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If I dip two pieces of metal in tap water I measure a potential difference between the two electrodes with a common digital voltmeter. For example, for Cu and Al I measured 0.55 V and for Cu and Fe I measure 0.86 V. Is there any way to relate these potential differences to the standard electrochemical potential given in tables? (for Al i found -1.66 V, for Cu +0.34 V and for Fe, +0.77 V).

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    $\begingroup$ Unfortunately, the system under test is ill-defined and ill-controlled: tap water should be potable, but that is not at all chemically specific. If you stick copper and iron electrodes into a lemon, then there is a rich body of literature we could consult. ;-) Tap water, not so much. $\endgroup$
    – Ed V
    May 31 at 17:01
  • $\begingroup$ What if it's a 5%solution of HCl in deionized water instead of tap water? $\endgroup$
    – nasu
    May 31 at 17:40
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    $\begingroup$ Then you need to start looking at the chemistry: aluminum and iron react with hydrochloric acid, but copper does not react with 5% HCl solution. So what are the possible redox half-reactions? Please note that this is a Q & A site and people are expected to do some investigation/research on the topic of their question. Just asking a question is generally not going to get a positive or useful response. $\endgroup$
    – Ed V
    May 31 at 17:55
  • $\begingroup$ Anyway you cannot compare the standard potential and the potential of a metal dipped n water, where the concentration of the corresponding ion is unknown. The tabulated redox potential are always defined in a one-molar solution of the ions. $\endgroup$
    – Maurice
    May 31 at 18:59

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