I have measured the cell potential between a number of metals e.g. Ni, Pb, Al, Cu and Zn by constructing a galvanic cell and then used a voltmeter to measure the potential.
Now, if I want to calculate the theoretical potential (if I have understood this correctly) I need to look at a standard reduction potential table and compare the two elements in question. Take Al and Cu for example. The element that will act as the anode where oxidation will occur will be the element with the more negative cell potential. In this case Al because it has a value of -1,66 V compared to +0,34 V for Cu. Then since it is a reduction table I need to flip the reaction of Al to show an oxidation with a value of +1,66 V.
The full cell potential will be 1,66 V + 0,34 V = 2,0 V
I have used this same logic and gotten results for Ni and Pb (0,13 V), Al and Ni (1,4 V) etc.
Now my teacher asks me how come you haven't gotten any negative theoretical values and to check my work again? But surely I can't get any negative cell potential values since this is a galvanic cell or am I mistaken?
She asked the same thing with the real measured potentials with the voltmeter and of course here I got a few negative readings, but isn’t that simply because I placed the positive and negative ends of the voltmeter on the opposite electrodes? So I made all negative measurements positive as if I HAD placed them correctly…
Am I wrong here, should there be any negative values theoretically?