Voltage potential of a metal in an electrolyte
What physical property determines the voltage generated when a metal is a half-cell in a standard electrolyte. For example, zinc, copper and iron generate different voltages in same type solution and referenced to the same type other electrode?

I was going to write a long-winded explanation based on my limited knowledge of electrochemistry, but I found a site which explains it better:

http://antoine.frostburg.edu/chem/senese/101/redox/faq/different-metals-different-voltages.shtml