ag+}$ ions to the solution. $\endgroup$ May 16 '21 at 15:07
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The silver-silver chloride is a half cell. When the silver wire coated with AgCl is dipped into HCl solutions with no Ag$^+$ cations, as confirmed by the OP, it is not a half cell with AgCl in equilibrium with silver and silver chloride, but a half cell that is a silver anode corroding in acid solution (with oxygen from air?).

After some time (hours?), with stirring of the solution, enough AgCl may dissolve, still leaving enough on the silver wire, dropping the 0.5-0.7 V to wind up with the desired 0.222 V. Perhaps the HCl solutions need to be saturated with AgCl and deoxygenated before the immersion of the wire electrode.

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  • $\begingroup$ Basically OP worked with a quasi AgCl wire. It should not be expected to have right the Ag+/AgCl potential. $\endgroup$
    – Alchimista
    May 16 '21 at 16:39

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