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I'm in kind of a pickle here. I made a galvanic cell of Lead and aluminium for a chemistry internal assessment but unfortunately, it didn't turn out so well and neither me nor my teacher has any clue why. The E0 value of the cell is 1.53v. I got a maximum reading of 0.29v

If you don't have time to read here is what I think the sources of error could be: 1. I used filter paper as a salt bridge and I don't think it was thick enough to allow ions to flow through it. 2. I used around a 1M concentration of KCl to dip the salt bridge in. 3. I suspect either of the metals was not correct but this seems improbable as I got a voltage just not the correct one. 4. The solutions of AlNO3 and PbNO3 were kept in round-bottomed flasks with a glass cork in them for a few days before I decided to do the experiment. 5. I got a negative reading for the voltage which was weird. 6. I got a higher voltage reading only when I touched the aluminium rod to the salt bridge in the solution

I used the following procedure:

  • I made 0.01M, 0.1M, 0.25M and 0.5M solutions of aluminium nitrate and a 0.1M solution of lead nitrate (both well dissolved) from lab equipment.
  • I poured each of them into separate 50ml beakers. I then placed an aluminium rod (that was 50% submerged) into the AlNO3 beaker and a thin piece of lead (that was well sanded before each reading) that was fully submerged in PbNO3.
  • I connected each of the metals to a multimeter via alligator clips
  • I then prepared a salt bridge using 5x25cm strips of filter paper which were dipped in a solution of KCL of about 1M concentration. The filter strips were folded and stapled.
  • I then placed the salt bridge in both the beakers and conducted the experiments.

I'm not in school right now and I don't have time to conduct the experiment again as the assessment is due very soon.

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  • $\begingroup$ You do know that SEM isn't the same as E0 and depends on concentrations, time, and other stuff? $\endgroup$
    – Mithoron
    Mar 27, 2019 at 18:54
  • $\begingroup$ Another question is possible passivation of lead and mainly aluminium by oxidized layers acting against electrode kinetics. Potentials are thermodynamic quantities, they do not say, how fast are the reactions ongoing. If the reaction is very slow, then the cell may act externally as a power source with huge internal resistance. Additionally, the potentials in open state may not be established. Salt bridges are used to have at least one common ion with electrolyte, so KNO3 Is preferred. The forming of PbCl at solution/paper contact can be an issue as well. $\endgroup$
    – Poutnik
    Apr 27, 2019 at 5:09

3 Answers 3

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Just keep in mind that Eo is determined under highly controlled conditions and concentrations. I sense two key problems here. The major problem is already stated above that the resistance of a wet filter paper must be high, and this is giving you lower voltage. Evaporation of water from the filter paper would make the situation worse. Since you are not repeating the experiment anyway, make sure to identify the causes of lower voltage. Secondly, both lead and aluminum nitrate tend to form hydroxides in water if you don't add little bit of acid in them. Did you notice that lead solutions were cloudy or they were clear and transparent?

As a side note, negative voltage reading of the battery just means that your connections of the lead are the other way round on the voltmeter. Test your voltmeter with a battery keeping the signs on the battery in your mind. Once you get a positive reading of a simple AAA cell you have correctly made the connections.

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  • $\begingroup$ +1. All good calls and the lead cathode should not have been totally submerged with the alligator clip and its attached wire. I wish the filter paper/paper towel salt bridge would go away. Also, the suggestion by @Poutnik (to use potassium nitrate rather than KCl in the salt bridge) is good: no need to risk clogging the ersatz salt bridge with lead chloride. $\endgroup$
    – Ed V
    Aug 25, 2019 at 1:32
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Eliminate the salt bridge. Test Al and Pb in a) Al(NO3)3 solution and b) (separately) Pb(NO3)2 solution. The salt bridge is for purity of thought and separation of the electrochemical reactions. A third possibility is to put an Al and a Pb electrode in HNO3 solution: any output voltage?

Just try to get a cell going. Aluminum does not dissolve easily in HNO3, and may require some catalyst, like fluoride or chloride. In other words, your aluminum may be passivated. If the lead surface is oxidized (most lead I see is dull, oxidized), it may require some activation to dissolve off the oxide before it is really a good electrode.

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I think, there is still something wrong with the salt bridge. It works like a resistor I guess. You loose voltage according the law of Ohm. U = R*I. Did you try for a short time to use a wire between your two cells. If yes how is the voltage reading. Other possibilities are oxidising of the lead and aluminium as well AND KCl means chloride ions , which can form precipitation of PbCl2. Blocks the saltbridge. Did you immerse both eletrodes in KCl solution for test? Exchange the saltbridge to potassium nitrate. KNO3.

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  • $\begingroup$ Using a wire would create a serie of 2 cells. Al wire||wire Pb. $\endgroup$
    – Poutnik
    Aug 25, 2019 at 3:53

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