# What is the meaning of signs in electrode potentials?

## Question

Suppose you have a cell set up between a copper metal/copper(II) ion electrode and a reference electrode.

Under standard conditions, the emf of this cell was −0.07 V. The standard electrode potential of the copper metal / copper(II) ion electrode is +0.34 V. Hence the standard electrode potential of the reference electrode is:

A: -0.41 V

B: -0.27 V

C: +0.27 V

D: +0.41 V

## My attempt:

I was taught this equation: $\mathscr{E} _{cell} = \mathscr{E} _{a} - \mathscr{E} _{b}$

where:

$\mathscr{E} _{a}$ is the emf of the more positive electrode potential

$\mathscr{E} _{b}$ is the emf of the less positive electrode potential

But using this equation, none of the combinations of electrode potentials give -0.07V as the emf.

Thanks